ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - ECOLOGY OF PRIMARY TERRESTRIAL CONSUMERS potx

253 ECOLOGY OF PRIMARY TERRESTRIAL CONSUMERS 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 © 2006 by Taylor & Francis Group, LLC 254 ECOLOGY OF PