Ecology of Primary Terrestrial Consumers

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or, if natural logarithms are used:— NNkt0⫽ exp( ). These equations have led to the alternative and preferred descriptions of the response as being exponential or logarith-mic. Since the number of viable cells decreases throughout the process, then the rate constant, k, is always negative in character. The two curves shown in Figures 2(b) and 2© illus-trate similar, though opposite, deviations in response from the straight line case. These are described, according to their shape, as concave or convex, and qualified by the additional designation upward or downward, to indicate orientation. The graphs indicate that the rate of disinfection changes gradu-ally in the early stages, but then assumes a more steady state of change similar to that in the straight case. In Figure 2(b), the rate of disinfection is relatively slow at first, but gradu-ally increases to a steady, limiting value. Various interpre-tations have been suggested to account for this change; among them, that the distribution of resistances between the individual cells exhibits a relative deficiency of cells of low resistance; or alternatively, that the cells must pass through one or more intermediate stages before becoming sensitive to the disinfection process in question. The weakness of such interpretations is underlined by the fact that changes in the experimental conditions often result in a change in the shape of the graphical response. Figure 2© illustrates a relatively fast rate of disinfection initially, which gradually decreases to a steady, limiting value. In Figure 2(d) is illustrated the most common deviation from the straight line response, the sigmoid curve. Responses of this type are more easily demonstrated in systems employ-ing a moderate rate of disinfection. As the rate of disinfection is increased, the limitations of viable counting techniques make it more and more difficult to monitor the progress of the process with any precision. This results in the apparent response becoming indistinguishable from the straight line case. It is sometimes suggested that the sigmoid response is the most common situation encountered, but that it is often unrecognized due to the practical difficulties experienced. The sigmoid response is usually interpreted as an indication that the distribution of resistances between the individual cells is of the log normal type. Complete agreement with this model distribution is indicated when the sigmoid curve is symmetrical. Figure 2(e) illustrates a response of particular interest. The graph consists of two parts, both of which are linear but of different slope, with a fairly sharp transition between the two. This would appear to indicate a fairly rapid rate of disinfection initially, followed by a fairly sharp transition to a lower, but steady, rate. Such a sudden transition naturally engenders interest, if not suspicion. It has been suggested that this type of response indicates the presence of two distinct groups of cells, each of which exhibits its own characteristic distribution of resistances. Experiments with a mixture of two bacterial cultures of different identity, whether obtained different species, or consisting of spores and vegetative cells of the same species, can be shown to yield this type of response. The first part of the graph corresponds to the usual response of the more sensitive component, and the second part to that of the more resistant component. However, at relatively low temperatures and humidities, exposure of nominally homogeneous cultures to ethylene oxide gas often yields this type of response. While it is sometimes suggested that this indicates the presence of two distinct groups of cells, as discussed above, it must also be considered that not only can this phenomenon be demonstrated with cultures appar-ently homogeneous to other sterilization methods, but also that this two part response reverts to the exponential type on increasing the temperature or humidity of the system. As indicated in the foregoing discussion, consideration of the shapes of survivor curves may provide useful circum-stantial evidence on which to base hypotheses relating to the response of cell populations to disinfection processes. However, as also indicated in the discussion, the operative word is “circumstantial.” Empirical Parameters While a survivor curve illustrates the response of a cell popu-lation in terms of variation in number of survivors with time (or dose) of disinfection treatment, this is essentially a static time(Log)Surviving Fraction1.0(a) (b)© (d)(e)FIGURE 2C004_002_r03.indd 226C004_002_r03.indd 226 11/18/2005 10:19:27 AM11/18/2005 10:19:27 AM

227situation since all other factors are held constant. Often, it is the variation in response of the system to changes in experi-mental conditions which is of particular interest. This varia-tion in response may be monitored by constructing families of survivor curves, one curve for each level of the factor being varied. Comparison of curves within these families indicates the nature of the change in response. The two factors most prominent in the regulation of disinfection processes are temperature and concentration of disinfectant. For both of these factors, it is found that their influence on the disinfection process varies in a regular manner over a fairly wide range of conditions. As a result, empirical parameters have been derived which enable the influence of these two factors to be characterized in a con-venient form. Temperature Coefficient In general, it is found that the activity of a disinfectant varies directly with the temperature, i.e., the higher the tempera-ture the greater the activity. This change in activity may be quantified by expressing the disinfection rates observed at two different temperatures as a ratio. The value of this ratio is found to remain reasonably constant over a wide range of temperature. This may be expressed mathematically: uTTkk2121⫺⫽ / where k 1 and k 2 are the rate constants at temperatures T 1 °C and T 2 °C, respectively, with T 2 greater than T 1 . The con-stant, u, is termed the temperature coefficient, and assumes a numerical value which is characteristic of the agent and, to a certain extent, of the organism, employed. The super-script ( T 2 − T 1 )°C is necessary to indicate the temperature difference. Since the disinfection rate is inversely related to killing or extinction time, the latter may be used instead. The expression then becomes uTTtt2112⫺⫽ / where t 1 and t 2 are the extinction times at temperatures T 1 °C and T 2 °C, respectively. At high value for u indicates that the process is relatively sensitive to temperature changes. It is usual to find two versions of the temperature coeffi-cient in most common use: the coefficient for a 1°C tempera-ture change, u, when the superscript is usually omitted; and the coefficient for a 10°C temperature change, u 10 , which may be sometimes expressed as Q 10 . The popularity of the 10°C coefficient follows from its use to characterize changes in reaction rates in chemical systems. This enables interest-ing comparisons to be made between disinfection processes and non-living chemical reactions. Attempts have been made to deduce modes of death of the organisms by such compari-sons. However, subsequent biochemical investigations have tended to disagree with these deductions. Dilution Coefficient The activity of disinfectants under otherwise constant condi-tions is found to vary directly with the concentration of dis-infectant employed over a considerable concentration range. As before, killing or extinction times are usually employed as a measure of disinfection rate. The effect of concentration may be expressed mathematically as: c ht ⫽ a constant or: h log c ⫹ log t ⫽ a constant where c ⫽ concentration of disinfectant t ⫽ extinction time h ⫽ dilution coefficient. A high value for the dilution coefficient indicates that the process is relatively sensitive to changes in concentration of disinfectant. The dilution coefficient is sometimes referred to as the concentration exponent. Other Factors Influencing Activity The antimicrobial activity of several disinfectants is influenced to a considerable extent by changes in pH. For example, a rise in pH results in a decrease in the activity of phenols (Bennett, 1959), organic, acids, compounds liberating chlorine, benzoic acid and iodine, although iodine is less affected by acidity than is chlorine. An increase in pH increases the dissociation of phenols and benzoic acid (Wedderbern, 1964); chlorine in water forms HClO, which is dissociated with a rise in pH with a concomitant loss of activity. In contrast to the above, how-ever, there is an increased microbicidal activity of the quater-nary ammonium compounds (QACs) and of acridines (Foster and Russell, 1971); in the case of QACs, the effect of pH is considered by Salton (1957) to be on the cell rather than the disinfectant molecule, since the number of negatively-charged groups on the bacterial surface will be increased as the pH rises, thus influencing the number of positively charged mol-ecules which can be attached. Another factor which influences the activity of certain antimicrobial agents is organic matter, e.g., the presence of blood, serum, pus, etc. In general terms, the more chemically reactive a compound, the greater the effect of organic matter on its activity. This is particularly true with the hypochlorites. other examples are provided under individual compounds later. Mathematical Models A considerable number of mathematical models have been derived at various times, in attempts to reconstruct the disinfec-tion process. These have utilized deterministic, probabilistic, and thermodynamic approaches to the problem, and have been reviewed in detail by Prokop and Humphrey (1970). In general, these models are based on attempts to recon-struct the survivor curves obtained, even though, as already discussed, these are liable to be changed by changes in C004_002_r03.indd 227C004_002_r03.indd 227 11/18/2005 10:19:27 AM11/18/2005 10:19:27 AM
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experimental conditions. In addition, they usually involve preliminary assumptions concerning the nature of the disinfection process. Consequently, attempts to deduce mechanisms of killing from these models tend to be rather disappointing. They are, however, useful in terms of con-cise descriptions of the results of the process; and as fur-ther biochemical information on modes of killing becomes available, their usefulness and reliability should increase further. DISINFECTANT TESTING The testing of disinfectants is a topic with a long history of controversy. Differing opinions on the merits and relevance of various methods are legion, and have been the cause of many heated exchanges between those holding them. The wide variety of methods may most conveniently be consid-ered under the headings of screening, standardization and “in-use” tests, although there is a degree of overlap between the three categories. The interested reader should consult the papers by Forsyth (1975), Miner et al. (1975) and Reybrouck (1982) for further information. Screening Tests These are usually of the simplest type, since they are specula-tive by nature, and often involve the testing of large numbers of disinfection agents or formulations; both time and cost usually dictate this simplicity. The actual methods employed vary according to the physical characteristics of the disinfec-tant and the type of use envisaged. Ideally, any test should be as realistic as possible, although for screening purposes this will be subject to the requirements of simplicity discussed above. Disinfectants which are soluble or miscible in water are often incorporated in microbiological culture media which are then inoculated with suitable microorganisms. After incubation under appropriate conditions, the inoculated media are examined for growth of the organisms, absence of growth indicating that the inoculum has been inhibited. Where the continuous presence of the disinfectant is inap-propriate, then the method is modified to include a suitable means of removing or inactivating the disinfectant after a predetermined exposure time. Other disinfectants may be tested in a similar manner, by arranging for inocula of suitable organisms to be sub-jected to standardized exposure to them. After exposure, the organisms are inoculated into suitable culture media, and incubated as before. Since the end-point in these tests is the death of all the organisms involved, this type of test is often referred to as an Extinction Test. The phrase “suitable organisms” used above, represents one of the key factors in the testing of disinfectants. A disin-fectant is expected to kill all undesirable organisms, which usually refers to organisms injurious to health (see previous section). Test organisms may be chosen, therefore, either for their resistance to disinfection or, like Salmonella typhi, for their medical significance. Although the designation “undesirable organisms” covers a wider range of life-forms, most disinfectant testing has employed various species of bac-teria for reasons of convenience. In retrospect, this does not appear to have been a significant disadvantage, since activity against bacteria usually coincides with that against the other life-forms. However, although it is recognized that bacterial spores are much more resistant to disinfection than the veg-etative forms, the latter have, with few exceptions, invariably been used in testing. This is partly due to convenience, but it should be noted that there are many disinfectants marketed which under normal conditions of use are incapable of killing bacterial spores. This has been countered, in certain quarters, by re-defining a disinfectant as a chemical agent capable of killing bacteria but not necessarily bacterial spores (see pre-vious section). Standardization Tests This is the area which has given rise to the greatest amount of contention. In most cases, this has been due to misin-terpretation and misapplication of the results of testing in situations where they have little, if any, relevance. The standardization of disinfectants usually requires the use of microbiological techniques rather than chemical assay. Even when chemical characterization of the active agent(s) involved is possible, the activity of a disinfectant will usu-ally be significantly influenced by factors concerned with the formulation and method of use of the product. However, although the disinfectant may be standardized in terms of its intended biological effect, this is carried out under specific, controlled conditions. Depending on the similarity of their circumstances, the results from standardization tests may, or may not, be capable of extrapolation to practical situations. This is where the controversy tends to arise. The most popular methods of testing for standardiza-tion purposes have been extinction tests. These are basically similar to the general method discussed in the previous section, except that the procedure and materials used are rigidly standardized in order to achieve best reproducibility of results. In addition, the most widely used tests employ the pure chemical, phenol, as a control of the resistance of the test organisms used. Since results are expressed by comparison of the activities of phenol and the disinfectant being tested, these tests are referred to as Phenol Coefficient methods. The most popular and official versions in use are the Rideal–Walker Method (as modified by B.S. 541), the Chick–Martin Method (as modified by B.S. 808), and the Association of Official Agricultural Chemists (AOAC) Phenol Coefficient Method (1970). Details of these tests may be found in the appropriate publications. While the use of phenol as a control on the resistance of the test organisms is extremely valuable, it has led to widespread assumptions that the ratio of activities of disin-fectant and phenol indicated by the phenol coefficient will hold true in all circumstances. This, of course, is far from true. The AOAC (1970) attempted to improve this situation by introducing a Use-Dilution Method. This test is based on C004_002_r03.indd 228C004_002_r03.indd 228 11/18/2005 10:19:27 AM11/18/2005 10:19:27 AM

229the hypothesis that disinfectants in use shall be at least as efficient as 5% phenol, and that a dilution of twenty times the phenol coefficient should achieve this. Accordingly, this dilution is tested against standardized cultures in order to confirm this, or alternatively, in order to determine a correc-tion factor. Whereas the extinction methods discussed above evaluate the extinction concentration corresponding to predetermined exposure times, Berry and Bean (1954) devised a test for evaluating extinction times for chosen disinfectant concentra-tions. While the test is only applicable to phenolic and other easily inactivated disinfectants, it has been claimed to be at least as reproducible as other methods in common use (Cook and Wills, 1954). The method of assessing the end-point of the reaction has been further improved by Mathei (1949). Methods other than extinction tests for standardizing dis-infectants include various methods based on assessment of cessation of vital enzyme activities. The enzymes involved have generally been certain oxidases or dehydrogenases (Sykes, 1939; Knox et al. , 1949). In a less specific manner, inhibition of respiration has been used as a method of assess-ment (Roberts and Rahn, 1946). The controversy surround-ing these methods centers around the problem of correlating enzyme activity with viability. Other methods which have been proposed include mea-surement of post-incubation opacity corresponding to stan-dard survivor levels (Needham, 1947), and also measurement of cell volume increase following post-exposure incubation (Mandels and Darby, 1953). The range of tests discussed above is by no means exhaustive, and only covers general purpose tests appli-cable to water-misible disinfectants. Any number of alter-native tests could be devised by appropriate selection and standardization of the various parameters. In addition, there are numerous tests which can be, and have been, devised in order to standardize disinfectants intended for specific uses such as sporicidal or tuberculocidal duties (AOAC 1970). Methods of standardizing disinfectants other than those to which the above discussion applies are usually designed more closely around the particular use envisaged. Thus, a considerable element of actual, or simulated, in-use testing is usually involved. For convenience, they are best discussed in the following section. A recent test is the Kelsey–Sykes test (Kelsey and Maurer, 1974), which is a form of capacity test. In this, incremental additions of test organisms are made to appropriate dilutions of test disinfectant and aliquots are removed for detecting survival immediately prior to the next addition of organisms. On the basis of this method, use-dilutions of the test dis-infectant (which need not necessarily be a phenolic) under clean and dirty conditions can be recommended to hospitals, which should then check them during in-use tests (see also Coates, 1977; Cowen, 1978). In-Use Tests The previous two sections have dealt with testing methods applied in the laboratory, which yield information primarily of value to the disinfectant manufacturer. The “consumer,” however, is almost solely concerned with the performance of disinfectant materials under the conditions of use which are associated with the application envisaged. To this end, he is more interested in testing methods which resemble, as closely as possible, these practical conditions. However, the foregoing remarks should not be taken to imply that labo-ratory standardization methods are completely arbitrary. In view of the greater difficulty experienced in killing organ-isms which are present as dried surface films, as opposed to those in fluid suspension, many standardization tests have employed films on various surfaces as their inocula. Surfaces used have included silk thread (Koch, 1881), garnets (Kronig and Paul, 1887), glass cover-slips (Jensen and Jensen, 1933), glass cylinders (Mallmann and Hanes, 1945), glass slides (Johns, 1946), stainless steel cylinders (AOAC Use-Dilution Confirmatory Test, 1960), and glass tablet tubes (Hare, Raik and Gash, 1963). It should be noted that while these sur-faces represent a step toward the practical situation, they nevertheless comprise a collection of laboratory “artifacts” when compared with real situations. A nearer approach was achieved by use of surfaces such as rubber strips (Goetchins and Botwright, 1950) and glazed, waxed, and rubber tiles (Rogers, Mather and Kaplan, 1961). Where the physical size of articles required to be disin-fected is fairly small, it is quite feasible to carry out in-use testing in the laboratory. Hence metal trays were used by Neave and Hoy (1947), 10 gallon milk churns by Hoy and Clegg (1953), and small drinking glasses by Gilcreas and O’Brien (1941). Similarly, scalpels, syringes and similar small items may conveniently be subjected to in-use testing in the laboratory. Where the physical size of the system is somewhat greater, then the laboratory must be forsaken in order to carry out on-site testing. Typical examples of such situations include hospital walls and floors, and industrial or dairy processing machinery. The outstanding value of in-use testing arises from the fact that results obtained are directly applicable to the system without the need for interpreta-tion and extrapolation, and that practical difficulties such as short contact times or inaccessibility of certain areas can be accounted for. The choice of test organisms usually reflects either of two main approaches. The simplest approach is to inoculate the system artificially with organisms considered to be of practical significance in the particular application. This sig-nificance may be due to the resistance to disinfection of the organism, or alternatively, to its practical effect on the system should it survive the disinfection process. In order to simu-late the practical system, the organisms may be suspended in appropriate materials before inoculation. An example would be the use of milk as a suspending fluid in tests on dairy disinfection. A slight variation on this approach is to inoculate with indeterminate mixtures of likely organisms obtained from natural sources, such as low quality, raw milk. The second approach involves the use of the normal, pre-existing flora of the system which has arisen during normal use. Investigations of this flora before and after the disin-fection process would provide direct evidence as to whether C004_002_r03.indd 229C004_002_r03.indd 229 11/18/2005 10:19:27 AM11/18/2005 10:19:27 AM
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the process has achieved its object. One difficulty with this approach is that of providing suitable growth conditions for all of the possible species of organisms which may be pres-ent. Also, the normal flora is likely to vary with time, and so, ideally, this method is best applied in the form of a routine monitoring procedure. Both of these difficulties produce an increase in the financial cost of this type of approach, but the reliability of the results obtained is correspondingly high. The ideal method would involve a combination of these two approaches: an initial test with known resistant organisms in order to indicate the upper levels of activity which may be required; this to be followed by routine monitoring tests to guard against both unsuspected resistance amongst the normal flora, and unforeseen breakdowns in the method of application. Any testing method which involves the use of surface films is subject to the problem of physical recovery of the organisms. In the case of small surfaces, the articles can often be placed in or on a suitable nutrient medium, and provided this allows contact between the medium and the organisms in the film, then growth takes place. Where larger or less accessible surfaces are concerned, then the simple direct method will have to be replaced by some kind of sampling technique. Sampling techniques vary widely and not all are applicable in all situations. In the case of accessible surfaces, simple methods such as wiping with sterile swabs may be used; or alternatively, flooding the surface with a sterile liquid, some or most of which is then removed by swab or pipette. An interesting alternative consisting of a sterile, agar medium, “sausage” was devised by Ten Cate (1965). The exposed transverse surface of this sausage is pressed against the surface to be sampled. After sampling a slice is cut from the sausage and incubated with the exposed side uppermost. A simple method which is used for sampling skin surfaces may also be used in other applications. This method employs adhesive cellophane tape. The adhesive surface is pressed onto the surface to be sampled, and then removed for trans-fer to a suitable medium. Where surfaces are inaccessible, as in pipes and processing machinery, then a rinsing tech-nique is usually most convenient. The resulting liquid may be added directly to suitable nutrient media, or, if present in large bulk, may be filtered through membrane filters to remove the organisms. The resulting filter membrane is then transferred to a suitable medium. One final problem which is of importance in all meth-ods of testing which involve assessment of viability, is that of recovery and growth of the test organisms following exposure to the disinfectant. Organisms which have survived a disin-fection process often show altered requirements for optimal growth. Response to physical conditions such as incubation temperature, as well as to biochemical conditions such as dependence on certain nutrients, may be completely altered from that of unexposed cells. While considerable effort has been made to derive efficient recovery methods (Flett et al. , 1945; Jacobs and Harris, 1960; Harris, 1963; Russell, 1964) the problem is so variable and so many combinations of fac-tors must be considered, that it is far from being solved. There is general agreement that recovery methods should be selected which will allow maximum recovery of exposed organisms; but putting this into effect can be extremely difficult. LIQUID DISINFECTANTS Several chemical agents have long been employed for destroy-ing microorganisms, although it is frequently asserted that such substances are without effect on bacterial spores. With many agents, however, this is untrue (Sykes, 1970; Russell, 1971, 1982). The most important substances are phenols and cresols, biguanides, chlorine-releasing compounds and other halogens, aldehydes, alcohols, quaternary ammonium com-pounds, mercury compounds, strong acids and alkalis and hydrogen peroxide. The majority of these are considered in detail below. Further information is provided by Hugo and Russell (1982). Phenols and Cresols Although Kronig and Paul (1897) and Chick (1908) showed that phenol was active against spores, the concentrations, 5%, employed, were considerably higher than those needed to kill vegetative bacteria. More recent studies have indicated that bacterial spores are not killed even after long exposure to phenols (Sykes, 1958, 1965; Loosemore and Russell, 1963, 1964; Russell and Loosemore, 1964; Russell, 1965, 1971; Rubbo and Gardner, 1965; Briggs, 1966). Of the bacterial spores. Bacillus stearothermophilus is the most resistant to phenol and B. megaterium the most sensitive (Briggs, 1966). However, in contrast to its lack of sporicidal activity, phenol is sporostatic at low concentrations. Several factors influence the antimicrobial activity of phenols and cresols (Bennett, 1959; Cook, 1960; Bean, 1967): 1) Concentration. These compounds have high con-centration exponents, h, which, as described above, indicates that they rapidly lose their antibacterial activity on dilution. This also means that dilution procedures can be used to prevent the carryover of inhibitory concentrations into recovery media when viable counts or sterility tests are being carried out (Russell, 1982; Russell et al. , 1979). Studies from Bean’s laboratory (Bean and Walters, 1955; Bean and Das, 1966; Bean, 1967) are of interest, for they show that with dilute solutions of disinfectants with high intrinsic activity, e.g. benzylchlorphenol, the high proportion taken up by the cells means that the concentration remaining is only weakly bacte-ricidal, so that the surviving cells do not meet lethal conditions. 2) Temperature. The bactericidal activity of the phe-nols and cresols increases rapidly with an increase in temperature. Examples of temperature coeffi-cient ( u 10 ) against E. coli at 30–40° (Tilley, 1942) are: phenol 8.4, o -cresol 6.9, p -cresol 5.6. C004_002_r03.indd 230C004_002_r03.indd 230 11/18/2005 10:19:27 AM11/18/2005 10:19:27 AM

231 Of importance, also, is the finding that these substances are sporicidal at elevated temperatures (Berry et al. , 1937; Russell and Loosemore, 1964). As a result of the findings of Berry et al. (1937), one method of sterilizing certain injections by heating them with 0.2% w/v chlorocresol is still an official method in Britain (British Pharmacopoeia, 1980). 3) pH. The phenols are more active at an acid pH than in alkaline solution, as phenates (phenox-ides) are formed at high pH. Acid pH also results in a more effective, although still slow, sporicidal action (Sykes and Hooper, 1954). 4) Organic matter. The presence of organic matter may decrease the antimicrobial activity of these compounds; this was early recognized in the design of the Chick–Martin test for evaluating phenolic disinfectants (Chick and Martin, 1910; Garrod, 1934, 1935). The results do, however, depend on the actual phenol used and on the kind of organic matter, and the interference is less than with other disinfectants such as the quaternary ammonium compounds (Cook, 1960). 5) Oxygen tension. Anaerobic bacteria are generally more resistant than aerobes to phenols. Moreover, facultative organisms, e.g., E. coli, are more resis-tant when grown under anaerobic conditions. 6) Type of organism. As described above, these compounds are bactericidal and sporostatic at low concentrations, and sporostatic and not spo-ricidal even at high concentrations. As a group, however, they are also fungicidal to several moulds, and use is made of this in the inclusion of cresol and chlorocresol as preservatives in creams which are liable to fungal contamination (Wedderbern, 1964). Morris and Darlow (1971) have pointed out that phenolic compounds with high R-W coeffi-cients are effective against some viruses, but that they are generally too variable in their activity to be suitable as general virucidal agents. 7) Chemical nature. Dihydric and trihydric phenols (Figure 3) are generally less active than phe-nols, and alkylation of monohydric phenols to give cresols potentiates the antimicrobial activ-ity. Also halogenation of the phenols increases their activity (although to a lesser extent if the halogenation is in the ortho- than in the parapo-sition) and this is even more pronounced when accompanied by the introduction of aliphatic or aromatic groups into the nucleus, e.g., p -chloro- m -cresol (chlorocresol). This increase in anti-microbial activity is, however, paralleled by a decrease in water solubility. To overcome this decrease in aqueous solubility, vari-ous soaps have been used to render water-soluble (solubi-lize) these substances. However, the effect of soap on the biological efficacy of the phenols depends on two factors, firstly the nature of the soap, and second the proportion of soap to phenol. Solution of cresol with soap (Lysol, BP), for example, contains 50% of cresol in a saponaceous solvent and has a bactericidal activity which depends on the nature and amount of soap used. The ratio of cresols to soap may be critical, the optimal cresol–soap ratio being of the order of 2:1. In lysol, the soap content is c. 22%, and the ratio is thus 2.2:1. The soap solutions are able to solubilize insoluble phenols in the micelles; the critical micelle concentrations (cmcs) of different soaps vary, and this explains differences in bactericidal action noted above. Lysol and the so-called “black-fluids,” which consist of the lower coal tar phenols, are formulated in sufficient soap so that they are retained in solution when diluted with water. In contrast, the white fluids consist of concentrated emul-sions of high boiling phenols stabilized with protective col-loids; they can be diluted with hard or soft waters, whereas black fluids should be diluted with soft waters only. Micelles have been considered as being reservoirs of phe-nols, so that when the concentrated solution is diluted before use, the phenols are released by dilution below the cmc to give a highly active solution. Two types of system have been investigated experimentally in attempts to assess the exact role of the micelles; these are (a) constant phenol concentra-tion, and (b) a constant phenol/soap ratio (Berry, 1951; Berry and Briggs, 1956; Berry, Cook and Wills, 1956; Cook, 1960). With a constant phenol system, there is a rapid increase in bactericidal activity below the cmc of the soap which could be the result of an increased uptake of phenol together with an increased permeability of the bacterial surface (Mulley, 1964); however, above the cmc in this system, there is a decrease in bactericidal activity, as the increasing numbers of micelles being formed compete for the phenol with the cell surface. In systems where there is a constant phenol/soap ratio, there is likewise an increase in activity below the cmc, OHOHOHOHOHOHPyrogallolResorcinolm-Cresol ChlorocresolPhenolOHOHCH3CH3ClFIGURE 3C004_002_r03.indd 231C004_002_r03.indd 231 11/18/2005 10:19:28 AM11/18/2005 10:19:28 AM
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and a decrease in activity at the cmc. However, in this system the activity again increases at higher soap concentrations; thus, this increased bactericidal activity parallels a saturation of the soap micelles with the phenol. The reasons for this remain unclear, for although the soap itself could contrib-ute to or be responsible for the increased activity, systems in which the soap has a low activity give similar results. The actual role of the micelles is thus difficult to assess. Of the two systems described, it is apparent that the system with the constant phenol/soap ratio is more important from the viewpoint of practical disinfection. It is necessary to use the lowest possible proportion of soap to phenol, thus giving a comparable situation to bactericides in two-phase systems, in which the bactericidal efficiency is related to the concentra-tion in the aqueous phase (Bean and Heman-Ackah, 1965). Phenol itself is an effective bactericide and anti-fungal agent, which is used as a preservative in some injections and creams; it is also the standard reference substance in some methods of testing phenolic bactericides (see earlier). Cresol, a mixture of o -, m - and p -cresol, in which the meta -isomer predominates, and of other phenols obtained from coal tar, is highly bactericidal and fungicidal. Apart from being the active constituent of Lysol, it is also used as a preservative in certain injections and creams. Chlorocresol is a power-ful antimicrobial agent which, in conjunction with heat, is employed for the sterilization of certain injections. It is also used as a preservative in certain cosmetic creams and lotions, and is included in Sodium Benzoate and Chlorocresol Solution, which may be used for the storage of sterilized surgical instruments, the sodium benzoate delaying rusting. Chloroxylenol has low water solubility and is solubilized by means of soap, the solution being known as Roxenol. Its antimicrobial activity is markedly reduced in the presence of organic matter. Thymol is a potent bactericide and fungicide which, in the form of Glycerin of Thymol, is employed as an oral antiseptic. Hexachlorophane [2,2⬘-methylenebis(3,4,6-trichloro-phenol)] is most frequently used as a soap contain-ing 2% of the substance; the effectiveness of this soap as a skin disinfectant may depend upon the accumulation of hexachlorophane on the skin. Pentachlorophenyl dodecano-ate is extensively used as a fungicide and insecticide in the textile and packing industries. Unlike the parent molecule, pentachlorophenol, it is nontoxic to humans. None of the above compounds can be relied upon to kill bacterial spores at ordinary temperatures. Another group, related to the phenols, consists of the esters (parabens) of p -hydroxybenzoic acid (3-hydrobenzoic acid). Unlike benzoic acid, the dissociation of which increases with increasing pH with a corresponding decrease in activity, the parabens are active against bacteria and fungi over a fairly wide pH range. Their bactericidal and fungicidal properties increase within the homologous series, but this is paralleled by a decrease in aqueous solubility. The parabens are not spo-ricidal, and although on their discovery they were heralded as being the ideal preservatives, they are now used (often in combination of two or more) mainly as preservatives in various pharmaceutical and cosmetic products (Russell, Jenkins and Harrison, 1967; Parker, 1982). The phenols and parabens have an effect on the cytoplas-mic membranes of bacteria and fungi (Hugo, 1976a,b). Biguanides and Bisbiguanides Biguanides have the general formula R1R2H .NHNHNHNCCN(5)(1)R2 Three distinct antimicrobial actions of N 1 , N 5 -substituted biguanides have to date been recognized (Weinberg, 1968): germicidal, antiviral and antimalarial. Unfortunately, no one compound is generally active in more than one of these three categories. The requirements for each type of activity are fairly unique, e.g., for maximum broad-spectrum germicidal activity, both N 1 and N 5 should have a halogen-substituted aralkyl substituent. Bisbiguanides have the general formula R1.NHNHNH NHNH NHNHNH NHNHCCCC(CH2)6R For maximum broad spectrum germicidal activity R and R 1 should consist of a halogen-substituted aryl group. A number of bisbiguanides are known to be germicidal, and the most important member of this group is chlorhexidine (R and R 1 are both C 6 H 5 Cl). Chlorhexidine was first described in 1954 (Davies et al. , 1954), and it is bacteriostatic in low concentrations; higher concentrations are bactericidal to several bacterial species, including strains of the genus Proteus, of which Pr. mirabilis is the most resistant, and Pr. rettgeri and Pr. morganii are the most sensitive to chlorhexidine, with Pr. vulgaris occu-pying an intermediate position. Some strains of Ps. aerugi-nosa may be highly resistant. Chlorhexidine also possesses antifungal activity. It is a membrane-active agent (Hugo, 1976a,b, 1982). Chlorhexidine, the active constituent of “Hibitane,” is used in surface disinfection, as an antimicrobial agent in eye-drops, and, in the presence of sodium nitrite to prevent corrosion, for the storage of surgical instruments. Chlorine-Releasing Compounds It was observed by Dakin (1915, 1916) that the commercial hypochlorites then in use were not of constant composition and contained free alkali and sometimes free chlorine. He thus developed a solution (Dakin’s Solution or Chlorinated Soda Solution, Srugical) which is still in use today. The sta-bility of free available chlorine in solution is dependent on a number of factors, in particular on the chlorine concentration, pH of the solution, the presence of catalysts, temperature, the presence of organic matter, and light (Dychdala, 1977). C004_002_r03.indd 232C004_002_r03.indd 232 11/18/2005 10:19:28 AM11/18/2005 10:19:28 AM

233 The types of chlorine compounds which are frequently used are (1) Hypochlorites. These are cheap and convenient to use, and have a wide antibacterial spectrum (Davis, 1963). They possess potent sporicidal activity (Truman, 1971; Kelsey et al. , 1974; Waites, 1982) which may be potentiated by alco-hols (Coates and Death, 1978; Death and Coates, 1979). The hypochlorites are moderately effective against animal viruses. The antibacterial activity of the hypochlorites decreases with increasing pH (Charlton and Levine, 1937; Weber, 1950; Ito et al. , 1967; Hays et al. , 1967), e.g., whereas 99% of spores of B. cereus are killed after 2.5 min at pH 6 by a solution containing 25 ppm available chlorine, nearly 8 hours are required for a comparable kill at a pH of c. 13 (Rudolph and Levine, 1941). At constant pH, the time to kill bacteria depends on the concentrations of available chlorine. The spo-ricidal activity of sodium hypochlorite may be potentiated by various compounds, e.g., by the addition of ammonia (Weber and Levine, 1944) or 1.5–4% sodium hydroxide (Cousins and Allan, 1967), notwithstanding the earlier comment about pH. In the presence of bromide, hypochlorite has an enhanced effect in bleaching cellulosic fibres as compared with hypochlorite alone, possibly because of a continuous generation of hypobromite when hypochlorite is in excess. A potentiation of the bactericidal effect of hypochlorite has been achieved by the addition of small amounts of bromide (Farkas–Himsley, 1964). The antimicrobial activity of hypochlorites is considerably reduced by organic matter. However, the hypochlorites are used in the disinfection of water, dairy equipment and eating utensils. (2) Chloramine-T (sodium p -toluene sulphonchlo-ramide). Dakin et al. (1916) considered that chloramine-T had a powerful germicidal action. It is bactericidal and sporicidal, although the rate of kill is slower than with the hypochlorites. Its activity is considerably higher at acid than at alkaline pH (Weber, 1950), and a drop of 10°C in the reaction tempera-ture results in a 3–4 fold increase in the time necessary to kill microorganisms (Weber and Levine, 1944). Chloramine-T is employed as a wound “disinfectant,” and as a general surgi-cal disinfectant. It is nonirritant and nontoxic, in contrast to Dichloramine-T (toluene- p -sulphon-dichloramide) which although a powerful disinfectant is not used because of its toxicity and instability. The mode of action of chlorine compounds is unknown, although several proposals have been made, e.g., the informa-tion of chloramines as a result of combination of chlorine with bacterial protoplasm, halogenation or oxidation reactions of chlorine with bacterial cells, changes in cellular permeability and an effect on enzyme systems. It has also been found, however (Bernarde et al. , 1967) that chlorine dioxide causes a marked and immediate cessation of protein synthesis in growing cells. Iodine and Iodophors Iodine in aqueous or alcoholic solution is considered by most authors (Gershenfeld and Witlin, 1950; Gershenfeld, 1956; Report, 1965; Sykes, 1970) to be a reliable and effective germicide which is lethal to vegetative bacteria, bacterial spores and acid-fast bacilli. Spaulding et al. (1977), however, consider that alcoholic iodine (0.5% iodine in 70% alcohol) possesses good activity against non- sporing bacte-rial and M. tuberculoses but none against bacterial spores, whereas Rubbo and Gardner (1965) state that bacterial spores are moderately resistant to iodine. Viruses are consid-ered by Rubbo and Gardner (1965) to be moderately sensi-tive to iodine. Iodine has a high fungicidal or fungistatic activity against yeasts and various moulds, but its antimicrobial properties are to a great extent inhibited in the presence of organic matter, since iodine is a highly reactive element. Iodine is sparingly soluble in cold water, but more solu-ble in hot water. Stronger solutions can be made in potassium iodide solution or in aqueous alcohol. Iodine is more effective as a germicide at acid than at alkaline pH, but is less affected by pH than are chlorine compounds. The concentration of iodine to disinfect does not vary greatly with different types of microorganisms. Various types of iodine solution are used for the first-aid treatment of small wounds and abrasions, and in pre-operative skin “disinfection.” Iodine has also been employed for the sterilization of surgical catgut (although this method is now little used) and is nowadays used for the disinfection of drinking and swimming pool water, the disin-fection of instruments and of clinical thermometers, and the sanitization of eating and drinking utensils. Unfortunately, iodine solutions stain fabrics and tissues and tend to be toxic. However, certain non-ionic surface-active agents can solubilize iodine to form compounds, the iodophors (Blatt and Maloney, 1961; Davis, 1962, 1963, 1968) which retain the germicidal activity of iodine, but not its undesirable properties; these iodophors are literally “iodine-carriers.” They are active against bacterial spores, including pathogenic anaerobic spores (Lawrence et al. , 1957; Gershenfeld, 1962). It is the concentration of free iodine in an iodophor which is responsible for its microbial action; this has been well demonstrated by Allawala and Riegelman (1953) who made a log-log plot of killing time against amount of free iodine, and found that the 99% killing time of B. cereus spores was a function of the concentration of free-iodine in the presence or absence of added surface-active agent. The bactericidal properties of the iodophors are increased at low pH values, but their stability is unaffected (cf. hypochlorites). They may thus be employed with acids, e.g., phosphoric acid, to enhance their microbial action and also to assist in preventing the formation of film or milkstone (see later) in the dairy industry. The iodophors are consid-ered (Davis, 1968) to be powerful detergents, although they do not dissociate protein as readily as do alkalis. The formu-lation of acidic solutions of iodophors is particularly useful when calcium or magnesium scale is encountered, but they can be corrosive, especially with galvanized iron. Surface-Active Agents Surface-active agents have 2 regions in their molecular structure, one a hydrocarbon, water-repellent (hydrophobic) C004_002_r03.indd 233C004_002_r03.indd 233 11/18/2005 10:19:29 AM11/18/2005 10:19:29 AM
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group, and the other a water-attracting (hydrophilic or polar) group. Depending on the basis of the charge or absence of ionization of the hydrophilic group, surface-active agents are classified into (James, 1965): 1) Anionic agents, which usually have strong deter-gent, but weak antimicrobial properties, e.g., sodium lauryl sulphate. 2) Cationic agents, which have strong bactericidal, but weak detergent, properties. The term “cationic detergent” usually signifies a quarternary ammo-nium compound (QAC, onium compound), but this is not strictly accurate, as the concentration at which a QAC is germicidal is so low that its detergent activity is negligible. 3) Non-ionic agents, which consist of a hydrocar-bon chain attached to a non-polar water-attracting group, which is usually a chain of ethylene oxide units, e.g., cetomacrogols. Non-ionic agents have no antimicrobial properties. 4) Ampholytic (amphoteric) agents, which are com-pounds of mixed anionic–cationic character, in which the charge can be similarly positive and negative. They are effective antimicrobial com-pounds, which are extensively used in the dairy industry. The QACs are organically substituted ammonium com-pounds, in which the nitrogen atom has a valency of 5, four of the substituent radicals (R 1 –R 4 ) are alkyl or het-erocyclic radicals, and the fifth is a small anion. The sum of the carbon atoms in the 4R groups is more than 10. For a QAC to have a high activity, at least one of the four organic radicals must have a chain length in the range C 8 to C 18 . The germicidal activities of the QACs were origi-nally recognized in 1916, but they did not attain promi-nence until Domagk’s work in 1935. They are primarily active against Gram-positive bacteria, with concentrations as low as about 1 in 200,000 being lethal; higher concentra-tions ( c. 1 in 30,000) are lethal to Gram-negative bacteria, although Pseudomonas aeruginosa is highly resistant. The QACs possess antifungal properties, are sporostatic and not sporicidal (Russell, 1971) and are inactive against the tubercle bacillus. Their antimicrobial activity is markedly affected by organic matter, and they are incompatible with anionic surface-active agents, some of the non-ionic agents (such as Lubrols and Tweens) and the phospholipids, e.g., lecithin and other fat-containing substances (Russell, 1982; Russell et al. , 1979). The QACs exert their maximal bac-tericidal activity under alkaline conditions. Although the QACs are effective bactericidal and fungicidal agents, it has been found (Grossgebauer, 1970) that viruses are rather more resistant than bacteria or fungi. R1R4R3R2N+X– Although several hundred QACs have been prepared and tested for antimicrobial activity, only a few are regularly used. These are: 1) Cetrimide (cetyltrimethylammonium bromide, CTAB), a mixture of dodecyl, tetradecyl- and hexadecyl-trimethylammonium bromide. In addi-tion to it uses for pre-operative skin disinfec-tion and treatment of seborrhoea of the scalp, cetri mide is also employed, in conjunction with sodium nitrite, which delays or prevents rusting, for the storage of sterilized surgical instruments, although this practice should be discontinued (Rubbo and Gardner, 1965). 2) Benzalkonium chloride (Zephirol, Zephiran) is the active constituent of a general purpose and skin sterilizing agent, “Roccal.” It is also used to alleviate or prevent napkin rash caused by urea-splitting organisms, as a preservative in eye-drop preparations (Pharmaceutical Codex, 1979) and for the disinfection of blankets. Other important QACs are domiphen bromide and cetyl-pyridinium chloride. The value of QACs in the disinfection of woollen blan-kets has been observed by various authors (Frisby, 1957; International Wood Secretariat, 1961), although they cannot be incorporated in the wash with an anionic compound, because of incompatibility. In addition, it must be emphasized that res-idues of anionic agents on blankets from previous washings could interfere with the action of QACs. To overcome this, blankets may be washed in an appropriate non-ionic deter-gent, with a final prolonged rinse in a QAC, or they may be treated with a non-ionic detergent incorporating a QAC. The QACs are also of considerable value as disinfectants in food and dairy plant. If an alkali detergent containing anionic surface-active wetting agents is used prior to sanitiz-ing, then utensils and equipment must be thoroughly rinsed (Barrett, 1969). The QACs at their normally used concentra-tions are odourless and noncorrosive, but many are not free-rinsing, and undesirable traces may remain on equipment or even be present in dairy food (Clegg, 1967, 1970). However, a combination of a free-rinsing type of QAC and a suitable non-ionic agent may be usefully employed for washing instruments and cutlery, etc. (Barrett, 1969). The cytoplasmic membrane of bacteria and fungi is the site of action of the QACs. The membrane is composed of lipoprotein, and it is considered (Russell, 1971) that the lipid moiety is involved in the lethal action of these compounds (for more recent information, see Hugo, 1976a,b). There appears to be a clear relationship between the ther-modynamic and antibacterial activities of QACs, with solu-tions having equal antimicrobial activity against an organism having surface concentration values of the same order of magnitude. Thermodynamic activities of QACs at two bac-terial survivor levels (1% and 0.01%) have been shown to be sufficiently constant (Laycock and Mulley, 1970) to sup-port the Ferguson (1939) principle that compounds with the C004_002_r03.indd 234C004_002_r03.indd 234 11/18/2005 10:19:29 AM11/18/2005 10:19:29 AM

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235same thermodynamic activity will have an equal bactericidal activity. Amphoteric compounds, as already stated, are of mixed anionic-cationic character, and they combine the detergent properties of anionic compounds with the bacterial proper-ties of the cationic substances; their bactericidal properties remain virtually constant over a wide pH range (Barrett, 1969) and they are less readily inactivated by proteins than are the QACs (Clegg, 1970). Examples of amphoteric surface-active agents are dodecyl- b -alanine, dodecyl- b -aminobutyric acid and dodecyldi(aminoethyl)-glycine (Davis, 1960a,b, 1968), the last named being a “Tego” compound. The Tego series of com-pounds have a high molecular weight, and in addition to being recommended for use in pre-operative hand cleansing and pre-operative skin preparation it has also been found that they are suitable for the cleansing of surgical operating theatre floors, walls and equipment and for ward cleansing (Frisby, 1959, 1961). It has, however, recently been shown that Tego 103S in 1% solution is less active than a 0.5% solution of chlorhexidine in 70% alcohol (Kuipers and Dankert, 1970). Amphoteric sur-face-active agents are inactivated by soaps and other anionic compounds (Frisby, 1959), but they are non-irritating and non-corrosive. Unfortunately, they tend to be expensive. Aldehydes The two most important aldehydes are glutaraldehyde (Pentanedial) and formaldehyde (methanal). CH2 · CHOCH2 · CHOHO O OHCH2Hydrated Ring Structure Glutaraldehyde is a dialdehyde which has been used for several years as a fixative in electron microscopy investigations and its antimicrobial activity has been comparatively recent (see Rubbo and Gardner, 1965; Rubbo et al. , 1967; Borick, 1968), but they do indicate that this substance has a valuable role to play. A 2% solution of glutaraldehyde buffered with sodium bicarbonate (0.3% w/v is considered to be the optimum bicarbonate concentration) is effective in killing nonsporing bacteria within 2 min, M. tuberculosis, fungi and viruses in 10 min, and spores of Bacillus and Clostridium spp. in 3 hours. Aqueous solutions of glutaraldehyde are acid, and are consider-ably less active against microorganisms than are alkaline ones (Pepper and Chandler, 1963; Stonehill et al. , 1963; Snyder and Cheatle, 1965; Lane et al. , 1966; Rubbo et al. , 1967; Munton and Russell, 1970a,b), but solutions become progressively less stable at pHs above 7. Concentrated solutions of glutaraldehyde (25%) can be purchased, diluted to the required concentration (2%) and “activated” by the addition of sodium bicarbonate. (Alternatively, 2% solutions ready for use when “activated” can also be purchased.) When made alkaline, glutaraldehyde solutions gradually undergo polymerization with a consequent loss of activity, this polymerization proceeding rapidly at pH values above 9. At pH 7.5–8.5, however, activity is maintained for at least 2 weeks. Serum does not affect the antimicrobial activity of glu-taraldehyde, but the dialdehyde is considerably less active in nutrient broth at pH 7.5 than it is at the same pH in buffer (Rubbo et al. , 1967; Munton and Russell, 1970a), the reason being that glutaraldehyde combines with the peptone present in broth (which is thereby discolored). Glutaraldehyde is used as a fixative in the preparation of microbial cells for electron microscopy. It is a useful hospital disinfectant, particularly for articles which cannot be sterilized by physical means (Report, 1965). It has been employed in the sterilization of cytoscopes in urology (Lane, McKeever and Fallon, 1966) and of endoscopic instruments, such as bronchoscopes (Snyder and Cheatle, 1965), as it has no deleterious effect on the cement or lens coating. Glutaraldehyde is also employed as a tanning agent in pref-erence in glyoxal and formaldehyde (Fein et al. , 1959), and has been shown to inactivate rapidly influenza virus and a coliphage in mouse tissue blocks (Sabel et al. , 1969). Glutaraldehyde is non-corrosive, and does not affect rubber and plastic articles or the sharpness of cutting instru-ments; because it does not coagulate protein matter, such as blood and mucus, it does not render the cleaning of blood-covered instruments more difficult. It is obvious, therefore, that glutaraldehyde is most useful. Rubbo et al. (1967) have proposed that the microbicidal activity of glutaraldehyde is due to the presence of two free aldehyde groups in the molecule. In solution, glutaraldehyde exists in an equilibrium between the open chain structure and the hydrated ring structure (see above), and there is a com-plete loss of activity if one or both of the aldehyde groups is altered, whereas a substitution elsewhere in the molecule reduces, but does not abolish, its activity. It is thus essential to have free aldehyde groups, which may react with cell sul-phydryl or amino groups. Glutaraldehyde is about 10 times as active as glyoxal, with succinaldehyde occupying an interme-diate position (Pepper and Chandler, 1963). Certain bacteria treated with glutaraldehyde become pink in color (Munton and Russell, 1970b) as a result of cell-aldehyde interaction. Formaldehyde has long been employed as a disinfectant. Formaldehyde solution is rapidly sporicidal to B. subtilis (Ortenzio et al. , 1953; cf. Klarmann, 1956, 1959) but not to various Clostridia (Ortenzio et al. , 1953; Klarmann, 1956, 1959). Ethanol (Rubbo et al. , 1967) and methanol (Willard and Alexander, 1964) cannot be recommended as vehicles for formaldehyde, as there is a reduction in antibacterial activity. Formaldehyde is an important virucidal agent which finds its greatest use in the preparation of certain sterile vac-cines, e.g., Poliomyelitis Vaccine (Inactivated). As a result of the experimental evidence accumulated over several years, a considerable amount of information is now available on the kinetics of the virus inactivation by formaldehyde. This particular vaccine consists of poliovirus Types I, II and III, and each is inactivated separately and then blended to give the trivalent vaccine. It is thus essential that formaldehyde treatment be sufficient to destroy the viruses without affect-ing their antigenicity; prolonged exposure to the aldehyde C004_002_r03.indd 235C004_002_r03.indd 235 11/18/2005 10:19:30 AM11/18/2005 10:19:30 AM
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will, in fact, destroy the antigenic potency also (Morris and Darlow, 1971). The inactivation of poliovirus by formalde-hyde has been considered to be a first-order reaction so that extrapolation of the death curve would give a point at which the probability of any infective particles remaining would approach zero. However, first-order kinetics cannot be used with any degree of safety to extrapolate the inactivation curve. Similarly, although the inactivation of SV 40 virus by formaldehyde has been shown by Sweet and Hilleman (1960) to be linear, it is now known that this linear inactiva-tion is followed by a flattening of the curve indicating the persistence of a residual fraction which resists inactivation. Formaldehyde is rapidly lethal to vegetative bacteria, and is sometimes used for this purpose in the preparation of inactivated bacterial vaccines. Metals Because of their antibacterial and antifungal activity, com-pounds of mercury, silver, copper and tin are of importance from both medical and industrial points of view (Hugo and Russell, 1982). Mercury Compounds These are of two types, the inorganic mercuric and mercurous salts and the organic substances. Mercuric salts are primarily bacteriostatic and fungistatic and contrary to earlier findings are not sporicidal (see Russell, 1971). Because of their toxicity, the mercuric salts do not find widespread use in modern medicine, but are extensively employed as industrial preservatives, e.g., in the preservation of wood, leather and paper, and in the control of fungal infections in seeds and bulbs. Mercurous salts have no application as preservatives. The most important organic mercury compounds are the phenylmercuric salts (nitrate, acetate and borate) and thi-omersal. Phenylmercuric nitrate (PMN) and acetate (PMA) are now mainly employed as preservatives in various phar-maceutical and cosmetic products. PMN is also used as a spermicide in certain contraceptive formulations, as a plant fungicide and for the disinfection of leather and timber. However, because of their lack of sporicidal activity at ordi-nary temperature, the organic mercury compounds cannot be recommended as sterilizing agents. Some plasmid-containing gram-negative bacteria are resistant to mercury compounds, which are vaporized (Chopra, 1982). Various sulphydryl compounds, such as cysteine and thioglycollic acid, can reverse mercury-induced bacteriosta-sis, which led Fildes (1940) to propose that these compounds combined with, and displaced, mercury from its combina-tion with the —SH group of an enzyme (E). EEESSSHSHSHSH+ HgHgH2S+ HgS+ Silver Compounds Silver compounds have long been used in medicine for their antimicrobial activity, which extends to Gram-positive and Gram-negative bacteria and fungi. Of the silver compounds available, silver protein and silver nitrate are the most important. The latter, in the form of compresses, is highly effective in preventing the colonization of burns with Ps. aeruginosa and Proteus species. Copper Compounds These are bactericidal and fungicidal. They have been used for the latter purpose for more than 200 years, and their sole use nowadays is as industrial preserva-tives against fungal spoilage. The most frequently used sub-stances are copper naphthenate, oxinate, 1-phenylsalicylate and sulphate; the last-named, in combination with a lime mix-ture, is known as Bordeaux mixture. Dialkyldithiocarbamates are considered (Albert, 1963) to be converted into active bactericides and fungicides in the presence of copper. Such salts are highly successful, widely used, agricultural fungicides (Owens, 1969). Tin Compounds Stannous and stannic salts have little antimicrobial ctivity. However, when tin is coupled with organic radicals, forming what are known as the organo-tins, potent antimicrobial activity results. If R represents the organic radical linked directly to a tin atom, by C—Sn bond, and X an inorganic or organic radical not so linked, various types of compounds can be obtained, of which R—SnN 3 is most active. Gram-negative bacteria are less sensitive than Gram-positive bacteria to organotin compounds. Triphenyltin acetate and hydroxide are important agricultural fungicides. Dyes The acridines have held a valuable place in medicine for sev-eral years, although with the advent of the antibiotics and other chemotherapeutic agents, they are now less widely used than hitherto. The acridines are active against several Gram-positive and Gram-negative bacteria, and have been used mainly in treating infected wounds. Their uses and mode of action have been reviewed (Foster and Russell, 1971). The most important members of the triphenylmethane group of dyes are crystal violet, brilliant green and malachite green. These have mainly been employed for local applica-tion to burns and wounds. Some members of a third group of dyes, the quinones, are important agricultural fungicides. The quinones are natural dyes which impart color to many forms of plant and animal life. Chemically, the quinones are diketocy-clohexadienes, the simplest of which is 1,4-benzoquinone (Figure 4). Naphtaquinones are the most toxic to bacteria, moulds and yeasts, followed (in this order) by phenanthren-equinones, benzoquinones and anthraquinones (Figure 4). Antimicrobial activity is increased by halogenation, and two powerful antimicrobial agents employed as fungicides are chloranil (tetrachloro-1, 4-benzoquinone) and dichlone (2,3-dichloro-1,4-naphthaquinone). Alcohols Ethyl alcohol, although active against Gram-positive and Gram-negative bacteria, is devoid of lethal activity against bacterial spores, and thus cannot be relied upon as a steril-izing agent (Russell, 1971). Methyl alcohol is likewise not C004_002_r03.indd 236C004_002_r03.indd 236 11/18/2005 10:19:30 AM11/18/2005 10:19:30 AM

237sporicidal. Moreover, alcohols may reduce the sporicidal activity of aldehydes. GASEOUS ANTIMICROBIAL AGENTS Gaseous agents may play an important role in sterilization of certain types of medical equipment and of components used in outer space research. However, only two gases (ethylene oxide and formaldehyde) are used extensively for the ster-ilization of medical products. Other gases (methyl bromide, b -propiolactone and propylene oxide) are not used as routine methods. Appropriate measures must be taken to counteract toxicity to humans (Christensen and Kristensen, 1982). Ethylene Oxide Ethylene oxide (EO) exists as a gas which is soluble in rubber, water, oils and several organic solvents. Chemically, it is CH2CH2.O Its inflammability when in contact with air is overcome by using mixtures of EO with carbon dioxide or with fluoro-carbon compounds (Phillips and Kaye, 1949; Barwell and Freeman, 1959; Freeman and Barwell, 1960). EO diffuses freely through paper, cellophane, cardboard and some plas-tics, but less rapidly through polythene; it cannot penetrate crystalline materials (Opfell and Miller, 1965). The antimi-crobial activity of EO has been dealt with in many papers and reviews (Phillips, 1949, 1958, 1961, 1977; Phillips and Kaye, 1949; Kaye, 1949, 1950; Kaye and Phillips, 1949; Kaye et al., 1952; Phillips and Warshowsky, 1958; Thomas, 1960; Bruch, 1961; Sutaria and Williams, 1961; Russell 1971; 1976; Kelsey, 1961, 1967; Sykes, 1970; Hoffman, 1971; Kereluk, 1971; Ernst, 1974, 1975; Hugo and Russell, 1982). Several factors are known to influence the antimicrobial activity of EO (Christensen and Kristensen, 1982): 1) Concentration. As would be expected, the higher the concentration of EO (expressed as mg/l, which refers to the actual amount present in the sterilizer) the more rapid is the rate at which microorganisms are killed. However, even at high concentrations, EO is only slowly lethal, and long periods may, therefore, be needed for sterilization to be achieved. 2) Temperature. The lethal activity of EO increases with a rise in temperature. It has a temperature coef-ficient of 2.74 for each 10°C rise in temperature. 3) Type of organism. EO gas will kill bacteria and their spores, yeasts, moulds and their spores, and viruses (Griffith and Hall, 1938), and resis-tant strains have not been developed. In contrast to many other chemical substances which are considerably less effective against spores than against vegetative cells, bacterial spores are only about 2–10 times as resistant as the latter to EO (Toth, 1959; Phillips, 1958) and in some cases, e.g., B. stearothermophilus (Thomas et al., 1969) even less resistant. These results imply that EO can freely penetrate the outer layer(s) of the bacterial spore, although experimental results in support of this contention are sadly lacking. In addition to its antibacterial and antifungal activity, ethylene oxide is also effective against rickettsiae and viruses (Hoffman, 1971). 4) Relative humidity (RH). Of all the factors which influence the activity of EO, RH is probably the most important. The optimum RH is considered to be c. 28–33% (Schley et al., 1960), and EO is considerably less microbicidal at high RH and in relatively dry air. The correct RH may be achieved to prehumification of the steriliza-tion chamber (Halowell et al. 1958; Ernst and Schull, 1962). However, the moisture content of microorganisms themselves, as well as the RH of the environment, is also important. Bacterial cells which have been desiccated and then equil-ibrated to successively high RH values contain less water and are more resistant to EO than cells which have not been desiccated but have instead been allowed to dry naturally until equilibrated to the same RH values. To overcome this resistance OOOOOOOONaphthaquinone9,10-Anthraquinone9,10-PhenanthrenequinoneBenzoquinoneFIGURE 4C004_002_r03.indd 237C004_002_r03.indd 237 11/18/2005 10:19:30 AM11/18/2005 10:19:30 AM
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to EO produced by dehydration, the cells have to be actually wetted (Perkins and Lloyd, 1961; Winge-Heden, 1963; Gilbert et al., 1964). Alkylating agents act through alkylation of sulphydryl, amino, carboxyl, hydroxyl and phe-nolic groupings, with the loss of a hydrogen atom and the production of an alkyl hydroxyethyl group (Phillips, 1952, 1958; Kelsey, 1967), and it seems likely that EO kills microorganisms by an alkylation of protein molecules (Gilbert et al. , 1964). The influence of RH on the microbicidal activity of EO is related to this, since the pres-ence of insufficient water prevents alkylation, whereas excess water causes hydrolysis of EO to ethylene glycol, CH 2 OHCH 2 OH (Wilson and Bruno, 1950). 5) Effect of drying medium on spore resistance. Organisms dried from salt solution always show some survivors after exposure to EO (Royce and Bowler, 1961; Beeby and Whitehouse, 1965). Moreover, organisms enclosed within crystals are protected from the action of EO (Abbott et al. , 1956; Royce and Bowler, 1961) as a result of the inability of the gas to penetrate crystalline materials. EO has several uses in the pharmaceutical and medical fields. These include the sterilization of ophthalmic instruments, anaesthetic equip-ment, heart-lung machines, disposable syringes (Rubbo and Gardner, 1968) and hospital blankets (Kaye, 1950), and as a decontamination proce-dure for articles handled by tuberculous patients. There may, however, be a “crazing” of disposable syringes, and EO treatment is a slow, costly pro-cess, with a “quarantine” period necessary at the end of the process to ensure that complete dissipa-tion of the gas has been achieved. b -Propiolactone b -Propiolactone (BPL) is a colorless liquid at room tem-perature. It has been employed as a chemosterilizer in both the liquid and gaseous states, and is microbicidal in both forms (Allen and Murphy, 1960). Its antimicrobial activity depends on its concentration and on the temperature and RH at which the vapor is used (Spiner and Hoffman, 1960), a temperature above 24°C being required for optimal activ-ity. As with EO, RH is of considerable importance in deter-mining the activity of BPL vapor, although with the latter a RH in excess of 70% is required for rapid microbicidal acivity (Hoffman and Warshowsky, 1958). Again, however, it is not necessarily the atmospheric RH which is of greatest importance but the location and content of water within the microbial cell. B. globigii spores equilibrated to 98% are rapidly killed by BPL at 45% RH, whereas they are more resistant if preconditioned at 75% RH before treatment with BPL at 45% RH, and a small proportion of spores equilibrated at 1% RH are subsequently highly resistant to BPL at 75% RH (Hoffman, 1968). Bacterial spores are more resistant to BPL than vegeta-tive cells, viruses or fungi (Lo Grippo et al. , 1955; Trafas et al. , 1954; Bruch and Koesterer, 1962; Toplin, 1962) although some strains of Staph. aureas may be almost as resistant as spores (Hoffman and Warshowsky, 1958). BPL is also highly active against viruses and rickettsiae (Hoffman, 1971). BPL has been used for the chemical sterilization of regen-erated collagen sutures (Ball et al. , 1961), for the decontamina-tion of enclosed spaces (Bruch, 1961b) and for the sterilization of a variety of instruments contaminated with various sporing and non-sporing bacteria (Allen and Murphy, 1960). However, its reported carcinogenic effects in rats and mice (Walpole et al. , 1954) mean that a considerable degree of caution is needed before BPL is employed as a chemosterilizer. Formaldehyde Formaldehyde vapor may be obtained by evaporating appro-priate dilutions of standardized batches of commercial for-malin (a 40% solution of formaldehyde in water) with 10% methanol added to prevent polymerization (Report, 1958). Temperature affects the activity of the gas, as does the RH, there being an increase in activity with increasing RH up to 50%, but little further increase in killing rate as the RH rises from 50 to 90% (Nordgren, 1939; Report, 1958). In contrast, gross wetting retards killing. Bacteria protected by organic matter, such as blood and sputum are less rapidly killed by formaldehyde vapor (Nordgren, 1939; Bullock and Rawlins, 1954). Micro- organisms may also be protected from it when they are included in a crystal mass, in contrast to surface-contaminated crystals (Abbot et al. , 1956). Although bacterial spores are more resistant than vegeta-tive cells to formaldehyde vapor, the degree of difference is not high (Phillips, 1952; Report, 1958; Sykes, 1965). The vapor only has weak penetration, and its application is thus normally limited to surface sterilization (Borick, 1968; Davis, 1968). However, the addition of formaldehyde vapor to steam under sub-atmospheric pressure at temperatures below 90°C results in deep penetration into fabrics with destruction of heat-resistant microorganisms (Alder et al. , 1966; Alder and Simpson, 1982). Formaldehyde vapor has long been used for the disin-fection of blankets, and is considered to be one of the best methods available for disinfecting woolen blankets that have not received a shrink-resist treatment (International Wool Secretariat, 1961). It is also used to decontaminate rooms, buildings and instruments (Hoffman, 1971). CH2CH2COOC004_002_r03.indd 238C004_002_r03.indd 238 11/18/2005 10:19:30 AM11/18/2005 10:19:30 AM

239 PRACTICAL USES This final section brings together some of the data presented in the preceding sections, and also provides information in certain specific instances. Disinfectants will be considered from two points of view, first their medical, and second their nonmedical uses. Some brief information on antiseptic and preservative use will also be supplied. Medical Uses The use and choice of disinfectants in hospitals have been extensively considered in the last decade in Britain by a spe-cial committee (Report, 1965) set up for this purpose. Our comments here are thus based on the recommendations of this report and on the findings presented by Kelsey and Maurer (1967). The Report (1965) recommended that two classes of disinfecting agents were needed, (a) for general disinfec-tion and (b) for surface disinfection of clean objects. Agents for general disinfection should have a wide spectrum, and at appropriate dilutions should remain active in the presence of organic matter; the main purpose of such disinfectants, e.g., phenolic disinfectants based on coal-tar acids, is not neces-sarily to kill all bacteria but to ensure that an object is free of significant numbers of organisms. Chemicals for surface disinfection must be quick-acting, have a wide spectrum, be non-harmful to materials and leave no objectionable odours. Such disinfectants, e.g., hypochlorites, should be used for the rapid disinfection of clean surfaces such as trolley tops, kitchen tables and clinical thermometers. Kelsey and Maurer (1967, 1972) have presented a list of the steps to be taken in drawing up a policy for the use of general purpose disinfectants in hospitals, and among the points they make is the non-usage of disinfectants in cer-tain cases, especially where sterilization is the objective or where other more reliable means are available. For further information, see Lynn (1980), Ayliffe and Collins (1982) and Lowbury (1982). Preoperative disinfection of the skin (including surgeon’s hands), disinfection of operation sites and topical prophylaxis, i.e., antisepsis in burns, are dis-cussed by Lowbury (1982). Ayliffe and Collins (1982) pro-vide a rational approach to hospital disinfection. Nonmedical Uses The main nonmedical uses of disinfectants occur in the food, dairy, brewing and fermentation industries (Foster et al. , 1958; Frazier, 1967). The maintenance of equipment for use in these industries in a proper sanitary condition cannot be overemphasized. This therefore means that the cleaning of such equipment is of considerable importance, since the presence of organic matter can reduce or virtually eliminate the effect of many disinfectants (page 164). In the dairy industry, milk stone—resulting from milk drying on equipment, and thus consisting of fat, protein and minerals—and milk film are a well-known problem in disinfection (Clegg, 1967). Chemicals which are of use against micro-organisms in liquid suspension in laboratory tests may be of little use against such organisms on a soiled surface if they are poor detergents (Cousins, Hoy and Clegg, 1960). To counteract the unwanted effects of organic matter, one of the following two methods may be employed (see also Davis, 1972a,b and BSI 1977): 1) detergent first, followed by a disinfectant; 2) combination of detergent and disinfectant (this corresponds to a sanitizer, or to the detergent-sterilant of Davis (1968), as described in the Introduction). Harris (1969) stresses the need for using two operations, i.e., the use of cleaning before disinfection. Cleaning is the first essential in the sanitary care of food equipment, and approximate sterility the last (Foster et al. , 1958). Steam under pressure is an obvious method of sanitization, but this is limited only to closed systems which can withstand pressure (Frazier, 1967). Theoretically, separate procedures would be expected to give a better result (Clegg, 1970), because of the inactivating effect of organic matter on disin-fectants; however, a finding made several years ago (Neave and Hoy, 1947) suggests that a detergent-disinfectant com-bination would be of greater use, because “the effect of the detergent on milk solids more than outweighs the effect of milk residues inactivating the disinfectant” (Clegg, 1970). An effective detergent should dissociate organic and inor-ganic solids, emulsify, saponify or suspend grease, fats and oils, have good wetting ability, be easily rinsed and be non-corrosive (Olivant and Shapton, 1970). Detergents are thus of considerable importance in this field, because they can also be responsible for the mechanical removal of bacteria (Gilbert, 1969). The most commonly used detergents are strong and mild alkalis, alkali salts, strong acids, anionic alkyl sulphates and aryl sulphonates and non-ionic conden-sates (Davis, 1968). An excellent descaling agent is nitric acid which can be used hot at concentrations of 0.25–0.5% or cold at 0.5–1%, and which is, in addition, a powerful disinfectant. It is, however, less effective than alkali in the removal of hardened protein films, and is normally employed with a corrosion inhibitor. In actual fact, many detergents are good disinfectants and vice versa, e.g., a detergent such as sodium hydroxide possesses considerable germicidal power (Whitehouse and Clegg, 1963), whereas hypochlorites have a useful detergent effect by disintegrating protein matter (Davis, 1968). Cleaning is an essential first part in high-speed food and beverage processing plant, and considerable economic benefit is achieved as a result of the production of stable liquid detergents and disinfectants which can be deliv-ered by tanker and then distributed to the cleaning areas by pipeline (Hill, 1969). However, even with good precleaning, traces of inactivating protein material may remain on equip-ment, and it is thus important to choose a disinfectant which has a high protein tolerance (Harris, 1969). The design of equipment to facilitate cleaning must also be stressed, and stainless steel is an obvious example, with glass pipelines to give a high degree of visual cleanliness (Harris, 1969). 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240
The disinfection of fermentation laboratories has been described by Darlow (1969). Gaseous disinfectants may be employed, and aerosols appear to have an important role to play. Local disinfection of bench tops, floors, etc. is also a standard practice. The importance of disinfectants in water conservation is emphasized by Fielden (1969), and in the pharmaceutical industry by Underwood (1980). The control of airborne microorganisms is of particu-lar importance in the fermentation industry, in laboratories where strict asepsis is essential (e.g., in the production of various sterile products in the pharmaceutical industry and in hospital pharmacy departments, as well as in the rearing of germ-free animals), in hospital wards to reduce the inci-dence of cross-infection, and in special wards set aside for patients with a rare disease (hypogammaglobulinaemia) who are particularly sensitive to infection. This control is normally achieved by the use of special air filters, often in conjunction with ultraviolet lamps to irradiate the upper atmosphere. Disinfectants in the form of aerosols are also of importance in aerial disinfection. To be effective for air disinfection, a chemical should ideally possess the following properties: 1) be odorless, cheap and stable 2) be without toxic or irritant properties 3) be capable of being dispersed in the air, with con-sequent complete and rapid mixing of infected air and chemical 4) be capable of maintaining an effective concentra-tion in the air 5) be highly and rapidly effective against airborne organisms 6) be unaffected by relative humidity (RH). Aerosols consist of a very fine dispersed liquid phase in a gaseous (air) continuous phase. The germicide must be nebulized in sufficiently fine spray (aerosols droplets of <1 µ m are the accepted standard) so that it will remain airborne and thus have ample opportunity to collide with any microorganisms present in the air. At low RH, particles are too dry for adequate condensation of the disinfectant that such organisms enclosed in particles, and thus bacteria occurring on dust or on surfaces are much less susceptible to the aerial disinfectant than such organisms enclosed in droplets (Sykes, 1965). The optimum RH is usually c. 40–60%. Chemical aerosols are often generated in the following manner: if the chemical is liquid, it may be sprayed directly into the air from an atomizer; if the chemical exists as a solid, it may be dissolved in an appropriate solvent, e.g., propylene glycol, and atomized, or alternatively the solid may be vaporized by heat from a thermostatically-con-trolled hot-plate. Chemicals which have been used as aerial disinfectants include hexylresorcinol, lactic acid, propylene glycol (this possesses antimicrobial activity in its own right), hypo-chlorous acid, formaldehyde gas and sulphur dioxide. Other Uses of Antimicrobial Agents Antimicrobial agents are widely employed as preservatives in pharmaceutical and cosmetic products. Factors influenc-ing their activity, as well as those governing their choice in different classes of sterile or non-sterile products have been well considered by Bean (1972), Croshaw (1977), Parker (1978, 1982), Kazmi and Mitchell (1978a,b), Allwood (1980) and Beveridge (1980). Preservation is also required in other specialized areas, e.g., in cutting oils (Hill, 1982a), fuels (Hill, 1982b), paper and pulp (Weir, 1982), wood (Richardson, 1982), paint (Springle and Briggs, 1982) and textiles (Hugo, 1982b) and in the con-struction industry (Bravery, 1982). 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243 212. Sykes, G., 1958, J. Pharm. Pharmac., 10, 40T. 213. Sykes, G., 1965, Disinfection and Sterilization, 2nd Edition, Spon, London. 214. Sykes, G., 1970, J. Appl. Bact., 33, 147. 215. Sykes, G. and M.C. Hooper, 1954, J. Pharm. Pharmac., 11, 235T. 216. Ten Cate, L., 1965, J. Appl. Bact., 28, 221. 217. Thomas, C.G.A., 1960, Guy ’ s Hospital Rep., 109, 57. 218. Thomas, C.G.A., B. West, and H. Besser, 1959, Guy ’ s Hospital Rep., 108, 446. 219. Tilley, F.W., 1942, J. Bact., 43, 521. 220. Toplin, I., 1962, Biotechn. Bioeng., 4, 331. 221. Toth, L.Z.J., 1959, Arch. Mikrobiol., 32, 409. 222. Trafas, P.C., R.E. Carlson, G.A. LoGrippo, and C.R. Lam, 1954, Arch. Surg., 69, 415. 223. Truman, J.R., 1971, in Inhibition and Destruction of the Microbial Cell, Ed., W.B. Hugo, Academic Press, London and New York. 224. Underwood, E., 1980, in Pharmaceutical Microbiology, 2nd Edition, Ed., W.B. Hugo, and A.D. Russell, Blackwell Scientific Publications, Oxford. 225. Waites, W.M., 1982, in Principles and Practice of Disinfection, Pres-ervation and Sterilisation, Ed., A.D. Russell, W.B. Hugo, and G.A.J. Ayliffe, Blackwell Scientific Publications, Oxford. 226. Walpole, A.L., D.C. Roberts, F.L. Rose, J.A. Hendry, and R.F. Homer, 1954, Br. J. Pharmac., 9, 306. 227. Weber, G.R., 1950, Publ. Health Rep., 65, 503. 228. Weber, G.R. and M. Levine, 1944, Am. J. Publ. Health, 34, 719. 229. Wedderbern, D.L., 1964, Adv. Pharm. Sci., 1, 195. 230. Weinberg, E.D., 1968, Ann. NY Acad. Sci., 148, 587. 231. Weir, B., 1982, in Principles and Practice of Disinfection, Preser-vation and Sterilisation, Ed., A.D. Russell, W.B. Hugo, and G.A.J. Ayliffe, Blackwell Scientific Publications, Oxford. 232. Whitehouse, R.L. and L.F.L. Clegg, 1963, J. Dairy Res., 30, 315. 233. Willard, M. and A. Alexander, 1964, Appl. Microbiol., 12, 229. 234. Wilson, A.R. and P. Bruno, 1950, J. Exp. Med., 91, 449. 235. Winge-Heden, K., 1963, Acta Path. Microbiol. Scand., 58, 225. A.D. RUSSELL P.J. DITCHETT Welsh School of Pharmacy University of Wales Institute of Science & Technology Cardiff, UK DRAINAGE-SURFACE: see URBAN RUNOFF C004_002_r03.indd 243C004_002_r03.indd 243 11/18/2005 10:19:32 AM11/18/2005 10:19:32 AM
244EEARTH–SPACE ORGANIZATION: see THE TERRESTRIAL SYSTEMECOLOGY: see
THEORYECOLOGY OF FISH: see POLLUTION EFFECTS ON FISH; THERMAL EFFECTS ON FISH ECOLOGYECOLOGY OF HUMANS: see
; HAZARDOUS WASTES
Ecology is the study of the relationship between organisms and the environment and the interrleationships between organisms. The response of an organism to the environ-ment is determined by the effects of the environment on the processes in the organism. Stated in another way, all environmental factors affect the growth and distribution of organisms only by affecting rates of processes in the organisms. The study of the effects of environmental fac-tors on processes in organisms is physiological ecology, or in other words, it is the study of the physiological basis of ecological behavior. This field obviously represents a mar-riage between ecology and physiology, and is sometimes called environmental physiology. The field of physiological ecology is extremely broad: thus, this paper is restricted to certain aspects of physiological plant ecology. ENVIRONMENTAL FACTORS The environment is very complex, consisting of all influences external to an organisms. Many systems have been devised to classify the environment but space does not allow a dis-cussion here. The following list of specific environmental factors is a satisfactory one although the list could obviously be subdivided in many ways: 1) water 2) temperature 3) radiant energy 4) essential elements 5) aeration 6) toxins 7) wind 8) topography 9) nature of geologic strata 10) altitude 11) organic matter content of soil 12) texture and structure of soil 13) base exchange capacity of soil 14) color of soil 15) plant growth regulators 16) soil organisms 17) parasites 18) plants and animals other than above. Even cursory examination of this list makes it clear that the effects of all these factors on processes in plants could not be discussed in a brief survey. For example, thorough coverage of the effects of the various essential elements on plant processes would require a sizeable manuscript. I have decided, therefore, to discuss a few factors in sufficient depth to give the reader a feeling for the area. C005_001_r03.indd 244C005_001_r03.indd 244 11/18/2005 10:20:10 AM11/18/2005 10:20:10 AM

245 INTERRELATIONSHIPS BETWEEN ENVIRONMENTAL FACTORS In addition to having a direct effect on many plant functions, the water factor affects other factors through the amount and type of precipitation involved. It affects: (1) amounts of radiant energy, (2) temperature of air and soil, (3) amount of nitrogen brought down by rain, (4) amounts of minerals leached from the soil, (5) availability of various elements to plants, (6) topography of the region, (7) numbers and types of organisms in the soil, (8) toxins in the soil, (9) aeration of soil, and (10) numbers and types of parasites in a region. All other environmental factors have many indirect effects also. EFFECTS OF WATER FACTOR Warming (1909) stated that “no other influence impresses its mark to such a degree upon the internal and external struc-tures of the plant as does the amount of water present in the air and soil.” This is due primarily, of course, to the following roles of water: (1) it is important as a solvent system in the soil and in cells of all living organisms on earth, (2) it serves as the dispersion medium for the colloidal systems present in cells, (3) there is a close interaction between the nature of the hydration shell surrounding protein molecules and the physicochemical properties of the proteins (Klotz, 1958), (4) it is important as a raw material in photosynthesis, (5) it is important as a raw material in all hydrolytic processes, and (6) it has some influence on leaf temperatures through transpirational cooling. Klotz (1958) suggested that protein molecules in cells have hydration shells of lattice-ordered water which act like ice-shells around the molecules and that the maintenance of an active configuration is due to the shells. The effect of heat is thought to reduce the extent of the ice-shell, whereas lower temperatures increase the thickness of the shell. Urea is thought to be an effective denaturizer of proteins because its strong hydrogen bonding characteristics may break down the frozen structure of the water envelope. The observed effects of water stress on the viscosity of protoplasm sug-gests that reduced hydration has a definite effect on the ice-shell. Jacobson (1953) offered considerable evidence for the presence of an ordered-water lattice around DNA molecules also. He suggested that such an ice-shell would make sepa-ration of the two chains of DNA during replication possible with low energies. The models of Klotz and Jacobson have apparently been well received (Stocker, 1960; Slatyer, 1967). If these models are indeed correct, it is easy to see how water deficiency can have pronounced effects on all sorts of processes, because of the role of DNA in protein synthesis and the role of pro-teins in enzymes. It is probable that the integrity of specific water-protein structures is necessary for the continuance of organic processes at optimum rates. It is likely that the variations in reductions to different species to water stress are due chiefly to different degrees of sensitivity of essential metabolic systems. Buckman and Brady (1960) state that a slight lowering of the available water content in the soil below field capac-ity stimulates growth of plants generally. Blair et al. (1950) found that the rate of stem elongation in sunflower (a dwarf type from an inbred line, apparently of Helianthus annuus ) is markedly reduced before half the available water is used, and that zero growth results during the use of the last one-fourth of the available water. A reduction of 30–40% between field capacity and the wilting point probably slows growth in most plants. Probably the most direct effect of water on plant growth is its effect on the turgor pressure of the individual cells. Turgor pressure affects cell enlargement and stomatal clo-sure. Closing of the stomates due to reduced turgor pressure in the guard cells reduces both transpiration and photosyn-thesis and can ultimately reduce growth. Probably, many of the detrimental effects of moderate water deficits are due to stomatal closure. One important effect is that the tem-perature of leaves can rise to detrimental levels under some conditions when the transpiration rate is lowered markedly (Slatyer, 1967). It appears that cell division is influenced less by water deficits than cell enlargement (Slatyer, 1967). Gates and Bonner (1959) found that the amount of DNA in water stressed tomato leaves remained constant during the period of their experiments whereas the amount increased steadily in control leaves. This indicated that chromosomal multipli-cation was continuing to occur in the control leaves but had ceased in the test leaves. Apparently, cell number is linearly related to DNA content (Slatyer, 1967) so it appears that there was a reduction in rate of cell division in the water stressed leaves. Gardner and Nieman (1964) found that water stress in cotyledonary leaves of Raphanus sativus (radish) reduced the rate of increase of DNA. Gates and Bonner (1959) found that water stress pre-vented a net increase in RNA in Lycopersicum esculentum (tomato) leaves also. The water stressed leaves were able to incorporate P 32 -labeled phosphate into RNA apparently at the same rate as in control leaves. They concluded, there-fore, that the rate of destruction of RNA was greater in water stressed leaves than in controls. Kessler (1961) reported that drought stress causes a pronounced decrease in the RNA content of several plants. Despite this, the rate of incorpo-ration of uracil-C 14 was the same as in control leaves but had ceased in the test leaves—drought stressed plants. This was confirmed because the activity of RNase was found to increase markedly with water stress. A reduction in the water content of leaves generally results in a decrease in the rate of photosynthesis (Meyer and Anderson, 1939). This may seem strange because less than one per cent of the water absorbed by a plant is used as a raw material in photosynthesis. According to Slatyer (1967) however, there are two main modes of action of water stresses on photosynthesis: (1) stomatal closure and reduced rates of CO 2 exchange can influence the supply of CO 2 and (2) there is probably a direct effect of the water C005_001_r03.indd 245C005_001_r03.indd 245 11/18/2005 10:20:10 AM11/18/2005 10:20:10 AM
246

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deficit on the biochemical processes involved in photosyn-thesis. Gates (1964) maintained that the rate of photosyn-thesis is more closely related to the water content of leaves than to the diffusive capacity of the stomates. Certainly, if Klotz’s model of protein hydration and behavior is correct, one would expect Gate’s contention to be logical. Gates (1964) reported that during water stress, the respira-tion rate is accelerated but later declines. There is an increase in starch hydrolysis and an increase in sucrose which later declines. The respiration rate appears to be correlated with the sucrose content. Protein synthesis is impaired and active proteolysis may occur with prolonged wilting, especially in older leaves. Finally, if the degree of protoplasmic desicca-tion reaches critical levels, individual cells, tissues and even the entire organism may die. Flooding lowers the oxygen supply to the roots or roots and tops if the tops are covered with water also. Under these conditions, Crawford and Tyler (1969) found that plants intolerant of flooding have an increased rate of gly-colysis including alcohol dehydrogenase induction and the greatest production of acetaldehyde. On the other hand, malic acid accumulates in flood tolerant species and falls in intolerant species. Organic acids can accumulate in cells in much greater quantities than ethanol without damaging the plant. In fact, malic acid helps preserve the electrical neutrality of the cell. EFFECTS OF TEMPERATURE The cardinal temperatures (i.e., minimum, optimum, maxi-mum) often differ for the same function in different plants and different functions in the same plant. They often differ also for the same function at different stages of development, and often in different organs of the same plant at a given, stage. Moreover, they may differ with the general physi-ological condition of the plant, the duration of temperature regime, and changes in other environmental factors. Daubenmire (1959) states that the minimum temperature for growth of melons ( Citrullus ) and sorghums ( Sorghum ) is in the range of 15 to 18⬚C; and for peas ( Pisum ), rye ( Secale ), and wheat ( Triticum ), ⫺2 to 5⬚C. He further states that roots usually have a lower minimum for growth than shoots of the same plants. The optimum temperature for photosynthesis is below that for respiration in all known cases. This is prob-ably the reason peaches, apples, white potatoes, and many other plants do not accumulate normal food reserves in low latitudes. Seeds of many native plants of temperature or colder regions have a low temperature requirement for germination. The value of such a mechanism in seedling survival is obvi-ous. Work in my laboratory at the University of Oklahoma has shown that seeds of Ambrosia artemisiifolia (common ragweed), A. psilostachya (western ragweed) and Sporobolus pyramidatus (drop seed) require exposure to temperatures in the range of 4–7⬚C for about 6–10 weeks in moist condi-tions to break dormancy. Lane (1965) reported that seeds of Helianthus annuus (annual sunflower) require 90 days at 4⬚C in moist sand to completely break dormancy. Donoho and Walker (1957) reported that Elberta peach ( Prunus per-sica ) seeds usually required stratification for 60–100 days at 7⬚C or below to break dormancy. Overwintering buds of many, if not most, perennial plants of temperature or colder regions also have low temperature requirements for the breaking of dormancy. Weinberger (1956) found that Elberta peach buds require 950 hours at 7⬚C or below to break dormancy. Daubenmire (1959) stated that peaches in general require 400 or more hours of temper-atures at or below 7⬚C, blueberries ( Vaccinium spp.) require 800 hours at this level, and apples ( Pyrus malus ) require even more. The chemical changes which are necessary for the breaking of dormancy in seeds and buds, and which are stimulated by exposure to low temperatures, are not defi-nitely known. In fact, this is a fertile field for research. There is growing evidence, however, that the breaking of dormancy may be related to a change in balance between growth inhibitors and stimulators. Sondheimer, Tzou, and Galson (1968) reported that abscisic acid inhibits germi-nation of excised nondormant embryos and this can be reversed with a combination of gibberellic acid and kinetin. Natural amounts of abscisic acid in seeds and pericarps of several species of Fraximus (ash) were determined, and it was found that the amount in F. americana decreased 37% in the pericarp and 68% in the seed with chilling treatment. They felt that it is unlikely that the amount in the pericarp influences dormancy but that the inhibitor in the seed does appear to play a regulatory role. Junttila (1970) found that dormancy of seeds of Betula nana (birch) was effectively broken by low temperatures or treatment with gibberellic acid. Haber, Foard, and Perdue (1969), reported 84% ger-mination in dormant lettuce ( Lactuca sativa ) seeds treated with a 5 ⫻ 10⫺ 4 M gibberellic acid solution, 1.5% germi-nation in seeds treated with a 5 ⫻ 10⫺ 4 M gibberellic acid solution plus 100 mg/l of abscisic acid, and 0% in water controls. Donoho and Walker (1957) reported a 98% breaking of dormancy of Elberta peach buds after only 164 hours at 7⬚C when sprayed with a 4000 ppm gibberellic acid solution, despite the usual requirement of 950 hours at 7⬚C or below. Aung, Hertogh, and Staby (1969) found free gibberellic acid in Tulipa gesneriana (tulip) increased by 67% over the initial level in bulbs grown for 4 weeks at 18⬚C. Bound gibberel-lic acid showed a slight initial increase followed by a rapid decrease. In bulbs treated at 13⬚C, a marked decline, occurred in free gibberellic acid and a 2-fold increase occurred in the bound form. The rates of floral, shoot, and root development were accelerated at 18⬚C as compared with 13⬚C. Cold hardening of plants (i.e., improving the ability of plants to withstand low temperatures without severe damage or death) is probably just as important to plant survival in regions with periods of relatively low temperatures as dor-mancy. Actually, some of the same internal changes may be responsible for both. Irving (1969) found a buildup during cold hardening in Acer negundo (box elder) of an inhibitor similar or identical to abscisic acid which has, of course, been C005_001_r03.indd 246C005_001_r03.indd 246 11/18/2005 10:20:10 AM11/18/2005 10:20:10 AM

247associated with dormancy. He found that it or abscisic acid increased hardiness when applied to branches after a harden-ing period of 3 weeks at 5⬚C. There are several cellular chemi-cal changes associated with cold hardening (Klages, 1947); (1) increase in bound water, (2) decrease in total water, (3) increase in osmotic pressure, (4) change of starch to sugars, (5) increase in pentosans, and (6) change of certain proteins to amino acids. Nothing new has been added to this list since Klages’ publication in 1947. Temperature obviously affects rates of all organic pro-cesses in plants, but a comprehensive discussion of such effects is outside the scope of this discussion. The relationship of temperature conditions at the geographic point of origin of plants to the optimum temperatures for photosynthesis and respiration in those plants is very important in explaining their distribution. Mooney and Billings (1961) studied rela-tionships of temperature to photosynthesis and respiration in Oxyria digyna (alpine sorrel), a species with a circumbo-real distribution of wide latiudinal range. The sources of the populations studied varied from an elevation of 12,300 feet at 39⬚ 40⬘N latitude to a zero elevation at 68⬚ 56⬘ N. They found that plants of northern populations had higher respi-ration rates at all temperatures than did plants of southern alpine populations. Moreover, they found that plants of north-ern populations had higher photosynthetic rates at lower tem-peratures and attained maximum rates at lower temperatures than did plants of the southern alpine populations. Obviously, there are definite temperature ecotypes in this species. Mooney, Wright, and Strain (1964) investigated the gas exchange capacity of plants in relation to elevational zona-tion in the White Mts. of California. Field measurements over an elevational span of 4000 to 14,000 feet indicated that the ratio of photosynthesis to dark respiration decreased with altitude primarily due to increased respiration rates with increased elevation. When similar measurements were made on plants grown in uniform greenhouse conditions, from seeds collected at different elevations, no differences in res-piration rates correlated with elevation were obtained indi-cating considerable environmentally conditioned plasticity in gas exchange capacity. However, the high elevation plants generally displayed peak photosynthetic activity at temper-atures lower than the low elevation plants even though all were grown in the same conditions. Mooney and West (1964) studied the photosynthetic and respiratory acclimation of five species in the White Mts. of California (Table 1). Seeds were collected at the indicated ele-vations, planted in pots in the greenhouse, and after establish-ment of the seedlings, they were taken to stations at various elevations in the mountains for acclimation (Table 2). They were kept in the same pots and soil and subirrigated at each place. Water was thus not limiting and the soil type was constant. Gas exchange measurements were made after a minimum of 3 weeks acclimation. The dark respiration rates of plants of all species from all stations were similar. Plants of all species acclimated to the colder subalpine environment were more efficient in photosynthesis at colder temperatures than were plants grown at the warmer environment of the lowest elevation. Conversely, plants grown in the desert were more efficient in photosynthesis at high temperatures than were plants grown in the subalpine region. Plants of the two species with the widest natural distributional range in eleva-tion in the White Mts. showed the greatest plasticity in their photosynthetic response as indicated by shifts in the optimum photosynthetic temperatures dependent on treatment. Miller (1960) compared the effects of temperature on the photosynthetic rate in Agrostis palustris (creeping bent) which is adapted to cooler regions of the US, and Cynodon dactylon (bermuda grass) which is adapted to the warmer regions. The relative rate of apparent photosynthesis in creeping bent increased from 64.6% at 15⬚C to a maximum of 100% at 25⬚C, then decreased to 62.2% at 40⬚C. On the other hand, the relative rate of apparent photosynthesis in bermuda grass increased form 54.9% at 15⬚C to a maximum of 100% at 35⬚C, and only dropped to 97.7% at 40⬚C. In summary, it is evident that plants vary genetically in their photosynthetic and respiratory responses to temperature changes, and they vary also in such responses depending on the temperature conditions during acclimation. EFFECTS OF LIGHT According to Johnson (1954) solar radiation at the outer surface of the earth’s atmosphere has a virtually constant intensity of 1400 watts m⫺ 2 or 2cal cm⫺ 2 min⫺ 1 . About 98% of the energy is in the wavelength interval of 0.2–4.5 ␮ , including about 40–45% in the visible range of 0.4–0.7 ␮ . About one-third of the energy is reflected back to space by the earth’s atmosphere and smaller amounts are absorbed TABLE 1Natural distribution of experimental plants SpeciesElev. range(m)Elev. origin of seeds used (m)Artemisia tridentata1829–3200 3078Chamaebatiaria millefolium1980–3200 3200Haplopappus apargioides3048–3962 3658Artemisia arbuscula3200–3719 3475Encelia virginensis ssp. actoni1372–1829 1402TABLE 2Acclimation stationsMean air temp. ⬚CaNatural vegetation Elev. (m) Max. Min.Desert scrub 1402 38.9 11.7Pinyon woodland 2408 33.4 10.3Subalpine forest 3094 19.5 –2.4a Based on weekly maximum and minimum temperatures during period of June 17 to Aug. 12, 1963.C005_001_r03.indd 247C005_001_r03.indd 247 11/18/2005 10:20:11 AM11/18/2005 10:20:11 AM
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and scattered by various components of the atmosphere. As a result, only about 50% of the total radiation reaches the earth on the average. This figure may be greater than 70% in dry regions with little cloud cover and lower than 40% in tropical rainy areas (Slatyer, 1967). Obviously, on very cloudy days the energy reaching the surface of the earth may be very low. Of the solar energy reaching the earth, about 40–45% is still present in the visible range. Radiation in the visible range is most important in affecting organic processes in plants, so the rest of this dis-cussion will be concerned primarily with visible light. The chief processes in plants which are affected by light are: (1) chlorophyll synthesis, (2) photosynthesis, (3) phototro-pism, (4) photoperiodism, and (5) transpiration. Light Intensity Effects. Many plants are able under most conditions to develop well in less than full sunlight. Red kidney beans ( Phaseolus vulgaris ) grow and fruit well in growth chambers in a light intensity of 700–1000 ft-c. Intensity of light has a definite effect on the distribution of plants. Many ferns grow well under trees because such ferns attain their maximum rate of photosynthesis at light inten-sities considerably below full sunlight. Species which char-acteristically grow in shaded locations are able to remain alive at very low light intensities. Redwood ( Sequoia sem-pervirens ) seedlings normally grow in the deep shade of the forest floor and they are able to grow at intensities of as little as 100 ft-c (Bonner and Galston, 1952). Pinus taeda (loblolly pine) and Pinus echinata (shortleaf pine) seed-lings are found in great profusion along road cuts and other open areas in the southeastern United States, but they fail to survive under dense forest canopies. Many hardwood seedlings such as those of Quercus alba (white oak) and Quercus rubra (red oak) survive nicely under such cano-pies and thus replace the pines in succession. Kramer and Decker (1944) found that photosynthesis in loblolly pine increased with light intensity up to the highest intensity used which was almost that of full sunlight. On the other hand, photosynthesis in white oak and red oak reached a maximum at one-third or less of full sunlight and showed slight decreases at higher light intensities. These results indicate that lack of sufficient light for photosynthesis may be a significant factor in the failure of pine seedlings to become established under forest stands. Seedlings of many other climax forest dominants such as Fagus grandifolia (beech) and Acer saccharum (sugar maple) have very low light requirements for photosynthesis also (Daubenmire, 1959). After a long series of studies, Blackman (1950) con-cluded that the distribution of Scilla nonscripta (bluebell) in English forests is determined primarily by light intensity. Björkman and Holmgren (1963) investigated the pho-tosynthetic response to light intensity of sun and shade plants of Solidago virgaurea (golden-rod). They found that the shade plants had a higher chlorophyll content per unit leaf area in weak light (3 ⫻ 10 4 ergs cm⫺ 2 sec⫺ 1 ) than in strong light (15 ⫻ 10 4 ergs cm⫺ 2 sec⫺ 1 ) and more than did sun plants in weak light. On the other hand, the sun plants had lower concentrations of chlorophyll in weak light than strong light. The gross rate of photosynthesis (apparent rate plus respiration rate) in sun plants at light saturation was 25.5 ⫾ 2.0 mg CO 2 dm⫺ 2 h⫺ 1 when the plants were grown in strong light (same as above) prior to testing whereas it was only 17.9 ⫾ 0.6 mg CO 2 dm⫺ 2 h⫺ 1 in shade plants grown in strong light. The gross rate of photosynthesis at light saturation was higher in the shade plants than in the sun plants when the plants were grown in weak light prior to testing, 2.93 ⫾ 0.08 in shade plants versus 2.31 ⫾ 0.04 in sun plants. The difference of the means in the plants grown in low intensity was highly significant statistically. The authors concluded that the photosynthetic behavior was consistent with light intensities prevailing in the natural environment indicating that the behavior was a result of a genetic adaptation to the habitat. Hybridization experiments supported this conclusion. Björkman (1966) reported on additional comparative studies of sun and shade plants of several species. Plants of shaded habitats consistently reached a higher percent-age of the maximum photosynthetic rate at light saturation, when tested in a low light intensity of 10 4 ergs cm⫺ 2 sec⫺ 1 (Table 3). Plantago lanceolata (plantain) was found only in open meadows and Lamium galeobdolon (henbit) only in dense beech forests. Björkman found that the activity of Photosystem I (far red, 700 m ␮ ) in Solidago virgaurea was about the same when grown in weak or strong light, regardless of the original habitat. On the other hand, the action of Photosystem II (⬎625 m ␮ used) was impaired in shade plants grown in high light intensity and in sun plants grown in low light intensity. The Emerson enhancement effect (increased photosynthesis from combination red–far red light over sum of photosynthesis in each light quality separately) was drastically reduced in shade plants grown in high light intensities. Milner and Hiesey (1964) studied the photosynthetic responses to light intensity of several races of Mimulus car-dinalis (monkey-flower) collected from an elevation of 45 m to 2220 m in California. They found that the saturating light intensity for the races increased with elevation of their origi-nal habitats. At 0⬚C, the saturating light intensity varied from 290 to 570 ft-c for different races, and at 40⬚C from 3500 to 5900 ft-c. Light intensity, of course, generally increases with elevation. Mooney and Billings (1961) in their study of ecotypes of Oxyria digyna found that the high elevation, low latitude plants attained photosynthetic light saturation at TABLE 3 Photosynthesis at a low light intensity of 104 ergs cm⫺2 sec⫺1 in percentof the photosynthetic rate at light saturationaSpecies Sun plants Shade plantsPlantago lanceolata 8 —Solidago virgaurea 9 17Rumex acetosa 11 16Geum rivale 14 16Lamium galeobdolon — 25 a From Björkman (1966). C005_001_r03.indd 248C005_001_r03.indd 248 11/18/2005 10:20:11 AM11/18/2005 10:20:11 AM

249a higher light intensity than did low elevation, high latitude plants even when preconditioned under the same conditions. Light Quality All visible light is effective in the syn-thesis of chlorophyll except wavelengths above 680 m ␮ (Sayre, 1928). Chlorophyll has a maximum absorption peak in the blue-violet (430 m ␮ ) and a secondary maximum in the red (660 m ␮ ), and chlorophyll b has a maximum absorption peak in the blue (455 m ␮ ), and a secondary maximum in the orange-red (640 m ␮ ) (Salisbury and Ross, 1969). Hoover (1937) reported that wheat ( Triticum sp.) has a maximum rate of photosynthesis at 655 m ␮ in the red and a second-ary maximum at 440 m ␮ in the blue. Warburg and Negelein (1923) gave a general curve for photosynthesis showing a maximum rate in the red and a secondary maximum in the blue. It is now known that photosynthesis actually consists of two photosystems which work together to complete the light reactions which result in the production of ATP and NADPH. Photosystem I has an absorption peak at 700 m ␮ and Photosystem II has an absorption peak around 672 m ␮ (Salisbury and Ross, 1969). Phototrophic responses (movements in response to light) are chiefly due to wavelengths in the blue-violet, 400–490 m␮ , but there may be some response to UV light (Johnston, 1934). The most efficient light quality to interrupt the dark period to prevent a short-day photoperiodic response is in the wavelength range of 620–640 m ␮ . Effects of Daylength The response of organisms to the relative lengths of the light and dark periods is termed photoperiodism. Garner and Allard (1920) discovered this phenomenon while working with Maryland Mammoth Tobacco ( Nicotiana tabacum ). They extended the work to many other species and found that flowering in many plants is regulated by the relative lengths of the light and dark peri-ods. They classified plants into three categories on the basis of their photoperiodic flowering responses: (1) short-day plants—those plants which flower only when the day length is below a certain critical maximum, (2) long-day plants—those which flower only when the day length is above a cer-tain critical minimum, and (3) day-neutral plants—those in which flowering is not determined by day length. Another category has been added, the intermediate-day plants—those which flower only between two critical day lengths. Larsen (1947) found that the strains of Andropogon sco-parius (little bluestem) from the area south of 36⬚N latitude were intermediate-day plants because they failed to flower on a 13-hour photoperiod and also on a 15-hour photope-riod, but flowered well between those daylengths. It is now known that numerous phenomena in plants are regulated by day length in many species, bulb formation, type of vegetative growth, dormancy, leaf abscission, seed germination, etc. The timing mechanism is still obscure, but it seems to involve a pigment called phytochrome which was discovered and isolated by Borthwick and his associates at the USDA laboratories in Beltsville, Maryland (Borthwick, Hendricks, and Parker, 1952). They found that phytochrome exists in two forms which can readily be converted from one form to another. One form has an absorption peak in the red at about 660 m ␮ and is designated as P r . The other form has an absorption peak at a longer wavelength of red (730 m µ ) and is designated as P fr , for far-red. When P fr absorbs light at 730 m ␮ it is converted to P r , and when P r absorbs light at 660 m ␮ it is converted to P fr . In addition to these rapid changes brought about by light, there is a slow conversion of P fr to P r in darkness. In sunlight and broad spectrum artificial light the conversion of P r to P fr predominates. The P fr form appears to be inhibitory to the flowering process, so darkness is impor-tant to eliminate all or part of this inhibitory form. Salisbury (1958) feels that the plant possibly synthesizes a postulated flowering hormone only during darkness also. Borthwick and Hendricks (1960) reported that the basic difference in long- and short-day plants may be in the ratio of P r to P fr required to initiate the flowering or other response. The presence of photoperiodic ecotypes has been well documented in many geographically wide-ranging species of plants. Olmsted (1944) was the first to demonstrate such ecotypes in his work with Bouteloua curtipendula (sideoats grama). He found that plants of this species from southern Texas and southern Arizona were generally short-day or intermediate-day plants, with an upper critical photope-riod for flowering between 14 and 16 hours. These strains flowered more vigorously on a 13-hour than on a 9-hour photoperiod. The North Dakota strains consisted largely of long-day plants with a critical photoperiod of about 14 hours. The strains from Oklahoma, Kansas, and New Mexico included mostly long-day individuals although the length of the critical photoperiod decreased with decrease in latitude of origin. Larsen (1947) found that only strains of Andropogon scoparius (little bluestem) originating south of 36⬚N latitude produced visible inflorescences on a 14-hour photoperiod. The northern strains flowered on a 15-hour photoperiod but not the southern strains. The northern strains were apparently long-day plants because they were able to initiate flowers on a long photoperiod but were inhibited from flowering on 13- and 14-hour photoperiods. As was previously indicated at the beginning of this section on effects of day-length, the strains originating south of 36⬚N latitude were apparently intermediate-day plants. Vaartaja (1959) collected seeds from 19 genera of trees, 38 species and 82 origins of widely different latitudes in Europe and North America. Plants derived from these seeds were grown in uniform conditions except for photoperiod. Photoperiods of 12, 14, 16, and 18 hours were used with the total light energy being identical in all cases. He found that the farther north the origin of the strain, the longer was the critical daylength below which height growth of seedlings was retarded. Moreover, the more northerly the source, the shorter was the critical dark period which induced dormancy in several species. In some plants from the far north, photo-periods that inhibited elongation permitted great increases in weight of stems and roots, and in numbers of buds. Cambial growth thus would continue later in the growing season than elongation. Irgens-Moller (1957) collected approximately 100 plants (2–6 year old) of Pseudotsuga menziesii (douglas-fir) from each of seven locations in Oregon at elevations of 60 feet to C005_001_r03.indd 249C005_001_r03.indd 249 11/18/2005 10:20:11 AM11/18/2005 10:20:11 AM
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4000 feet. The plants were potted and kept in a cold frame at Corvallis, Oregon, during two winters after collection so that all were exposed to the same winter conditions. On February 1 after the second winter, half of the plants from each locality were placed in a 9-hour photoperiod and half on an 18-hour photoperiod with all other conditions the same. There were no effects of photoperiod on time of bud bursting in the low altitude plants, but the long days hastened bud bursting strik-ingly in high altitude plants as compared with effects of short days. The selection probably results from the killing of seed-lings by low night temperatures at high altitudes if they break dormancy in the short days of winter or early spring. EFFECTS OF PLANT PRODUCED TOXINS De Candolle first suggested the existence of naturally- occurring plant growth-inhibitors in 1832 (Bonner 1950). He noted from field observations that Euphorbia (spurge) was apparently inhibitory of flax ( Linum usitatissimum ), thistles (probably Cirsium ) to oats ( Avena sativa ) and rye to wheat. Very little research was done on the subject for almost 100 years at which time Cook (1921) described the characteristic wilting of potato ( Solanum tuberosum ) and tomato plants grown near Juglans nigra (black walnut). Davis (1928) extracted juglone from mature hulls and roots of black walnut and found it to be very toxic to tomato and alfalfa ( Medicago sativa ) plants when injected into the stems. The juglone was identified as 5-hydroxynaphthaquinone. Proebsting and Gilmore (1940) related the problem of re-establishing peach ( Prunus persica ) trees in old peach orchards to the presence of toxic substances in the soil. Bonner and Galston (1944) reported that the edge rows in guayule ( Parthenium argentatum ) plantings in California had much larger plants than the center rows and the differ-ences could not be eliminated by heavy watering and min-eral application. Experiments indicated that leachates from pots of year-old guayule plants were very inhibitory to gua-yule seedlings. The inhibitor was subsequently identified as transcinnamic acid. Muller, Muller, and Haines (1964) observed areas free of vegetation around certain shrubs in California grass-lands. They found that at least three shrubs, Salvia leuco-phylla (clearleaf sage), S. apiana (bee sage), and Artemisia californica (California sagebush) produced volatile materi-als which inhibited various test plants. Muller and Muller (1964) identified several terpenes which were produced by leaves of three species of Salvia, and two of these, camphor and cineole, were found to be very inhibitory to test plants. Muller, Hanawalt, and McPherson (1968) related the disap-pearance of herbaceous species in the open California chap-arral during the fire cycle to the production of inhibitory chemicals by chaparral species. My students and I have obtained good evidence that the rapid disappearance of the pioneer weed stage (2 to 3 years) in our revegetating old-fields in central Oklahoma is due to the inhibition of seedlings of species of that stage by several plants of the pioneer stage (Abdul-Wahab and Rice, 1967; Parenti and Rice, 1969; Wilson and Rice, 1968). Triple-awn grass ( Aristida oligantha ), the dominant of the second stage, is generally not inhibited by species of the pioneer weed stage and can grow well in the infertile soil, so it is able to invade the area. There is some evidence that antibiotics (substances produced by microorganisms which inhibit other micro- organisms) may be important in the growth and distribution of the higher plants as well as microorganisms. Certain soil bacteria inhibit the root-nodule bacteria which are important in adding nitrogen to the soil (Konishi, 1931). Iuzhina (1958) reported that many soil bacterial, fungi and actinomycetes are antagonistic to Azotobacter, a free living bacterium that adds nitrogen to the soil. Some soil microorganisms have been found to be inhibitory to certain microorganisms that cause diseases of higher plants (Cooper and Chilton, 1950). Another level of inhibition is that of microorganisms against higher plants. There is no question that many, if not most, pathogenic microorganisms produce abnormal symp-toms in the host plants through the production of toxins. Very few of these compounds have been identified and this remains an important and fruitful field for research. Microorganisms are sometimes responsible for changing certain non- inhibitory metabolites of higher plants into inhibitory compounds (Börner, 1960). The remaining level of inhibition is that of higher plants against microorganisms. There is considerable evidence that resistance of many plants to fungal, viral, and bacterial diseases may be associated with the production by resistant varieties of inhibitors of the pathogens (Farkas and Kiraly, 1962; Hughes and Swain, 1960; Schaal and Johnson, 1955). Ferenczy (1956) found that the seeds or fruits of 52 species and varieties from 19 plant families contained anti-bacterial compounds and that seeds of some species were attacked intensely by molds whereas others were attacked slightly or not at all. Lane (1965) found that fruits of the native sunflower, Helianthus annuus, rotted more frequently during germination tests if certain inhibitors were leached from them before the tests. Those species in which decay of the seeds during germination is less likely to occur would certainly stand a better chance of becoming established. I have found that many of the low-nitrogen requiring early plant invaders of abandoned cultivated fields pro-duce inhibitors of the nitrogen-fixing bacteria (Rice, 1964; 1965b; 1965c). Triple-awn grass, the dominant of the second stage, is one of the inhibitory species. This would give such plant species a selective advantage in competition over plants with higher nitrogen requirements. This prob-ably slows succession in old-fields, because Rice, Penfound, and Rohrbaugh (1960) found that the order in which spe-cies invade revegetating fields in central Oklahoma, begin-ning with the second stage, is the same as the order based on increasing nitrogen requirements. Muller (1965) found that the volatile terpenes from Salvia leucophylla reduce cell elongation and cell division in the radicles and hypocotyls of germinating Cucumis sativus (cucumber) seeds. Muller et al. (1969) reported that two vol-atile terpenes, cineole and dipentene, which emanate from C005_001_r03.indd 250C005_001_r03.indd 250 11/18/2005 10:20:12 AM11/18/2005 10:20:12 AM

251 Salvia leucophylla leaves markedly reduce oxygen uptake by suspensions of mitochondria from Avena fatua (wild oats) or cucumber. The inhibition appears to be localized in that part of the Krebs cycle where succinate is converted to fumarate, or fumarate to malate. They also reported that the permeability of cell membranes appears to be decreased on exposure to terpenes. Most of the inhibitors that my students and I have identified from our old-field species are phenolics (Abdul-Wahab and Rice, 1967; Parenti and Rice, 1969; Rice, 1965a, 1965b; Rice and Parenti, 1967; Wilson and Rice, 1968). Some of the inhibitors identified are: chlorogenic acid, p-coumaric acid, ferulic acid, gallic acid, p-hydroxybenzal-dehyde, isochlorogenic acid complex, scopoletin, scopolin, sulfosaliyclic acid, and several tannins. Sondheimer (1962) reported that chlorogenic acid is a strong inhibitor of several enzyme systems and it is pos-sible that this may be its chief role in growth inhibition. Schwimmer (1958) found that a 10⫺ 3 M concentration of chlorogenic acid caused a 50% inhibition of potato phosphorylase activity. Mazelis (1962) reported that a peroxidase-catalyzed oxidative decarboxylation of methio-nine is strongly inhibited by a 5 ⫻ 10⫺ 6 M concentration of chlorogenic acid. Sondheimer and Griffin (1960) reported that indoleacetic acid oxidase is inhibited by chlorogenic acid and its isomers, and by isochlorogenic acid and dihydrochlorogenic acid. Zenk and Müller (1963) reported that p-coumaric and ferulic acids increase decarboxylation of indoleacetic acid (IAA), resulting in reduced plant growth. Henderson and Nitsch (1962) found a pronounced inhibiting effect of p-coumaric acid on IAA induced growth also. Bendall and Gregory (1963) found that phenol oxidases which are important in the formation of lignin-like polymers from coniferyl alcohol and oxidation products of tannins such as quinones react with each other to produce a complex that may make phenol oxidase catalytically inactive, or may produce an active protein of modified properties. Hulme and Jones (1963) and Jones and Hulme (1961) found that tannins caused considerable reduction of Krebs cycle succin oxidase and malic enzyme activity. Gallic and tannic acids were shown also to inhibit IAA induced growth (Zimsmeister and Hollmuller, 1964). Hall (1966) and Williams (1963) reported that relatively low concentrations of tannic acid were very inhibitory to pectolytic enzymes. Gallic and tannic acids are very inhibitory to the nodulation of heavily inoculated legumes and to hemoglobin formation in the nodules (Blum and Rice, 1969). The hydroxyl groups of tannins are known to have a strong affinity for the peptide linkages of proteins, so it is likely that reactions readily occur between the two compounds when they come in contact at a time when the ice-shell around the protein is appropriately modified. This would undoubtedly change the nature of the protein. Einhellig et al. (1970) found that sunflower ( Helianthus annuus ) and tobacco plants treated with 10⫺ 4 and 5 ⫻ 10⫺ 4 scopoletin concentrations had significant increases in scopole-tin and scopolin in their tissues. Net photosynthesis in tobacco plants treated with a 10⫺ 3 M concentration of scopoletin was depressed to as low as 34% of that of the controls. Reduced growth in leaf area over a 12 day experiment correlated well with the significant reduction in the rate of net photosynthesis. The surface has just been scratched in identifying plant produced inhibitors of other plants. Moreover, the field is wide open for research on the mechanisms of action of the presently known inhibitors. GENERAL CONCLUSIONS I have discussed briefly the presently known physiological basis for some of the ecological responses of plants to the environmental factors of water, temperature, radiant energy, and toxins. It would obviously require most of the present volume to discuss the presently known effects of all envi-ronmental factors. Even so, our knowledge of the effects of most factors is superficial. Many of the known physiological effects may be indirect ones also. In closing, I emphasize that the physiological ecologist is searching for the physiological basis for ecological behavior. It is essential that research in this area be vigorously pursued. REFERENCES 1. Abdul-Wahab, A.S. and E.L. Rice, Bull. Torrey Bot. Club, 94, 486, 1967. 2. Aung, L.S., A.A. de Hertogh and G. Staby, Plant Physiol., 44, 403, 1969. 3. Bendall, D.S. and R.P.F. 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Klages, K.H.W., Ecological Crop Geography, MacMillan Co., New York, 1947. 45. Klotz, I.M., Science, 128, 815, 1958. 46. Konishi, K., Mem. Col. Agr., Kyoto Imp. Univ., 16, 1, 1931. 47. Kramer, P.J. and J.P. Decker, Plant Physiol., 19, 350, 1944. 48. Lane, F.E., Dormancy and germination in fruits of Sunflower, Ph.D. Dissertation, University of Oklahoma, Norman, 1965. 49. Larsen, E.C., Bot. Gaz., 109, 132, 1947. 50. Mazelis, M., J. Biol. Chem., 237, 104, 1962. 51. Meyer, B.S. and D.B. Anderson, Plant Physiology, D. Van Nostrand Co., Inc., New York, 1939. 52. Miller, V.J., Proc. Amer. Soc. Hort. Sci., 75, 700, 1960. 53. Milner, H.W. and W.M. Hiesey, Plant Physiol., 39, 208, 1964. 54. Mooney, H.A. and W.D. Billings, Ecol. Monogr., 31, 1, 1961. 55. Mooney, H.A. and M. West, Amer. J. Bot., 51, 825, 1964. 56. Mooney, H.A., R.D. Wright and B.R. Strain, Amer. Midland Naturalist, 72, 281, 1964. 57. Muller, C.H., W.H. Muller and B.L. Haines, Science, 143, 471, 1964. 58. Muller, W.H. and C.H. Muller, Bull. Torrey Bot. Club, 91, 327, 1964. 59. Muller, W.H., Bot. Gaz., 126, 195, 1965. 60. Muller, C.H., R.B. Hanawalt and J.K. McPherson Bull. Torrey Bot. Club, 95, 225, 1968. 61. Muller, W.H., P. Lorber, B. Haley and K. Johnson, Bull. Torrey Bot. Club, 96, 89, 1969. 62. Olmsted, C.E., Bot. Gaz., 106, 46, 1944. 63. Parenti, R.L. and E.L. Rice, Bull. Torrey Bot. Club, 96, 70, 1969. 64. Proebsting, E.L. and A.E. Gilmore, Proc. Amer. Soc. Hort. Sci., 38, 21, 1940. 65. Rice, E.L., W.T. Penfound and L.M. Rohrbaugh, Ecology, 41, 224, 1960. 66. Rice, E.L., Ecology, 45, 824, 1964. 67. Rice, E.L., Physiol. Plant, 18, 255, 1965a. 68. Rice, E.L., Proc. Okla. Acad. Sci., 45, 43, 1965b. 69. Rice, E.L., Southwestern Naturalist, 10, 248, 1965c. 70. Rice, E.L. and R.L. Parenti, Southwestern Naturalist, 12, 97, 1967. 71. Salisbury, F.B., Scientific Amer., 198, 108, 1958. 72. Salisbury, F.B. and C. Ross, Plant Physiology, Wadsworth Publishing Co., Inc., Belmont, Calif., 1969. 73. Sayre, J.D., Plant Physiol., 3, 71, 1928. 74. Schaal, L.A. and G. Johnson, Phytopathology, 45, 626, 1955. 75. Schwimmer, S., J. Biol. Chem., 232, 715, 1958. 76. Slatyer, R.O., Plant-Water Relationships, Academic Press, New York, 1967. 77. Sondheimer, E. and D.H. Griffin, Science, 131, 672, 1960. 78. Sondheimer, E., The chlorogenic acids and related compounds, in Plant Phenolics and Their Industrial Significance, V.C. Runeckles, Ed., Imperial Tobacco Co. of Canada, Montreal, 1962. 79. Sondheimer, E., D.S. Tzou and E.C. Galson, Plant Physiol., 43, 1443, 1968. 80. Stocker, O., UNESCO Arid Zone Res., 15, 63, 1960. 81. Vaartaja, O., Ecol. Monogr., 29, 91, 1959. 82. Warburg, P. and E. Negelein, Zeirschr. Phys. Chem., 106, 191, 1923. 83. Warming, E., Oecology of Plants; An Introduction to the Study of Plant Communities, The Clarendon Press, Oxford, England, 1909. 84. Weinberger, J.H., Proc. Amer. Soc. Hort. Sci., 67, 107, 1956. 85. Williams, A.H., Enzyme inhibition by phenolic compounds, in Enzyme Chemistry of Phenolic Compounds, MacMillan Co., New York, 1963. 86. Wilson, R.E. and E.L. Rice, Bull. Torrey Bot. Club, 95, 432, 1968. 87. Zenk, M.H. and G. Müller, Nature, 200, 761, 1963. 88. Zimsmeister, H.D. and W.H. Hollmuller, Planta, 63, 133, 1964. GENERAL REFERENCES 1. Alexander, M., Introduction to Soil Microbiology, 2nd Ed. John Wiley, New York, 1977 2. Bainbridge, R., G.C. Evans and O. Rackham, Light as an Ecological Factor, John Wiley, New York, 1966. 3. Baker, F.B. and W.C. Snyder, Ed., Ecology of Soil-borne Plant Patho-gens, Univ. of Calif. Press Berkeley, 1965. 4. Burges, A. and F. Raw, Soil Biology, Academic Press, New York, 1967. 5. Evans, L.T., Ed., Environmental Control of Plant Growth, Academic Press, New York, 1963. 6. Fried, M. and H. Broeshart, The Soil-Plant System, Academic Press, New York, 1967. 7. Geiger, R., The Climate Near the Ground, Harvard Univ. Press, Cambridge, 1965. 8. Gray, T.R.G. and D. Parkinson, Ed., The Ecology of Soil Bacteria, Univ. of Toronto Press, 1968. 9. Hilman, B., Crop Losses from Air Pollutants, Environ. Sci. and Tech. , 16, 495, 1982. 10. Hoagland, D.R., Inorganic Nutrition of Plants, Chronica Botanica, Waltham, Massachusetts, 1948. 11. Jeffrey, D.W., Soil-Plant Relationships, Croom Helm, London, 1987. 12. Keller, T., Air Pollutants Deposition and Effects on Plants (B. Ulrich and J. Pankrath, Eds.), Reidel, Dordrecht, 1983. 13. Kononova, M.M., Soil Organic Matter, Pergamon Press, New York, 1966. 14. Kramer, P.J., Plant and Soil Water Relationships, McGraw-Hill, New York, 1949. 15. Leopold, A.C., Plant Growth and Development, McGraw-Hill Book Co., New York, 1964. 16. Nason, A. and W.D. McElroy, Modes of action of the essential min-eral elements, in Plant Physiology, A Treatise, Vol. III, Academic Press, New York, p. 451–536, 1963. 17. Stewart, W.D.P., Nitrogen Fixation in Plants, Athlone Press, London, 1966. 18. Sutcliffe, J.F., Mineral Salts Absorption in Plants, Pergamon Press, New York, 1962. 19. Treshow, M., Environment and Plant Response, McGraw-Hill Book, Co., New York, 1970. 20. Treshow, M., Effects of toxicants on terrestrial plants, in Environmental Toxicology, WHO, Copenhagen, 1983. 21. Whyte, R.O., Crop Production and Environment, Faber and Faber, London, 1946. ELROY L. RICE University of Oklahoma C005_001_r03.indd 252C005_001_r03.indd 252 11/18/2005 10:20:13 AM11/18/2005 10:20:13 AM
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BASIC CONCEPTS There are many approaches to the study and appreciation of the natural world. The ecologist looks at it with his interest focused on the relations between living things and their sur-roundings. For purposes of quantitative analysis, he ﬁ nds it useful to think of nature as organized into ecological systems ( ecosystems ), in which the living units interact with their envi-ronments to bring about the ﬂ ow of energy and the cycling of matter wherever life is found. In this conceptual framework, organisms can proﬁ tably be considered according to their major roles in the handling of matter and energy. Thus, living things have ecologically classiﬁ ed (Thienemann, 1926) as producers if they are autotrophic, i.e., able to manufacture their own food from simple inorganic substances with energy obtained from sunlight (photosynthetic green plants) or from the chemical oxidation of the inorganic compounds (chemosynthetic bacte-ria), or as consumer if they are heterotrophic, i.e., required to depend on already synthesized organic matter as the source of food energy. A special and very important group of consum-ers are the decomposers, which break up the complex organic substances of dead matter, incorporating some of the decom-position products in their own protoplasm and making avail-able simple inorganic nutrients to the producers. Decomposers consist chieﬂ y of fungi and bacteria, which absorb their food through cell membranes and thus differ signiﬁ cantly from the larger consumers, which ingest plant and animal tissue into an alimentary tract. There are, however, many different modes of nutrition, and it has recently been suggested (Wiegert and Owen, 1970) that energy ﬂ ow and the cycling of matter may be better understood if heterotrophic consumer organisms are classiﬁ ed on the basis of their energy resources rather than in terms of their feeding habits. Thus we may recognize two major groups of consumers: biophages if they obtain their energy from living matter, and saprophages if they derive their energy from dead and decaying materials. These basic classi-ﬁ cations do not accommodate such organisms as Euglena and the Venus ﬂ y-trap which are capable of both photosynthesis and heterotrophy, but they provide a reasonable framework for the majority of plant and animal species. FOOD CHAINS AND FOOD WEBS The concept of food-chain was developed by Elton (1927) to represent the series of interactions that occur between organ-isms in their efforts to obtain nourishment. Thus a hierarchy of relationships is formed as green plans or their products are eaten by animals and these are in turn eaten by other animals. Organisms, then, are functionally related to one another as links in a chain, i.e., as successive components in a system for the transfer of energy and matter. An organism can be char-acterized ecologically in terms of the position it occupies in its food-chain, and consumer organisms that are one, two and three links removed from the producer are referred to as pri-mary, secondary and tertiary consumers, respectively. Thus primary consumers are those which subsist on green plants or their products, and are broadly termed herbivores, in contrast to secondary and higher-order consumers, which are termed carnivores. (Note again, as with the producer–consumer clas-siﬁ cation, that the herbivore–carnivore dichotomy is not per-fect; it does not allow for omnivores, which eat both plant and animal tissue, or for those organisms which are herbivorous at one state in their life history and carnivorous at another.) With each successive transfer, some of the energy that was incorporated by the producer organism (the initial link in the chain) is lost as heat, and for this reason food-chains generally do not involve more than four or five links from the beginning to the ultimate large consumer. A typical terrestrial food-chain consists of a green plant, e.g., grass, eaten by an insect, e.g., a grass-hopper, which is in turn eaten by a small bird, e.g., a spar-row, and this in turn by a larger bird, e.g., a hawk. However, food chains are generally linked to other chains at almost every point; the grass is likely to be eaten by numerous species of herbivore, each of which has its own set of predators, etc. The result is that the community ecosystem is made up, not of a set of isolated food-chains but of an interconnected food-web whose structure rapidly becomes very complex when more than a few species are considered. This complexity, however, is restricted in part by the limited length of food-chains and in part by the unidirectional flow of energy in these chains. Food-webs are made up of two principal types of food-chains: grazing food-chains, which involve the direct consumption and transformation of living tissue, as in the grazing of rangeland by cattle or sheep, and detritus food-chains, which involve the disintegration and conversion of dead matter, both plant and animal, by a sequence of decomposer organisms. TROPHIC LEVELS AND ECOLOGIGAL PYRAMIDS The concept of the food-chain permits us to equate, in terms of ecosystem function, all organisms which occupy the same C005_002_r03.indd 253C005_002_r03.indd 253 11/18/2005 10:20:39 AM11/18/2005 10:20:39 AM
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feeding position, i.e., which are the same number of links removed from the producer organism. Organisms which are similar in this respect are said to occupy the same trophic level. Such levels form a natural hierarchy arranged from the producer level at the bottom, through the primary con-sumer (herbivore) level, to one or more successive levels of subsidiary consumers at the top. The trophic level concept was developed by Lindemann (1942) to compare the energy content of the different feeding groups in natural commu-nities and to evaluate the efﬁ ciency with which energy is transferred from level to level. If the number of individuals, the total biomass (weight of tissue), or the energy content of the organisms of succes-sive trophic levels in a natural community is examined, the quantity is generally found to decrease as one goes upward from producer to ultimate consumer levels. Diagrammatic models of this trophic structure thus tend to be pyramidal and have given rise to the concept of ecological pyramids. Comparisons of trophic levels based on numbers of individ-uals can be misleading, however, especially when species of very different size and rate of growth are involved: herbivo-rous insects are likely to be much more numerous per unit of suitable habitat than are grazing or browsing mammals (Evans and Lanham, 1960). This difficulty is partly resolved by measuring total weight of organisms present, thus replac-ing a pyramid of numbers with a pyramid of biomass. If they are constructed for parasite food-chains (in which one host can support many parasites) or for communities whose producers (such as diatoms and other minute algae) have a more rapid rate of replacement, or turnover, than its consum-ers, such pyramids may appear inverted or may show a very narrow base. Furthermore, because the number of calories varies considerably from tissue to tissue because of differ-ences in composition (fat averages about 9500, and carbo-hydrate and protein about 4000, calories per gram), biomass may prove a poor indicator of energy content. It is therefore desirable, whenever possible, to replace weights with calo-rific values and to convert the pyramid of biomass into a pyr-amid of energy. If the total amount of energy utilized at each trophic level over a set period of time is taken into account the quantity will always be less at each succeeding level and the upright pyramidal shape of the model will be maintained. Thus energy units provide the best basis for comparing the productivity (the rate at which energy and matter are stored in the form of organic substances) of different organisms, of different trophic levels, and of different ecosystems. They also offer the best means of evaluating the efficiency (the ratio of energy stored or put out in a process to that put in, usually expressed as a percentage) of organisms and eco-systems in carrying out their activities of transferring and transforming energy and matter. STANDING CROPS, PRODUCTION, AND ENERGY FLOW The quantity of living organisms present at a given time may be referred to as the standing crop or stock. For reasons already explained, this quantity is best expressed in terms of its energy content (in calories). The magnitude of the stand-ing crop will vary from place to place, depending basically on the available quantities of energy and nutrients, and in almost all places from season to season, being inﬂ uenced by all the factors affecting growth and reproduction. Relatively long-lived consumers like the large herbivorous mammals may accumulate and store considerable quantities of matter and energy in their bodies and thus achieve a large standing crop, but surprisingly high values can be reached by much smaller organisms, such as insects, which feed more efﬁ ciently and reproduce more rapidly. Unusually high standing crops are seen at times of population explosions, such as those of defo-liating insects and of periodically ﬂ uctuating species like snowshoe hares and lemmings, but these levels cannot be sustained for more than a short time. Inverted pyramids of biomass illustrate the possibility of a relatively large standing crop of consumers supported by a small standing crop of pro-ducer plants, when the latter are smaller, grow more rapidly, and are replaced more frequently than the consumers. Because organisms may be eaten or move away from an area, the standing crop often fails to be a good measure of the total quantity of tissue they have produced over a given period of time. This total quantity, or production, includes not only the new tissue added as growth to the bodies of individual organisms but also that resulting from the repro-duction of new individuals. That part of the production that is removed by man (or some other species) is known as the yield or harvest. Because of the limited efficiency of the meta-bolic processes required in the formation of new tissue, more energy and matter are needed by the organism than are stored in production. Thus, to evaluate the complete functioning of an ecosystem, we need a measure of the total energy (or matter) involved in metabolism; for consumer organisms, this is referred to as assimilation or energy flow. (For producer organisms, the total energy or matter used in their metabo-lism is called gross primary production, while that incorpo-rated as new tissue is called net primary production. ) ENERGY BUDGETS AND EFFICIENCIES Full understanding of increases or decreases in standing crop or energy ﬂ ow requires quantitative knowledge of the biological processes involved in the transfer of matter and energy. For consumer organisms, these consist of the pro-cesses of ingestion or intake, respiration —a measure of metabolic activity, and egestion (generally taken to include the elimination of both feces and urine). Use is made of the energy budget or balance described by the equations. Ingestion Production Respiration EgestionAssimilation Prod⫽⫹ ⫹⫽ uuction Respiraton⫹ and other expressions to calculate the values for processes whose quantities are not known or to determine the net change of energy transfer. Such calculations enable the ecologist to C005_002_r03.indd 254C005_002_r03.indd 254 11/18/2005 10:20:39 AM11/18/2005 10:20:39 AM
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255estimate the efﬁ ciency with which these processes are carried out, as for example assimilation efficiencycalories assimilatedcalories ingeste⫽ddgrowth efficiencycalories in growthcalories ingestedyield ⫽eefficiencycalories to man (or other exploiter)calories ingest⫽eed These and other efﬁ ciencies provide the means for compar-ing different trophic levels or other ecological units with respect to their functional roles in the ecosystem. For pri-mary consumers, the ratio calories consumed by herbivore populationcalories of available plaant food measures their food-chain efﬁ ciency, while the ratio calories of herbivores consumed by carnivorescalories of plant foodd consumed by herbivores measures their efﬁ ciency of energy transfer. DIVERSITY OF PRIMARY CONSUMERS All parts of plants—leaves, buds, ﬂ owers and their products (seeds, pollen, nectar), stems, sap, roots, bark and wood—are eaten by primary consumers, who have evolved a great diversity of life form and habits in response to their food. Large grazers and browsers, such as elephants and deer, can ingest plant tissue in bulk, but others are specialized to par-ticular products, e.g., the mylabrid weevils that feed on beans and peas, and the sap-sucking aphids and leaf-hoppers. The specialization may extend to particular kinds of plants; the Australian koala “bear,” for example, is limited to leaves of Eucalyptus, and the larva of the swallowtail butterﬂ y Papilio marcellus feeds exclusively on the foliage of the prickly ash ( Zanthoxylua ). Much less is known of the food habits of detritus-feeders, which never attain the large size of some grazers, but Petrusewicz and Macfadyen (1970) point out that “remarkably few species restrict their diets at all narrowly.” All of the principal phyla of land animals have developed forms which are partly or entirely herbivorous, and it is likely that the majority of terrestrial heterotrophic species are pri-mary consumers. About half of the known kinds of insects are plant-eaters (Brues, 1946), and in some habitats the pro-portion may be considerably higher: Menhinick (1967) found that herbivores made up about 80% of the insect species of a bush clover ( Lespedeza ) stand in South Carolina, and Evans and Murdoch (1968) reported that 85% of approximately 1500 insect species encountered on a 30-year-old abandoned field in Michigan were herbivorous. With such diversity, it is unlikely that any species of plant has escaped exploitation by herbi-vores. Indeed, most plants are host to a wide range of consum-ers; for example, at least 227 species of herbivorous insects are known to be associated with the oaks ( Quercus robur and Q. petraea ) of British woodlands (Elton, 1966, 197). The most important insect consumers of live veg-etation belong to the orders Orthoptera (grasshoppers), Hemiptera (true bugs), Homoptera (leaf-hoppers, aphids, scales), Coleoptera (beetles), Lepidoptera (moths, butter-flies), Diptera (flies), and Hymenoptera (bees, wasps, ants). Insects as a group share dominance as herbivores with the mammals, including such larger types as horses, pigs, deer, antelope, goats, sheep and cattle, and such smaller ones as hares, rabbits, squirrels, marmots, voles and lemmings. (In tropical regions, various monkeys and apes are also impor-tant herbivores, as are fruit-eating and nectarivorous bats.) There are significant herbivores among terrestrial birds, e.g., the Galliformes (grouse, quail, pheasants) and the “spe-cialist” seed-eating finches and sparrows, the fruit-eating parrots, and the nectar-sucking hummingbirds, and among land mollusks, such as the “garden” snails and slugs. Much less is known of the herbivores which feed on roots, underground stems, and other living plant parts in the soil. Studies by Bornebusch (1930), Van der Drift (1951), Cragg (1961) and Macfadyen (1961) indicate that the most important groups of herbivores in north temperate grassland and forest soils are the parasitic nematode worms, the molluscs, and the larvae of Diptera, Coleoptera, and Lepidoptera. They occur in close association with other primary consumers, the detritus feeders, which include the Oligochaeta (earthworms and enchytraeid worms), Isopoda (wood lice), Diplopoda (mil-lipedes), free-living Nematoda, Collembola (spring tails), and Acari (mites). The mechanical comminution of dead matter by these organisms provides a substrate of small particles which can be more easily attacked by fungi, protozoa, bacteria and other microorganisms (Phillipson, 1966). MEASUREMENT OF PRIMARY CONSUMPTION The measurement of primary consumption involves the assessment, in terms of number of individuals, biomass, and energy equivalence, of the quantity of herbivorous ani-mals present or produced over a period of time, the ener-getic cost of producing and maintaining that product, and the fate of the matter and energy that becomes incorporated in herbivore tissue. A detailed account of methods and tech-niques employed for these purposes is beyond the scope of this article, but a brief treatment is presented below. The mobility, abundance, and small size of many primary consumers make it difficult to assess their numbers. Total counts or censuses, as of large mammals by aerial photog-raphy (Parker, 1971) or of small ones by intensive trapping (Gliwicz et al., 1968), can only rarely be achieved, and it is general practice to rely on sample collections (e.g., Wiegert, 1964, 1965). The marking of individual organisms for subse-quent recapture has been employed to estimate population size in such herbivores as grasshoppers (Nakamura et al., 1971), C005_002_r03.indd 255C005_002_r03.indd 255 11/18/2005 10:20:40 AM11/18/2005 10:20:40 AM
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and indices of relative abundance based on fecal pellet counts (Southwood, 1966, 229) or on damage to vegetation (Odum and Pigeon, 1970, 1–69) are sometimes used. Estimation of change in numbers with time involves a knowledge of birth, death and migration rates, the calculation of which requires considerable lifetable information. Biomass values are commonly derived by multiplying the number of individuals by average weights obtained from sample specimens or calculated from formulas relating weight to body dimensions (Petrusewicz and Macfadyen, 1970, 51). More detailed studies require knowledge of the growth of individuals and the rate of weight gain. Ultimately, biomass should be converted to its energy equivalent by determining its calorific value; this requires the use of a calorimeter, such as that developed by Phillipson (1964), and because of the difficulty of obtaining complete combustion, it is generally carried out in the laboratory. Published tables of the caloric content of various plant and animal tissues (e.g., Golley, 1961) show a considerable range of values even for the same species, reflecting differences in life history stage, season, and environmental conditions. It is also clear that the dimensions of primary consumption cannot be accurately gauged without knowledge of the pro-cesses ( ingestion, egestion, assimilation, respiration ) associated with it. It is important, therefore, to investigate the food habits and feeding rates of herbivores, as well as to determine the quality of the food consumed, the proportion rejected as feces, the digestive efficiency, and the rates of respiration (oxygen consumption can be measured by a respirometer such as that of Smith and Douglas, 1949; see also the book on manometric techniques by Umbreit et al., 1957). Much of this work has had to be done on animals in confinement, and little is known as yet of activity levels and metabolic rates of herbivores in nature. Recent developments in the use of radioactive isotope tracers (Williams and Reichle, 1965) and the telemetric mea-surement of heart rate or other characteristics related to respira-tion (Adams, 1965) give promise that the study of metabolism under filed conditions will eventually be feasible. EFFICIENCY OF PRIMARY CONSUMERS With careful management and appropriate stocking large mammalian herbivores can consume a fairly high proportion of the available plant food. In Great Britain, sheep stocked at a density of 10 ewes per hectare on lowland pasture may ingest up to 70% of the annual production of grass (Eadie, 1970), and beef cattle on management rangeland in the United States, when stocked at the maximum recommended exploitation rate, will consume from 30% to 45% of the forage (Lewis et al., 1956). The efﬁ ciency of feeding will of course vary with the situation: sheep on hill pastures, where a stocking rate cannot exceed 0.8 ewes per hectare, may utilize no more than 20% of the available food (Eadie, 1970), and Paulsen (1960) indicates that on alpine ranges in the Rocky Mountains the propor-tion of herbage production removed by sheep was only 7%. This latter ﬁ gure is close to estimates of feeding efﬁ ciency obtained for large mammals in the wild: 10% for the Uganda kob (Buechner and Golley, 1967) and 9.6% for the African elephant (Wiegert and Evans, 1967). Where large, multi- species herds of ungulates occur, as on the savannas of Africa, their combined efﬁ ciency may be much higher: estimates of 60% for Uganda grassland and of 28% for Tanganyika grass-land were obtained from observations by Petrides and Swank (1965) and Lamprey (1964), respectively. At ordinary densities, small herbivores, both vertebrate and invertebrate, are much less efficient in their utilization of available food. Golley (1960) reports an efficiency of 1.6% for meadow voles ( Microtus ) in a Michigan grassland, and values of less than 0.5% are estimated for a variety of other small mammals and granivorous birds (Wiegert and Evans, 1967). Similar low efficiencies apparently characterize insect herbivores except when these are present in plague propor-tions. Wiegert (1965) found that grasshoppers (23 species of acridids and tettigoniids) consumed 1.3% of net plant pro-duction in a field of alfalfa, and Smalley (1960) obtained efficiency values of 1.6–2.0% for the meadow grasshopper Orchelimum in a Spartina salt marsh. Even if all invertebrate herbivores are considered together, their total consumption of net primary production seems rarely to exceed 10%; the following values appear to be representative: These values do not necessarily indicate the total damage done to the plant crop. For example, Andrzejewska et al. (1967) report that grasshoppers may destroy 4.8 times the amount of plant material they ingest, by gnawing the grass blades so that part of the leaf falls off. Such material does not enter the grazing food-chain, however, but drops to the ground and is consumed by detritus feeders. The nutritive value of most higher plants varies with age of the plant, season and environmental conditions, which also affect the palatability of the food. Few herbivores have the ability to digest cellulose without the assistance of symbiotic bacteria or protozoa, and much of the food that is eaten fails to be assimilated and is eliminated as feces. Assimilation therefore involves an important split in the flow of matter and energy through the ecosystem. High assimilation/consumption ratios can be achieved by herbivores which are selective feeders on concentrated foods such as nuts, seeds and fungi; assimilation efficien-cies of 85–95% have been recorded for such small mam-mals as Clethrionomys, Apodemus and Microtus (Drozdz, 1967; Davis and Golley, 1963, 81). Somewhat lower values are reported for large ruminants, ranging from 60–80% for Nature of ecosystem% net plant production used by invertebrates ReferencesBush-clover stand (S.C.)0.4–1.4 Menhinick, 1967Festuca grassland (Tenn.)9.6 Van Hook, 1971Spartina salt marsh (Ga.)7.0–9.2 Teal, 1962mesophytic woodland (Tenn.)1.5 Reichle and Crossley, 1967Mature deciduous forest (southern Canada)1.5–2.5 Bray, 1964C005_002_r03.indd 256C005_002_r03.indd 256 11/18/2005 10:20:40 AM11/18/2005 10:20:40 AM
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257sheep, cattle and deer down to 40% for moose and elephant (Graham, 1964; Dinesman, 1967; Davis and Golley, op. cit.). Interestingly, insect herbivores appear to be considerably less efficient feeders, reaching assimilation levels of 29% for the caterpillar Hyphantria, 27% for the grasshopper Orchelimum, 36% for the grasshopper Melanoplus, and 33% for the spittle-bug Philaenus (Gere, 1956; Smalley, 1960; Wiegert, 1964, 1965). Detritus feeders seem to have even lower efficiencies, as witness values of 20% for oribatid mites and 10% for the millipede Glomeris (Engelmann, 1961; Bocock, 1963). Thus, grazing food-chains and detritus food-chains appear to be characterized by quite different assimilation rates. Part of the energy assimilated by herbivores is stored as growth or production, while the rest is respired, dissipated as heat in the oxidation of organic matter. These respiratory losses may amount to as much as 98% of the energy assimi-lated (Petrusewicz and Macfadyen, 1970) but vary greatly, depending on such factors as environmental temperature, level of activity, and age of the individual. In mammals, there is a tendency for the weight-specific respiration rate to rise as body weight decreases, and this, in combination with the fact that growth and reproduction rates tend to be greater in small species than in large ones, apparently results in a rather constant low production/assimilation ratio of 1–2% for mammalian populations; values of this magnitude have been calculated for both elephants and mice (Wiegert and Evans, 1967). In contrast, herbivorous insect populations seem to show much higher assimilation efficiencies, on the order of 35–45% and, when only young life stages (larvae, nymphs) are involved, even of 50–60% (Macfadyen, 1967). Data are still too few to permit satisfactory generalizations, but the importance of further studies is evident when it is remembered that energy lost in respiration is irrevocably removed from the ecosystem and cannot be recycled. Synthesis of these process ratios leads to an evaluation of the overall efficiency with which herbivores convert the energy of primary (plant) production into their own tissue. The value of the production/consumption ratio for a fairly broad spectrum of vertebrate and invertebrate herbivores seems to range from less than 1% to 15–20% as a maximum (Petrusewicz and Macfadyen, 1970), with some evidence that the insects are generally more efficient than the non-domesticated mammals; this difference may be due in large part to the necessity for the latter to maintain a steady, high body temperature. When these values are considered along with the proportions of available food ingested (see above), the efficiency of energy transfer of herbivores is rarely found to exceed 15%, and this has been suggested as a likely maxi-mum level for natural ecosystems (Slobodkin, 1962). REGULATORY MECHANISMS OF PRIMARY CONSUMERS Although terrestrial herbivores are occasionally so abundant as to deplete the local supply of plant food, as sometimes happens when an introduced species like the Japanese beetle ( Popilia japonica ) or the Europena gypsy moth ( Porthetria dispar ) enters a new biotic community, such cases seem to be the exception rather than the rule, and, in contrast to many aquatic ecosystems, the bulk of net annual primary production on land is not eaten in the living state by primary consumers but dies and is acted on by decomposer organisms. Thus it appears that, on the whole, land herbivores usually occur at densities well below the level of their available plant food, and the reason or reasons for this are of great ecological interest. Despite the fact that sound generalizations about abstract concepts such as “trophic levels” are not easily arrived at (Murdoch, 1966), several hypotheses about the regulation of herbivore numbers have been suggested. The apparent rarity of obvious depletion of vegetation by herbivores, or of its destruction by meteorological catastrophes, has led Hairston, Smith and Slobodkin (1960) to the belief that herbivores are seldom limited by their food supply; after rejecting weather as an effective control agent, they conclude that herbivores are most often controlled by their predators and/or para-sites, interacting in the classical density-dependent manner. These views have been questioned on such grounds as (1) that much green material may often be inedible, unpalatable or even unreachable by the herbivores present, so that food limitation might occur without actual depletion (Murdoch, 1966) or (2) that native herbivores such as forest Lepidoptera and grasshoppers will often increase and cause serious defo-liation even in the presence of their predators (Ehrlich and Birch, 1967). Despite these and other criticisms, Slobodkin, Smith and Hairston (1967) find it unnecessary to modify their views in any essential respect. The possibility that herbivore (and other animal) popu-lations have some capacity for self-regulation has not been overlooked. Pimentel (1961, 1968) has suggested the concept of “ genetic feedback ,” according to which population density influences the intensity of selective pressure, selection influ-ences the genetic composition of the surviving individuals, and genetic composition influences the subsequent popula-tion density. The models thus far proposed to explain how this system works appear to be untenable (Lomnicki, 1971), but the general validity of the concept seems to gain support from the numerous instances of the co-evolution of plants and her-bivorous animals, e.g., the association between certain orchids and their insect pollinators (Van der Pijl and Dodson, 1966), or that between certain species of Acacia and ants (Janzen, 1966), which have been interpreted as involving reciprocal selective interaction. Another suggested mechanism for self-regulation involves the elaboration of social behavior patterns, e.g., territoriality, social hierarchies, and warning displays, which tend to main-tain animal populations at relatively low densities, thereby reducing the possibility of depletion of food supplies or other resources (Wynne Edwards, 1962, 1965). The evolution of such a mechanism seems to require that natural selection act on groups rather than on individual organisms, and for this reason Wynne Edwards’ hypothesis has been heavily criti-cized (Williams, 1966; Maynard Smith, 1964). The basic importance of self-regulatory mechanisms and other density-dependent interactions in limiting popula-tions of animals has also been questioned. Ehrlich and Birch C005_002_r03.indd 257C005_002_r03.indd 257 11/18/2005 10:20:40 AM11/18/2005 10:20:40 AM
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(1967), for example, deny that “the numbers of all populations are primarily determined by density-regulating factors” and emphasize the significant limiting effects that weather con-ditions often have on natural populations of herbivores. This view maintains that limiting factors which act on populations without relation to their density commonly maintain such populations at levels where self-regulatory, density-dependent mechanisms, if they exist at all, are not called into play. Wiegert and Owen (1971) suggest that the precise kinds of mechanisms limiting the population of a particular spe-cies of herbivore will depend firstly on (a) whether the pop-ulation, in making use of its plant food resources, directly affects the rate of food supply (either by destroying or by stimulating its capacity to produce new plant tissue), and secondly on (b) the life history characteristics of both the herbivore and its plant food. Most stable terrestrial ecosys-tems are dominated by relatively large, structurally complex plants that are long-lived, slow-growing and have low rates of population increase, and by herbivores that are smaller, more numerous and faster-growing than their food plants. In con-trast, open-water aquatic communities are often composed of small and structurally simple plants (phytoplankton) that are more abundant and multiply more rapidly than their con-sumers. In such aquatic communities, herbivores can attain high rates of consumption—approaching 100%—of net pri-mary production without danger of completely exhausting their nutritional resources, and their populations are likely to be limited by direct competition to levels dictated by the rate at which food is supplied to them. As already noted above, in terrestrial ecosystems, such high rates of consumption by herbivores are rare, and when they occur their influence on the food plants is usually severe. In forest, and to a lesser extent in grassland, communities the herbivores are less likely to be limited by direct competition for food; they tend, rather, to be subject to the effects of predation and parasitism and to have evolved behavioral patterns such as migrations which result in reducing grazing pressure. At the same time, the plants of these systems have developed an array of toxic compounds, unpalatable tissues, thorns and other protective devices to resist grazing. Thus Wiegert and Owen’s model of population control stresses the importance of trophic struc-ture (grazing food-chains vs. detritus food-chains) and the biological properties of the interacting populations. MANAGEMENT OF PRIMARY CONSUMERS Although energy ﬂ ow studies suggest that man should become predominantly herbivorous in order to make the most effective use of the solar radiation captured by plants, his need for protein, which he can obtain in more concentrated form from animal tissue, is likely to continue to motivate him toward increasing the production of primary consum-ers such as cattle, chickens, and ﬁ sh. In fact, the majority of man’s domesticated animals are herbivores. It is clear that if the conversion of solar energy into animal protein for human use is to be maximized herbivores with high growth efficiencies should be cropped. The energy that man himself must spend in order to secure his food must also be assessed, and the time, effort and other costs of cropping have therefore to be taken into consideration. Over the several thousand years in which man has attempted to domesticate plants and animals, his selective efforts have been remarkably successful in developing efficient herbivores for his own use. Modern methods of animal husbandry, where animals are kept in specially constructed buildings and are fed specially processed foods, have greatly increased the amount of food energy reaching the animals (Phillipson, 1966). However, these methods require the expenditure of much energy that goes as hidden cost, in the activities involved in the construc-tion and maintenance of facilities and in the production of the processed food. Are there ways of increasing the more direct conversion of plant tissue to animal protein useful to man? Macfadyen (1964) has indicated that in Great Britain beef cattle raised on grassland commonly consume only one-seventh of the net annual primary production, the rest going to other herbivores and to decomposers. More care in stocking and better management of range are two of the principal ways in which possible improvement can be sought. Less obvious is the extension of husbandry to other herbivorous species, perhaps even to insects, that may be more efficient feeders than the large warmblooded mammals and that might, through selection, be developed as a productive source of high protein nourishment. The use of faster-growing herbivores, such as rabbits, would yield greater efficiency in terms of meat production per unit of time (Phillipson, 1966). In East Africa, increasing use for human food is being made of antelope, zebra and other native ungulates which are relatively immune to the parasites and dis-eases that afflict European cattle, and other unproductive areas are being managed for their propagation. Increased production of plant food as a base for greater herbivore production is also possible. In this effort, man has tried particularly to exploit tropical regions with their higher average temperatures and longer growing seasons. Petrusewicz and Macfadyen (1970) point out, however, that primary productivity does not apparently increase propor-tionately to the increased light regime of tropical climates, largely due to higher respiration rates in tropical plants and to more rapid rates of decomposition; the tenfold increase in solar radiation experienced by some tropical forests seems not to result in any significant increase in energy available to primary consumers over that found in a temperate deciduous forest. It has long been apparent that agricultural practices developed in temperate regions are poorly adapted for use in the tropics. A great deal more needs to be learned about trop-ical ecology before marked improvement in environmental management can be expected (Owen, 1966). CONCLUDING REMARKS This article has attempted to outline the current view of nature as an interaction system in which organisms play a variety of roles in facilitating the circulation of matter and the ﬂ ow of energy within that thin layer of the earth’s surface known as the biosphere. Despite the relatively small quantities of C005_002_r03.indd 258C005_002_r03.indd 258 11/18/2005 10:20:40 AM11/18/2005 10:20:40 AM
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259 material and energy that are channeled through primary consumers, as compared with autotrophic plants and decom-poser organisms, the former play a vital role in the maintenance of such a system, in that they accumulate and concentrate inor-ganic nutrients such as nitrogen, phosphorus and potassium, as well as many trace elements, which are otherwise thinly dispersed in the environment. It has not been possible here to present more than a superﬁ cial account of the role of primary consumers, and indeed the study of these animals is still in its early stages. But further observation and experimentation in this aspect of ecology are clearly needed, if man is ever to understand and manage wisely the world he lives in. REFERENCES Adams, L. (ed.), Bio. Science, 15, 83–157, 1965. Andrzejewska, L., A. Breymeyer, A. Kajak, and Z. Wojcik, pp. 477–495 in Secondary Productivity of Terrestrial Ecosystems, edited by K. Petruse-wicz, Institute of Ecology, Warsaw, 1967. Bocock, K.L., pp. 85–91 in Soil Organisms, edited by J. Doeksen and J. Van der Drift, Amsterdam, 1963. Bornebusch, C.H., Forstl. Forsogskomission (Copenhagen), 11, 1–224, 1930. Bray, J.R., Ecology, 45, 165–167, 1964. Brues, C.T., Insect Dietary: An Account of the Food Habits of Insects, Har-vard University Press, Cambridge, 1946. Buechner, H.K., and F.B. Golley, pp. 243–254 in Secondary Productivity of Terrestrial Ecosystems, edited by K. Petrusewicz, Institute of Ecology, Warsaw, 1967. Cragg, J.B., J. Anim. Ecol., 30, 205–234, 1961. Davis, D.E., and F.B. Golley, Principles of Mammalogy, Reinhold Publish-ing Corp., New York, 1963. Dinesman, L.G., pp. 261–266 in Secondary Productivity of Terrestrial Eco-systems, edited by K. Petrusewicz, Institute of Ecology, Warsaw, 1967. Drozdz, A., pp. 323–330 in Secondary Productivity of Terrestrial Ecosys-tems, edited by K. Petrusewicz, Institute of Ecology, Warsaw, 1976. Eadie, J., pp. 7–24 in Animal Populations in Relation to Their Food Resources, edited by A. Watson, Blackwell Scientific Publications, Oxford, 1970. Ehrlich, P.R., and L.C. Birch, Amer. Nat., 101, 97–107, 1967. Elton, C., Animal Ecology, Sidgwick and Jackson Ltd., London, 1927. Elton, C.S., The Pattern of Animal Communities, Methuen and Co. Ltd., London, 1966. Engelmann, M.D., Ecol. Monogr. , 31, 221–228, 1961. Evans, F. C., and U.N. Lanham, Science, 131, 1531–1532, 1960. Evans, F.C., and W.W. Murdoch, J. Anim. Ecol., 37, 259–273, 1968. Gere, G., Acta Biol. Acad. Sci. Hungary, 7, 43–72, 1956. Gliwicz, J., R. Andrzejewski, G. Bujalska, and K. Petrusewicz, Acta The-riol., 13, 401–413, 1968. Golley, F.B., Ecol. Monogr., 30, 187–206, 1960. Golley, F.B. Ecology, 42, 581–584, 1961. Graham, N.M., Austral. J. Agric. Res., 15, 969–973, 1964. Hairston, N.G., F.E. Smith, and L.B. Slobodkin, Amer. Nat., 94, 421–425, 1960. Hutton, M., The Role of Wildlife Species in the Assessment of Biological Impact from Chronic Exposure to Persistent Chemicals, Ecotoxicol. Environ. Saf., 6, 471, 1982. Janzen, D.H., Evolution, 20, 249–275, 1966. Lamprey, H.F., East African Wildlife J., 2, 1–46, 1964. Legge, A.H. and S. V. Krupa, Air Pollutants and Their Effects on the Ter-restrial Ecosystem, John Wiley, New York, 1986. Lewis, J.K., G.M. Van Dyne., L.R. Albee, and F.W. Whetzal, Bull. S. Dak. Agric. Expt. Sta., 459, 44, 1956. Lindemann, R.L., Ecology, 23, 399–418, 1942. Lomnicki, A., Amer. Nat., 105, 413–421, 1971. Macfadyen, A., Ann. Appl. Biol., 49, 216–219, 1961. Macfadyen, A., pp. 3–20 in Grazing in Terrestrial and Marine Environments, edited by DJ. Crisp, Blackwell Scientific Publications, Oxford, 1964. Macfadyen, A., pp. 383–412 in Secondary Productivity of Terrestrial Eco-systems, edited by K. Petrusewicz, Institute of Ecology, Warsaw, 1967. Maynard Smith, J., Nature, 201, 1145–1147, 1964. Menhinick, E.F., Ecol. Monogr., 37, 255–272, 1967. Murdoch, W.W., Amer. Nat., 100, 219–226, 1966. Nakamura, K., Y. Ito, M. Nakamura, T. Matsumoto, and K. Hayakawa, Oekologia, 7, 1–15, 1971. Odum, H.T., and R.F. Pigeon, (eds.), A Tropical Rain Forest, US Atomic Energy Commission, Washington, DC, 1970. Owen, D.F., Animal Ecology in Tropical Africa, Oliver and Boyd, Edinburgh, 1966. Pandy, S.D., V. Misra, and P.M. Viswanathan, Environmental Pollutants and Wildlife—A Survey, Int. J. Env. Studies, 28, ⅔, 1986. Parker, G.R., Occas. Pap. No. 10, Canad. Wildlife Serivces, Ottawa, 1971. Paulsen, H.A., Jr., Iowa State J. Sci., 34, 731–748, 1960. Petrides, G.A., and W.G. Swank, Proc. Ninth Internat. Grasslands Con-gress, San Paulo, Brazil, 1965. Petrusewicz, K., and A. Macfadyen, Productivity of Terrestrial Animals: Principles and Methods, Internat. Biol. Program Handbook No. 13, Burgess and Son, Abingdon, Berkshire, England, 1970. Phillipson, J., Oikos, 15, 130–139, 1964. Phillipson, J., Ecological Energetics, St. Martin’s Press, New York, 1966. Pimentel, D., Amer. Nat., 95, 65–79, 1961. Pimentel, D., Science, 159, 1432–1437, 1968. Rasmussen, L., Ecotoxicology, Swedish Research Council, Stockholm, 1984. Reichle, D.E., and D.A. Crossley, Jr., pp. 563–587 in Secondary Produc-tivity of Terrestrial Ecosystems, edited by K. Petrusewicz, Institute of Ecology, Warsaw, 1967. Slobodkin, L.B., Adv. Ecol. Res., 1, 69–101, 1962. Slobodkin, L.B., F.E. Smith, and N.G. Hairston, Amer. Nat., 101, 109–124, 1967. Smalley, A.E., Ecology, 41, 672–677, 1960. Smith, A.H., and J.R. Douglas, Ann. Ent. Soc. Amer., 42, 14–18, 1949. Southwood, T.R.E., Ecological Methods, Methuen and Co. Ltd., London, 1966. Teal, J.M., Ecology, 43, 614–624, 1962. Thienemann, A., Verh. deutsch. Zool. Ges., 31, 29–79, 1926. Umbreit, W.R., R.H. Burris, and J.F. Stauffer, Manometric Techniques, 3rd ed., Burgess, Publishing Co., Minneapolis, 1957. Van der Drift, J., Tijdschr. Ent., 94, 1–68, 1951. Van der Pijl, L., and C.H. Dodson, Orchid Flowers: Their Pollination and Evolution, University of Miami Press, Coral Gables, Florida, 1966. Van Hook, R.I., Jr. , Ecol. Monogr., 41, 1–26, 1971. Walker, C.H., Pesticides and Birds; Mechanisms of Selective Toxicity, Agricultrue, Ecosystems and Environment, 9, 211, 1983. Wiegert, R. G., Ecol. Monogr., 34, 225–241, 1964. Wiegert, R.G., Oikos, 16, 171–176, 1965. Wiegert, R.G., Energy Transfer in Ecological Systems, Patterns of Life Series, Rand McNally and Company, Chicago, 1970. Wiegert, R.G., and F.C. Evans, pp. 499–518 in Secondary Productivity of Terrestrial Ecosystems, edited by K. Petrusewicz, Institute of Ecology, Warsaw, 1967. Wiegert, R.G., and D.F. Owen, J. Theoret. Biol., 30, 69–91, 1971. Williams, E.C., Jr., and D.E. Reichle, Oikos, 19, 10–18, 1968. Williams, G.C., Adaptation and Natural Selection, Chapter 4, Princeton University Press, Princeton, NJ, 1966. Wynne Edwards, V.C., Animals Dispersion in Relation to Social Behaviour, Oliver and Boyd, Edinburgh, 1962. Wynne Edwards, V.C., Science, 147, 1543–1548, 1965. FRANCIS C. EVANS University of Michigan ECOLOGY RADIATION: see RADIATION ECOLOGYC005_002_r03.indd 259C005_002_r03.indd 259 11/18/2005 10:20:40 AM11/18/2005 10:20:40 AM
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THEORY The concept of the ecosystem is not only the center of professional ecology today, but it is also the most relevant concept in terms of man’s environmental problems. During the mid 70s, the public seized on the root meaning of ecology, namely “oikos” or “house,” to broaden the subject beyond its previously rather narrow academic confines to include the “totality of man and environment,” or the whole environ-mental house, as it were. We are witnessing what is called a historic “attitude revolution” (Odum, 1969, 1970c) in the way people look at their environment for the very simple reason that for the first time in his short history man is faced with ultimate rather than merely local limitations. It will be well for all of us to keep this overriding simplicity in mind as we face the controversies, false starts and backlashes that are bound to accompany man’s attempts to put some negative feedback into the vicious spiral of uncontrolled growth and resource exploitation that has characterized the past several decades. Man has been interested in ecology in a practical sort of way since early in his history. In primitive society every individual, to survive, needed to have definite knowledge of his environment, i.e., of the forces of nature and of the plants and animals around him. Civilization, in fact, began when man learned to use fire and other tools to modify his environment. It is even more necessary than ever for man-kind, as a whole, to have an intelligent knowledge of the environment if our complex civilization is to survive, since the basic “laws of nature” have not been repealed; only their complexion and quantitative relations have changed, as the world’s human population has increased and as man’s power to alter the environment has expanded. Like all phases of learning, the science of ecology has had a gradual, if spasmodic, development during recorded history. The writings of Hippocrates, Aristotle, and other philosophers of the Greek period contain material which is clearly ecological in nature. However, the Greeks literally did not have a word for it. The word “ecology” is of recent coinage, having been first proposed by the German biologist Ernst Haeckel in 1869. Before this, many of the great men of the biological renaissance of the eighteenth and nineteenth centuries had contributed to the subject even though the label “ecology” was not in use. For example, Anton von Leeuwenhoek, best known as a pioneer microscopist of the early 1700s, also pioneered the study of “food chains” and “population regulation” (see Egerton, 1968), two impor-tant areas of modern ecology. As a recognized distinct field of biology, the science of ecology dates from about 1900, and only in the past decade has the word become part of the general vocabulary. Today, everyone is acutely aware of the environmental sciences as indispensable tools for cre-ating and maintaining the quality of human civilization. Consequently, ecology is rapidly becoming the branch of science that is most relevant to the everyday life of every man, woman, and child. As recently as 1960 the theory of the ecosystem was rather well understood but not in any way applied. The applied ecology of the 1960s consisted of managing compo-nents as more or less independent units. Thus we had forest management, wildlife management, water management, soil conservation, pest control, etc., but no ecosystem man-agement and no applied human ecology. Practice has now caught up with theory. Controlled management of the human population together with the resources and the life support system on which it depends as a single, integrated unit now becomes the greatest, and certainly the most difficult, chal-lenge ever faced by human society. As we have seen, an “anthropocentric” definition of the ecosystem might read something as follows: Man as a part of, not apart from, a life support system composed of the atmosphere, water, minerals, soil, plants, animals, and microorganisms that function together to keep the whole viable. Any unit including all of the organisms (i.e., the “com-munity”) in a given area interacting with the physical envi-ronment so that a flow of energy leads to a clearly defined trophic structure, biotic diversity and material cycles (i.e. exchange of materials between living and nonliving parts) within the system is an ecological system or ecosystem. The following formal definition is the one used in the third edition of Fundamentals of Ecology (E.P. Odum, 1971) The word ecology is derived from the Greek oikos, meaning “house” or “place to live.” Literally, ecology is the study of organisms “at home.” Usually ecology is defined as the study of the relation of organisms or groups of organisms to their environment, or the science of the interrelations between living organisms and their environment. Because ecology is con-cerned especially with the biology of groups of organisms and with functional processes on the lands, in the oceans, and in fresh waters, it is more in keeping with the modern emphasis to define ecology as the study of the structure and function of nature, it being understood that mankind is a part of nature. C005_003_r03.indd 260C005_003_r03.indd 260 11/18/2005 10:21:02 AM11/18/2005 10:21:02 AM

THEORY 261One of the definitions in Webster’s Unabridged Dictionary seems especially appropriate for the closing decades of the 20th century, namely, “ the totality or pattern of relations between organisms and their environment. ” In the long run the best defi-nition for a broad subject field is probably the shortest and least technical one, as for example, “environmental biology.” To understand the scope and relevance of ecology, the subject must be considered in relation to other branches of biology and to “ ologies ” in general. In the present age of specialization in human endeavors, the inevitable connec-tions between different fields are often obscured by the large masses of knowledge with the fields (and sometimes also, it must be admitted, by stereotyped college courses). At the other extreme, almost any field of learning may be so broadly defined as to take in an enormous range of subject material. Therefore, recognized “fields” need to have recog-nized bounds, even if these bounds are somewhat arbitrary and subject to shifting from time to time. A shift in scope has been especially noteworthy in the case of ecology as gen-eral public awareness of the subject has increased. To many, “ecology” now stands for “the totality of man and environ-ment.” But first let us examine the more traditional academic position of ecology in the family of sciences. For the moment, let us look at the divisions of biology, “the science of life.” Morphology, physiology, genetics, ecol-ogy, evolution, molecular biology, and developmental biol-ogy are examples of such divisions. We may also divide the subject into what may be called “taxonomic” divisions, which deal with the morphology, physiology, ecology, etc., of spe-cific kinds of organisms. Zoology, botany, and bacteriology, are large divisions of this type, and phycology, protozo-ology, mycology, entomology, ornithology, etc., are divisions deal-ing with more limited groups of organisms. Thus ecology is a basic division of biology and, as such, as also an integral part of any and all of the taxonomic divisions. Both approaches are profitable. It is often very productive to restrict work to cer-tain taxonomic groups, because different kinds of organisms require different methods of study (one cannot study eagles by the same methods used to study bacteria) and because some groups of organisms are economically or otherwise much more important or interesting to man than others. Ultimately, however, unifying principles must be delimited and tested if the subject field is to qualify as “basic.” Perhaps the best way to delimit modern ecology is to consider it in terms of the concept of levels of organization visualized as a sort of “biological spectrum.” Community, population, organization, organ cell, and gene are widely used terms for several major biotic levels. Interaction with the physical environment (energy and matter) at each level producing characteristic functional systems. By a system we mean just what Webster’s College Dictionary defines as “regularly interacting and interde-pendent components forming a unified whole.” Systems containing living components (biological systems or biosys-tems) may be conceived at any level in the hierarchy. For example, we might consider not only gene systems, organ systems, and so on, but also host-parasite systems as inter-mediate levels between population and community. HISTORICAL REVIEW OF THE
CONCEPT Although the term ecosystem was first proposed by the British ecologist, A.G. Tansley in 1935, the concept is by no means so recent. Allusions to the idea of the unity of organ-isms and environment (as well as the oneness of man and nature) can be found as far back in written history as one might care to look, and such an idea has been a basic part of many religions (less so in Christian religions as recently pointed out by historian Lynn White, 1967). Anthropologists and geographers have long been concerned with the impact of man on his environment and early debated the question: To what extent has man’s continuing trouble with deteriorated environments stemmed from the fact that human culture tends to develop independently of the natural environment? The Vermont prophet George Perkins Marsh wrote a classic treatise on this theme in 1864. He analyzed the causes of the decline of ancient civilizations and forecast a similar doom for modern ones unless “man takes what we would call today an ecosystematic” view of man and nature. In the late 1800s biologists began to write essays on the unity of nature, inter-esting enough in a parallel manner in German, English, and Russian languages. Thus Karl Möbius in 1877 wrote about the community of organisms in an oyster reef as a “biocoe-nosis,” while in 1887 the American S. A. Forbes wrote his classic essay on “The Lake as a Microcosm.” The Russian pioneering ecologist V. V. Dokuchaev (1846–1903) and his disciple G. F. Morozov (who specialized in forest ecology) placed great emphasis on the concept of “biocoenosis,” a term later expanded to geobiocoenosis (or biogeocoenosis) (see Sukachev, 1944), which can be considered a synonym of the word “ecosystem.” No one has expressed the relevance of the ecosystem concept to man better than Aldo Leopold in his essays on the land ethic. In 1933 he wrote: “Christianity tries to inte-grate the individual to society, Democracy to integrate social organization to the individuals. There is yet no ethic dealing with man’s relation to the land” … which is “still strictly economic entailing privileges but not obligations.” Thus, man is continually striving, with but partial success so far, to establish ethical relationships between man and man, man and government, and, now, man and environment. Without the latter what little progress has been made with the other two ethics will surely be lost. In the context of the 1970 scene Garrett Harden (1968) says it in another way when he points out that technology alone will not solve the popula-tion and pollution dilemmas; ethical and legal constraints are also necessary. Environmental science is now being called upon to help determine a realistic level of human popula-tion density and rate of use of resources and power that are optimum in terms of the quality of human life, in order that “societal feedback” can be applied before there are serious overshoots. This requires diligent study of ecosystems, and, ultimately, a judgment on the carrying capacity of the bio-sphere. If studies of natural populations have any bearing on the problem, we can be quite certain that the optimum density in terms of the individual’s options for liberty and C005_003_r03.indd 261C005_003_r03.indd 261 11/18/2005 10:21:02 AM11/18/2005 10:21:02 AM
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THEORYthe pursuit of happiness is something less than the maximum number that can be sustained at a subsistence level, as so many domestic “animals” in a polluted feed lot! My advanced ecology class recently attempted to deter-mine what might be the optimum population for the State of Georgia on the assumption that someday the state would have to have a balanced resource input-output (i.e., live within its own resources). On the basis of a per capita approach to land use the tentative conclusion was that a density of one person per five acres (2 hectares) represented the upper limit for an optimum population size when the space requirements for quality (i.e., high protein) food production, domestic ani-mals, outdoor recreation, waste treatment and pollution-free living space were all fully considered. Anything less than five acres of live support and resource space per capita, it was concluded, would result in a reduction in the individual person’s options for freedom and the pursuit of happiness, and, accordingly, a rapid loss in environmental quality. Since the 1970 per capita density of Georgia is 1 in 8 acres and for the United States as a whole, 1 in 10 acres no more than double the present US population could be considered opti-mum according to this type of analysis. This would mean that we have about 30 years to level off population growth. The study also suggested that permanent zoning of at least one-third of land and freshwater areas (plus all estuarine and marine zones) as “open space” in urbanizing areas would go a long way toward preventing overpopulation, overdevelop-ment and social decay that is now so evident in many parts of the world today. The results of this preliminary study have been published (E. P. Odum, 1970b). See also the pro-vocative symposium on The Optimum Population for Britain edited by Taylor (1970). THE TWO APPROACHES TO
STUDY G. Evelyn Hutchinson in his 1964 essay, “The Lacustrine Microcosm Reconsidered,” contrasts the two long-standing ways ecologists attempt to study lakes or other large eco-systems of the real world. Hutchinson cites E. A. Birge’s (1915) work on heat budgets of lakes as pioneering the holo-logical (from holos ⫽ whole) or holistic approach in which the whole ecosystem is treated as a “black box” (i.e., a unit whose function may be evaluated without specifying the internal contents) with emphasis on inputs and outputs, and he contrasts this with the merological (from meros ⫽ part) approach of Forbes in which “we discourse on parts of the system and try to build up the whole from them.” Each pro-cedure has obvious advantages and disadvantages and each leads to different kinds of application in terms of solving problems. Unfortunately, there is something of a “credibility gap” between the two approaches. As would be expected, the merological approach has dominated the thinking of the biologist-ecologist who is species-oriented, while the physicist-ecologist and engineer prefer the “black box” approach. Most of all, man’s environmental crisis has speeded up the application of systems analysis to ecology. The for-malized, or mathematical model, approach to populations, communities, and ecosystems has come to be known as systems ecology which is rapidly becoming a major science in its own right for two reasons: (1) extremely powerful new formal tools are now available in terms of mathematical theory, cybernetics, electronic data processing, etc. (2) Formal sim-plication of complex ecosystems provides the best hope for solutions of man’s environmental problems that can no longer be trusted to trial-and-error, or one-problem one- solution pro-cedures that have been chiefly relied on in the past. Again we see the contrast between merological and holological approaches in that there are systems ecologists who start at the population or other component level and “model up,” and those who start with the whole and “model down.” The same dichotomy is evident in the very rewarding studies of experimental laboratory ecosystems. One class of microecosystems can be called “derived” systems because they are established by multiple seeding from nature in contrast to “defined” microcosms which are built up from previously isolated pure cultures. Theoretically, at least, the approaches are applicable to efforts to devise life support systems for space travel. In fact, one of the best ways to visualize the ecosystems concept for students and laymen is to consider space travel, because when man leaves the bio-sphere he must take with him a sharply delimited enclosed environment that will supply all vital needs with sun energy as the only usable input from the surrounding very hostile space environment. For journeys of a few weeks (such as to the moon and back), man does not need a regenerative ecosystem, since sufficient oxygen and food can be stored while CO 2 and other waste products can be fixed or detoxi-fied for short periods of time. For long journeys man must engineer himself into a complete ecosystem that includes the means of recycling materials and balancing production, con-sumption and decomposition by biotic components or their mechanical substitutes. In a very real sense the problems of man’s survival in an artificial space craft are the same as the problems involved in his continued survival on the earth space ship. For example, detection and control of air, and water pollution, adequate quantity and nutritional quality of food, what to do with accumulated toxic wastes and gar-bage, and the social problems created by reduced living space are common concerns of cities and spacecrafts. For this reason the ecologist would urge that national and inter-national space programs now turn their attention to the study and monitoring of our spaceship earth. As was the case with Apollo 13, survival becomes the mission when the limits of carrying capacity are approached. THE COMPONENTS OF THE
From the standpoint of trophic energy an ecosystem has two components which are usually partially separated in space and time, namely, an autotrophic component (autotrophic ⫽ self nourishing) in which fixation of light energy, use of simple inorganic substances, and the buildup of complex sub-stances predominate; and secondly, a heterotrophic compo-nent (heterotrophic ⫽ other-nourshing) in which utilization, C005_003_r03.indd 262C005_003_r03.indd 262 11/18/2005 10:21:02 AM11/18/2005 10:21:02 AM

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THEORY 263rearrangement and decomposition of complex materials predominate. As viewed from the side (cross section) eco-systems consist of an upper “green belt” which receives incoming solar energy and overlaps, or interdigitates, with a lower “brown belt” where organic matter accumulates and decomposes in soils and sediments. It is convenient for the purposes of first order analysis and modelling to recognize six structural components and six processes as comprising the ecosystem as follows: A. Components 1) Inorganic substances (C, N, CO 2 , H 2 O, etc.) involved in material cycles. 2) Organic compounds (proteins, carbohydrates, lipids, humic substances, etc.) that link biotic and abiotic. 3) Climate regime (temperature, rainfall, etc.). 4) Autotrophs or producers, largely green plants able to manufacture food from simple substances. 5) Phagotrophs (phago ⫽ to eat) or macro-consumers, heterotrophic organisms, largely animals which ingest other organisms or particulate organic matter. 6) Saprotrophs (sapro ⫽ to decompose) or micro-consumers (also called osmotrophs), heterotro-phic organisms, chiefly bacterial, fungi, and some protozoa that break down complex compounds, absorb some of the decomposition products and release inorganic substances usable by the auto-trophs together with organic residues which may provide energy sources or which may be inhibitory, stimulatory or regulatory to other biotic compo-nents of the ecosystem. (Another useful division for heterotrophs: biophages ⫽ organisms that feed on other living organisms; saprophages ⫽ organisms that feed on dead organic matter.) B. Processes 1) Energy flow circuits. 2) Foods chains (trophic relationships). 3) Diversity patterns in time and space. 4) Nutrient (biogeochemical) cycles. 5) Development and Evolution. 6) Control (cybernetics). Subdivision of the ecosystem into these six “compo-nents” and six “processes,” as with most classification, is arbitrary but convenient, since the former emphasize struc-ture and the latter function. From the holistic viewpoint, of course, components are operationally inseparable. While different methods are often required to delineate structure on the one hand and to measure rates of function on the other, the ultimate goal of study at any level or organiza-tion is to understand the relationship between structure and function. It is not feasible to go into any detailed discussion of these component-processes in this brief introduction (one can refer to textbooks and review papers), but we can list a few key principles that are especially relevant to human ecology. Figure 1 is a schematic diagram that may be useful in picturing the basic arrangement and functional linkage of ecosystem components. 1) The living (items A4–6 above) and non-living (items A1–3) parts of ecosystems are so inter-woven into the fabric of nature that it is difficult to separate them, hence operational classifica-tions (B1–6) do not make a sharp distinction between biotic and abiotic. Elements and com-pounds are in a constant state of flux between living and non-living states. There are very few substances that are confined to one or the other state. Exceptions may be ATP which is found only inside living cells, and humic substances (resistant end-products of decomposition) which are not found inside cells yet are characteristic of all ecosystems. 2) The time-space separation of autotrophic and het-erotrophic activity leads to a convenient classifi-cation of energy circuits into (1) a grazing food chain (where term grazing refers to direct con-sumption of living plants or plant parts) and (2) an organic detritus (from deterere = to wear away) food chain which involves the accumulation and decomposition of dead materials. To build up a stable biomass structure there must be negative feedback control of grazing—a need too often neglected in man’s domesticated ecosystems. 3) As in well known available energy declines with each step in the food chain (so a system can sup-port more herbivores than carnivores; if man wants to keep his meat-eating option open there will have to be fewer people supported by a given food base). On the other hand, materials often become concentrated with each step in the food chains. Failure to anticipate possible “biologi-cal magnification” of pollutants, such as DDT or long-lived radionuclides, is causing serious prob-lems in man’s environment. 4) It is becoming increasingly evident that high bio-logical productivity (in terms of calories per unit area) in both natural and agricultural ecosystems is almost always achieved with the aid of energy subsidies from outside the system that reduces the cost of maintenance (thus diverting more energy to production). Energy subsidies take the form of wind and rain in a rain forest, tidal energy in an estuary (see Figure 1), or fuel, animal or human work energy used in the cultivation of a crop. In comparing productivity of different systems it is important to consider the complete budget —not just light input. 5) Likewise it is increasingly evident that both har-vest and pollution are stresses which reduce the C005_003_r03.indd 263C005_003_r03.indd 263 11/18/2005 10:21:02 AM11/18/2005 10:21:02 AM
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THEORYenergy available for self-maintenance. Man must be aware that he will have to pay the costs of added anti-thermal maintenance, or “disorder pumpout,” as H. T. Odum, 1971 (Chapter 5) calls it. It is dan-gerous strategy to try to force too much produc-tivity, or yield, from the landscape (as is being attempted in the so-called “green revolution”) because very serious “ecological back-lashes” can result from the following (1) pollution caused by heavy use of fertilizers and insecticides and the consumption of fossil fuels, (2) unstable or oscil-lating conditions created by one-crop systems, (3) vulnerability of plants to disease because their self-protection mechanisms have been “selected out,” in favor of yield, and (4) social disorder cre-ated by rapid shift of rural people to cities that C. ENERGY FLOWORGANICMATTERLIGHTCIRCULATION WORKA. VERTICAL ZONESENERGY TRANSFERWORK GATEB. MINERAL CYCLEBOTTOMCONSUMERSPOOL OFNUTRIENTSFOR PLANTGROWTHEXPORTEXPORTEXPORTIMPORTSEDIMENT SUBSYSTEM WORMS, CLAMS, BACTERIA, ETC.IMPORTIMPORTPLANTCELLORGANIC STORAGECONSUMERSCONSUMERSPRODUCERSEUPHOTIC(AUTOTROPHIC)SUBSYSTEMWATER COLUMN SUBSYSTEMZOOPLANKTON ANDSWIMMING CONSUMERS(FISH, ETC.)SUN LIGHTRRRPRRECYCLINGPRODUCERSCONSUM-ERSTIDE&WAVES FIGURE 1 Three aspects of the structure and function of ecosystems as illustrated by an estuarine system. A. Vertical zonation with photosynthetic production above (autotrophic stratum) and most of the respiration and decomposition below (heterotrophic stratum). B. Material cycle with circulation of plant nutrients upward and organic matter food downward. C. Energy flow circuit diagram showing three sources of energy input into the system. The bullet-shaped modules represent producers with their double metabolism, that is P (production) and R (respiration). The hexagons are populations of consumers which have storage, self maintenance and reproduction. The storage bins represent nutrient pools in and out of which move nitrogen, phosphorous and other vital substances. In diagrams B and C the lines represent the “invisible wires of nature” that link the components into a functional network. In diagram C the “ground” symbols (i.e., arrow into the heat sink) indicate where energy is dispersed and no longer available in the food chain. The circles represent energy inputs. The work gate symbols (large X) indicate where a flow of work energy along one pathway assists a second flow to pass over energy barriers. Note that some of the lines of flow loop back from “downstream” energy sources to “upstream” inflows serving various roles there including control functions (saprotrophs controlling photosynthesis by controlling the rate of mineral regeneration, for example). The diagram © also shows how auxil-liary energy of the tide (energy subsidy) assists in (1) recycling of nutrients from consumer to producer and (2) speeding up the movement of plant food to the consumer. Reducing tidal flow by dyking the estuary will reduce the productivity just as surely as cutting out some of the light. Stress such as pollution or harvest can be shown in such circuit models by adding circles enclosing negative signs linked with appropri-ate heat sinks to show where energy is diverted away from the ecosystem. Both subsidies (⫹) and stress (⫺) can be quantitated in terms of Calories added or diverted per unit of time and space. (From E.P. Odum, 1971 after H.T. Odum, and B.J. Copeland, 1969.) C005_003_r03.indd 264C005_003_r03.indd 264 11/18/2005 10:21:02 AM11/18/2005 10:21:02 AM

THEORY 265are not prepared to house or employ them (the tragedy here is that industrialized agriculture can result in increased food per acre but it can also widen the gap between rich and poor so that there are increasing numbers of people unable to buy the food!). 6) While we generally think of production and decom-position as being balanced on the biosphere as a whole, the truth is that this balance has never been exact but has fluctuated from time to time in geological history. Through the long haul of evo-lutional history production has slightly exceeded decomposition so that a highly oxygenic atmo-sphere has replaced the original reducing atmo-sphere of the earth. Man, of course, is tending to reverse this trend by increasing, decomposition (burning of fuels, etc.) at the expense of produc-tion. The most immediate problem is created by the increase in atmospheric CO 2 since relatively small changes in concentration can have large effects on the heat budget of the earth. 7) Ecological studies indicate that diversity is directly correlated with stability and perhaps inversely correlated with productivity, at least in many situ-ations. It could well be that the preservation of diversity in the ecosystem is important for man since variety may be a necessity, not just the spice of life! 8) At the population level it is now clear that the growth form of the human population will not conform to the simple sigmoid or logistic model since there will always be a long time lag in the effects of crowding, pollution and overexploita-tion of resources. Growth will not “automatically” level off as do populations of yeasts in a confined vessel where individuals are immediately affected by their waste products. Instead, the human popu-lation will clearly overshoot some vital resource, unless man can “anticipate” the effects of over-population and reduce growth rates before the deleterious effects of crowding are actually felt. Intelligent reasoning behavior seems now to be ProtistaMoneraCiliophoraSarcodinaPoriferaIngestionCoelenterataChaetognathaEchinodermataChordataArthropodaMolluscaAnnelidaAbsorptionTe ntoculataAschelminthesPlatyhelminthesMesozoaPhotosynthesisHyphochytridiomycoidPlasmodiophoremycoidSporozoaZoomastiginaCnidosporidiaMyaomycotaAcrasiomycotaLabyrinthylomycotaAnimaliaFungiPlantaeBacteriaCyanophytaEuglenophytaChrysophytaPyrrophytaZygomycotaBasidiomycotaAscomycotaOomycotaChytridiomycotaTrocheophytaBryophytaPhoeophytaCharophytaChlorophytaRhodophyta FIGURE 2 A five-kingdom system based on three levels of organization—the procaryotic (kingdom Monera), apearyotic unicelluar (kingdom Protista), and eucary-otic multicelluar and multinucleate. On each level there is divergence in relation to three principal modes of nutrition—the photosynthetic, absorptive, and ingestive. Many ecology and microbiology texts list four kingdoms by combining the “lower Protista” (i.e., Monera) with the higher “Protista” to form “Protista.” Evolutionary relations are much simplified, particularly in the Protista. Only major animal phyla are entered, and phyla of the bacteria are omitted. The Coelenterata comprise the Cnidaria and Ctenophora; the Tentaculata comprise the Bryozoa, Brachiopoda, and Phoronida, and in some treatments the Batoprocta. (From Whittaker, 1969.) C005_003_r03.indd 265C005_003_r03.indd 265 11/18/2005 10:21:03 AM11/18/2005 10:21:03 AM
III AII BIII-1AIII-1AIII-1BIII 2III 2III-3IVSUNENERGY FIGURE 3 Diagram of the pond ecosystem. Basic units are as follows? I, abiotic substances—basic inorganic and organic compounds; IIA, producers—rooted vegetation; IIB, producers-phytoplankton; III-1A, primary consumers (herbivores)— bottom forms; III-1B, primary consumers (herbivores)—zooplankton; III-2, secondary consumers (carnivores); III-3, tertiary consumers (secondary carnivores); IV, saprotrophs—bacteria and fungi of decay. The metabolism of the system runs on sun energy, while the rate of metabolism and relative stability of the pond depend on the rate of inflow of materials from rain and from the drainage basin in which the pond is located. (From Odum, 1971.) 266
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THEORY 267 FIGURE 4 Schematic design for a waste management park for the atomic power plant (PP) of the future which is located in a natural watershed basic (outlined by the dashed lines). Waste heat (i.e., thermal pollution) in the reactor cooling water (CW) flowing from a large storage reservoir ® is completely dissipated by evaporative cooling from the network of shallow ponds and spray irrigation systems. The warm ponds may be used for fish culture, sport fishing, or other recreation purposes. Irrigation of portions of the terrestrial watershed increases the yield of useful forest or agricultural products, while at the same time water recycles through the “living filter” of the land back into stream, ponds, and ground-water. Low-level nuclear wastes and solid wastes are contained within carefully managed land fill area (W); high-level nuclear wastes in spent fuel elements are exported to a special nuclear burial ground located outside of the management part. Stream flow, ground water, and stack gases are continuously monitored by hydrological weirs (HW), monitoring wells (MW), and stack gas control systems (SG) in order to make certain that no air or water pollution leaves the controlled area. The chief inputs and outputs for this environmental system include (see numbered marginal arrows): 1, input of sunlight and rainfall; 2, export of nuclear wastes to burial grounds; 3, electric power to cities, etc.; 4, input of nuclear and other fuels; 5, output of food, fibres, clean air, etc.; 6. downstream flow of clean water for agriculture, industry, and cities; 7, public and professional use for recreation, education, and environmental research. The size of such a complete waste management park will depend on regional climates and topography, and on the amount of electrical or other energy diverted to power cooling, but something on the order of 10,000 acres for a 2500 megawatt power plant would be the minimum needed to insure 100% pollution control and allow for accidents and mechanical malfunctions. However, such waste treatment capacity could also support a certain amount of light industry within the park. Heavy industry should be located within its own waste management park. (From Odum, 1971). C005_003_r03.indd 267C005_003_r03.indd 267 11/18/2005 10:21:03 AM11/18/2005 10:21:03 AM
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THEORY FIGURE 5 Pollution of a stream with untreated sewage and the subsequent recovery as reflected in changes in the biotic community. As the oxygen dissolved in the water decrease (curve to the left), fishes disappear and only organisms able to obtain oxygen from the surface (as in Culex mosquito larvae) or those which are tolerant of low oxygen concentration are found in zone of maximum organic decomposition. When bacteria have reduced all of the discharged material the stream returns to normal. (After Eliassen, Scientific American, Vol. 186, No. 3, March, 1952.) C005_003_r03.indd 268C005_003_r03.indd 268 11/18/2005 10:21:04 AM11/18/2005 10:21:04 AM

THEORY 269the only means to accomplish this, as emphasized at the beginning of the article. 9) Some of the most important “breakthroughs” in ecology are in the area of biogeochemical cycling. Since “recycle” of water and minerals must become a major goal of human society the recycle pathways in nature are of great interest; there seem to be at least four major ones which vary in importance in different kinds of eco-systems: (1) recycle via microbial decomposi-tion of detritus, (2) recycle via animal excretion, (3) direct recycle from plant back to plant via symbi-otic microorganism such as mycorrhizae associated with roots, and (4) autolysis, on chemical recycle, with no organism involved. Pathway 3 seems to be especially important in the humid tropics which suggests tropical agriculture might be redesigned to include plant foods with mycorrhizae. 10) The principles inherent in limiting factor analysis and in human ecology can be combined to formu-late the following tentative overview: In an indus-trialized society energy (power, food) is not likely to be limiting, but the pollution consequences of the use of energy and exploitation of resources is limiting. Thus, pollution can be considered the limiting factor for industrialized man—which may be fortunate since pollution is so “visible” that it can force us to use that reasoning power which is supposed to be a special attribute of man.
DEVELOPMENT Principles having to do with the development of ecosystems, that is ecological succession, are among the most relevant in view of man’s present situation. I have recently reviewed this subject (Odum, 1970); accordingly a brief summary will suffice for this piece. In broad view ecosystems develop through a rapid growth stage that leads to some kind of maturity or steady state (climax), usually an oscillating steady state. The early successional growth stage is characterized by a high Production/respiration (P/R) ratio, high yields (net produc-tion), short food chains, low diversity, small size of organ-ism, open nutrient cycles and a lack of stability. In contrast, mature stages have a high biomass/respiration (B/R) ratio, food web, low net production, high diversity and stability. In other words, major energy flow shifts from production to maintenance (respiration). The general relevance of the development sequence to land use planning can be emphasized by the following “mini-model” that contrasts in very general terms young and mature ecosystems: Young Mature Production Protection Growth Stability Quantity Quality It is mathematically impossible to obtain a maximum for more than one thing at a time, so one can not have both extremes at the same time and place. Since all six character-istics are desirable in the aggregate, two possible solutions to the dilemma suggest themselves. We can compromise so as to provide moderate quality and moderate yield on all the landscape, or we can plan to compartmentalize the landscape so as to simultaneously maintain highly produc-tive and predominantly protective types as separate units subjected to different management strategies. If ecosystem development theory is valid and applicable to land-use plan-ning (total zoning), then the so-called multiple-use strategy, about which we hear so much will work only through one or both of these cases, because in most cases, projected multiple uses conflict with one another. Some examples and sugges-tions for implementing compartmental plans are considered in the above mentioned paper. REFERENCES Birge, E.A. (1915), The heat budgets of American and European lakes, Trans. Wis. Acad. Arts. Lett. 18, 166–213. Cairns, J. Rehabilitating damaged ecosystems, CRC Press, Boca Raton, 1988. Dokuchaev, V.V., Teaching about the zones of nature, (in Russian). Reprinted 1948, Moscow. Forbes, S.A. (1887), The lake as a microcosm, Bull. Sc. H. Peoria, reprinted in: Ill. Nat. Hist. Surv. Bull. 15, 537–550 (1925). Hardin, G. (1968), The Tragedy of the commons, Science 162, 1243–1248. Hutchinson, G.E. (1964), The lacustrine microcosm reconsidered, Amer. Sci. 52, 331–341. Jerrers, J.N.R., Practitioners handbook in the modelling of dynamic changes in ecosystems, Wiley, Chichester, 1988. Jorgensøn, S.E., Modelling the fate and effect of toxic substances in the environment, Elsevier, Amsterdam, 1984. Leopold, Aldo (1933), The conservation ethic, Jour. Forestry 31, 634–643. MacArthur, R.H., Geographical ecology, Patterns in the distribution of spe-cies, Princeton University Press, 1984. Marsh, G.P. (1864), Man and Nature: or Physical Geography as Modifed by Human Action, reprinted 1965 by Harvard Univ. Press, Cambridge. Mellos, K., Perspectives on ecology, Macmillan, Basingstoke, 1989. Möbius, K. (1877), Die Auster und die Austernwirtschart. Berlin, Transl. 1880. Rept. U.S. Fish Comm., 1880, 683 to 751. Odum, H.T. (1970), Environment, Power and Society, John Wiley and Sons, N.Y., p. 331. Odum, H.T. (1967), Biological circuits and the marine systems of Texas, in Pollution and Marine Ecology (Olson and Burgess, Eds.), John Wiley and Sons, N.Y., pp. 99–157. Odum, H.T., B.J. Copeland, and E.A. McMahan (1969), Coastal Eco-logical Systems of the United States. A report to the Federal Water Pollution Control Administration, Marine Sci. Inst., Univ. Texas, Mimeograph. Odum, Eugene P. (1969), The attitude lag, BioScience 19, 403. Odum, Eugene P. (1970a), The strategy of ecosystem development, Science, 164, 262–270. Odum, E.P. (1970b), Optimum population and environment: A Georgia microcosm, Current History 58, 355–359. Odum, E.P. (1970c), The attitude revolution, in The Crisis of Survival, Scott, Foresman Co., Glenview, Ill., pp. 9–15. Odum, E.P. (1971), Fundamentals of Ecology, 3rd Edition, W.B. Saunders, Philadelphia. Polunin, N.V., Ecosystem theory and practice, Wiley, Chichester, 1986. Sukachev, V.N. (1944), On principle of genetic classification in bioeoenol-ogy, translated and condensed by F. Raney and R. Daubenmire. Ecol. 39, 364–367. (See also Silva Fennica, 105, 94, (1960)). C005_003_r03.indd 269C005_003_r03.indd 269 11/18/2005 10:21:05 AM11/18/2005 10:21:05 AM
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THEORY EFFECTS OF AIR POLLUTANTS: see AIR POLLUTANT EFFECTS Tansley, A.G. (1935), The use and abuse of vegetational concepts and terms, Ecol. 16, 284–307. Taylor, L.R. (Ed.), The Optimum Population for Britain, Academic Press, N.Y., p. 182, 1970. Walter, H. and S.W. Brecke, Ecological systems of the biosphere; Eco-logical principles in global perspective (Ökologie der Erde), Springer Verlag, Heidelberg, 1985. White, L. (1967), The historical roots of our ecological crisis, Science 155, 1203–1207. EUGENE P. ODUM University of Georgia C005_003_r03.indd 270C005_003_r03.indd 270 11/18/2005 10:21:05 AM11/18/2005 10:21:05 AM
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Almost daily one reads reports in the press about new dangers found or suspected in foods, medicines and other products. Public awareness of possible perils in everyday products has increased greatly. Smoking, asbestos, toxic and hazardous wastes (once called industrial wastes) are widely discussed. Problems resulting from technological advances are being uncovered at an alarming rate. While it is possible to predict, with some accuracy, benefits to be expected from improvements and new approaches in applied science, seldom have serious efforts been made to determine adverse effects resulting from these. The general public is increas-ingly aware of potential hazards from new products and processes and the trend in industrialized countries is toward greater control and regulation of hazardous undertakings. The question as to the more effective approach, gentle per-suasion to gain voluntary compliance or strong legislation for strict regulation, has not been satisfactorily answered. Probably there never will be agreement. In the United States, the Office of Technology Assessment was dissolved. The stated reason was that this Office did long-term studies which were not immediately useful to a legisla-tor. In remarks at the Conference on Technical Expertise and Public Decisions at Princeton University, the then Chairman of the House Science Committee said that it was more desir-able to depend on the views of those most interested in the topic—the lobbyists. The relationship among the air, water, and soil is not a static one. Effects of a pollutant may be demonstrated progressively in the various compartments into which the environment is divided. A substance may be initially present, without apparent ill effects, in one compartment. Later, the same substance, or a demonstrable derivative, may appear in a different part of the environment in a most undesirable way. An excellent example is the high mercury level found in fish. Initially, it was felt that disposal into the marine environment would conveniently remove a bother-some waste. However, by previously unsuspected paths the metal found its way into the systems of game and food fish. Food, thus, has become a secondary distributor of pollutant material. Chemical pollutants may be divided into four categories: 1) Natural chemicals in excess. 2) Naturally occurring toxins. 3) Mixtures of air and water pollutants which produce adverse effects but with only partially defined or undefined components. 4) Synthetic chemicals. The first group includes chemicals such as nitrates and nitrites. Nitrates, for example, can cause methemoglobinemia in infants by reduction of the capacity of the blood to carry oxygen. An intermediate reduction is involved. A famous case in New York involved accidental introduction of sodium nitrite into oatmeal and the resulting problems of “Eleven Blue Men.” The victims, all heavy users of alcohol, apparently tried instinctively to compensate for low salt in the body. Through an accident, sodium nitrite instead of sodium chloride, was placed in salt shakers in a public eating place. The eleven victims all became cyanotic, with the characteristic blue color giving the name to the episode. Nitrites may also react with secondary amines to form nitrosamines, some of the which have been shown to be carcinogenic, teratogenic, and mutagenic, all in microgram doses. Oxides of nitrogen, thought to be significant in smog production, can also form nitrosamines. The second group, natural fungal and plant toxins, usu-ally are introduced into the human ecosystem through acci-dent or carelessness. Conditions of harvesting, storage, and processing have been shown to be of possible importance. An excellent example of the third group has already been mentioned. Mercury was discharged to the receiving water as an apparent ultimate solution to a waste disposal problem. The two components of the system, mercury and water, were assumed to be non-reactive. Unfortunately, the two-component system was, in fact, a multicomponent system and no endeavor was made to determine the complete mechanism. The prob-lem was further complicated by the introduction of edible fish into the chain. Minimata Disease is named for the city in which it occurred. Inorganic mercury was discharged in the effluent of a local industrial plant. Through action of marine organ-isms, the mercury was converted to lipid-soluble methyl mercury, which was taken up in the food chain to fish, the staple of the local diet. 43 deaths and about 700 serious ill-nesses were acknowledged by local authorities in the 1950s. Some unofficial estimates have put the death toll as high as 800. In 1989 two former company officials were given prison sentences for the 1950s pollution. Lawsuits resulting from the pollution were finally settled in 1996. C005_004_r03.indd 271C005_004_r03.indd 271 11/18/2005 10:21:37 AM11/18/2005 10:21:37 AM
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In Japan in 1968 about 1800 people developed a malady similar to chloracne after ingesting rice oil contaminated with a chlorinated biphenyl. Known as Yusho or Rice Oil Disease, the rice oil used in cooking had been contaminated by a PCB which had leaked from a faulty heat exchanger. In 1978 an outbreak similar to Yusho Disease occurred in Taiwan and is known as the Yu-Cheng Incident. The cause was the same, a faulty heat exchanger. The last group, synthetic chemicals, includes pesticides and fertilizers used in agriculture, food additives, compounds containing heavy metals, plasticizers, fuel additives, house-hold chemicals, industrial chemicals, therapeutic and pro-phylactic drugs, and drugs of abuse. Food additives may be intentional or accidental. Anti-oxidants and dyes are added routinely to many foods. However, almost any of the afore-mentioned may be accidentally introduced into food, often with most unpleasant results. There is much controversy concerning synthetic agriculture chemicals. Advantages and disadvantages are numerous and no definite decision has been reached concerning continued use of many substances. NTA (Nitrilotriacetic Acid) as a substitute for phosphates in detergents, is another excellent example of conflicting use. The problem is far from restricted to simple direct physi-ological effects. Food additives may be classified as to function. They find use as coloring material, flavor enhancers, shelf life extenders, and in protection of food nutritional value. While valuable, color additives are not always essential. However, many of the foods now enjoyed by modern western society would not be possible, in their present form, without food additives. It is estimated by the World Health Organization that about one fifth of the food produced in the world is lost by spoilage. Preservation, or retardation of spoilage, can be accomplished by addition of chemical preservatives, or by physical means such as freezing, drying, souring, ferment-ing, curing or ionizing radiation. There is some concern that irradiation of food may have adverse effects and leave unwanted residues. Little is known about possible chemical chain reactions. Food additives have been classified by Kermode into five broad groups: 1) Flavors 2) Colors 3) Preservatives 4) Texture agents 5) Miscellaneous In Table 1 are displayed food additives declared by the United States Food and Drug Administration to be “gener-ally recognized as safe.” Not included in Table 1 is a large group of natural flavors and oils. To be on the list an additive must have been in use before 1958 and meet specifications for safety. Materials introduced after 1958 must be tested individually in order to quality for inclusion of the FDA list. Examples of materials formerly listed but now removed are cyclamate sweetners and saccharin. A ban on cyclamates ordered by the US government after tests revealed devel-opment of bladder cancer in laboratory rats fed on a diet containing cyclamates. Further testing, after the ban, found cancer development in the same test species at dosage rates one sixth as large as those which brought about the ban. TABLE 1 Additives listed by the U.S. Food and Drug Administration as “generally recognized as safe.” (Courtesy, Scientific American ) ANTICAKING AGENTSAluminum calcium silicateCalcium silicateMagnesium silicateSodium aluminosilicateSodium calcium aluminosilicateTricalcium silicateCHEMICAL PRESERVATIVESAscorbic acidAscorbyl palmitateBenzoic acidButylated hydroxyanisoleButylated hydroxytolueneCalcium ascorbateCalcium propionateCalcium sorbateCaprylic acidDilauryl thiodipropionateErythorbic acidGum guaiacMethylparabenPotassium bisulfitePotassium metabisulfitePotassium sorbatePropionic acidPropyl gallatePropylparabenSodium ascorbateSodium benzoateSodium bisulfiteSodium metabisulfiteSodium proponateSodium sorbateSodium sulfiteSorbic acidStannous chlorideSulfur dioxideC005_004_r03.indd 272C005_004_r03.indd 272 11/18/2005 10:21:38 AM11/18/2005 10:21:38 AM

273Thiodipropionic acidTocopherolsEMULSIFYING AGENTSCholic acidDesoxycholic acidDiacetyl tartaric acid esters of mono- and diglyceridesGlycocholic acidMono- and diglyceridesMonosodium phosphatederivatives of abovePropylene glycolOx bile extractTaurocholic acidNUTRIENTS AND DIETARY SUPPLEMENTSAlanineArginineAscorbic acidAspartic acidBiotinCalcium carbonateCalcium citrateCalcium glycerophosphateCalcium oxideCalcium pantothenateCalcium phosphateCalcium pyrophosphateCalcium sulfateCaroteneCholine bitartrateCholine chlorideChopper gluconateCuprous iodideCysteineCystineFerric phosphateFerric pyrophosphateFerric sodium pyrophosphateFerrous gluconateFerrous lactateFerrous sulfateGlycineHistidineInositolIron, reducedIsoleucineLeucineLinoleic acidLysineMagnesium oxideMagnesium phosphateMagnesium sulfateManganese chlorideManganese citrateManganese gluconateManganese glycerophosphateManganese hypophosphiteManganese sulfateManganous oxideMannitolMethionineMethionine hydroxy analogueNiacinNiacinamided-pantothenyl alcoholPhenylalaninePotassium chloridePotassium glycerophosphatePotassium iodideProlinePyridoxine hydrochlorideRiboflavinRiboflavin-5-phosphateSerineSodium pantothenateSodium phosphateSorbitolThiamine hydrochlorideThiamine mononitrateThreonineTocopherolsTocopherol acetateTryptophaneTyrosineValine (continued) TABLE 1 (continued) Additives listed by the U.S. Food and Drug Administration as “generally recognized as safe.” (Courtesy, Scientific American ) C005_004_r03.indd 273C005_004_r03.indd 273 11/18/2005 10:21:38 AM11/18/2005 10:21:38 AM
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Vitamin AVitamin A acetateVitamin A palmitateVitamin B12Vitamin D2Vitamin D3Zinc sulfateZinc gluconateZinc chlorideZinc oxideZinc stearateSEQUESTRANTSCalcium acetateCalcium chlorideCalcium citrateCalcium diacetateCalcium gluconateCalcium hexametaphosphateCalcium phosphate monobasicCalcium phytateCitric acidDipotassium phosphateDisodium phosphateIsopropyl citrateMonoisopropyl citratePotassium citrateSodium acid phosphateSodium citrateSodium diacetateSodium gluconateSodium hexametaphosphateSodium metaphosphateSodium phosphateSodium potassium tartrateSodium pyrophosphateSodium pyrophosphate, tetraSodium thiosulfateSodium tripolyphosphateStearyl citrateTartaric acidSTABILIZERSAcacia (gum arabic)Agar-agarAmmonium alginateCalcium alginateCarob bean gumChrondrus extractGhatti gumGuar gumPotassium alginateSodium alginateSterculoia (or karaya) gumTragacanthMISCELLANEOUS ADDITIVESAcetic acidAdipic acidAluminum ammonium sulfateAluminum potassium sulfateAluminum sodium sulfateAluminum sulfateAmmonium bicarbonateAmmonium carbonateAmmonium hydroxideAmmonium phosphateAmmonium sulfateBeeswaxBentoniteButaneCaffeineCalcium carbonateCalcium chlorideCalcium citrateCalcium gluconateCalcium hydroxideCalcium lactateCalcium oxideCalcium phosphateCaramelCarbon dioxideCarnauba waxCitric acidDextransEthyl formateGlutamic acid(continued) TABLE 1 (continued) Additives listed by the U.S. Food and Drug Administration as “generally recognized as safe.” (Courtesy, Scientific American ) C005_004_r03.indd 274C005_004_r03.indd 274 11/18/2005 10:21:38 AM11/18/2005 10:21:38 AM

275Glutamic acid hydrochlorideGlycerinGlyceryl monostearateHeliumHydrochloric acidHydrogen peroxideLactic acidLecithinMagnesium carbonateMagnesium hydroxideMagnesium oxideMagnesium stearateMalic acidMethylcelluloseMonoammonium glutamateMonopotassium glutamateNitrogenNitrous oxidePapainPhosphoric acidPotassium acid tartratePotassium bicarbonatePotassium carbonatePotassium citratePotassium hydroxidePotassium sulfatePropanePropylene glycolRennetSilica aerogelSodium acetateSodium acid pyrophosphateSodium aluminum phosphateSodium bicarbonateSodium carbonateSodium citrateSodium carboxymethylcellulose Sodium caseinateSodium citrateSodium hydroxideSodium pectinateSodium phosphateSodium potassium tartrateSodium sesquicarbonateSodium tripolyphosphateSuccinic acidSulfuric acidTartaric acidTriacetinTriethyl citrateSYNTHETIC FLAVORING SUBSTANCESAcetaldehydeAcetoinAconitic acidAnetholeBenzaldehydeN-butyric acidd- or l-carvoneCinnamaldehydeCitralDecanalDiacetylEthyl acetateEthyl butyrateEthyl vanillinEugenolGeraniolGeranyl acetateGlycerol tributyrateLimoneneLinaloolLinalyl acetate1-malic acidMethyl anthranilate3-Methyl-3-phenyl glycidicacid ethyl esterPiperonalVanillin TABLE 1 (continued) Additives listed by the U.S. Food and Drug Administration as “generally recognized as safe.” (Courtesy, Scientific American ) C005_004_r03.indd 275C005_004_r03.indd 275 11/18/2005 10:21:38 AM11/18/2005 10:21:38 AM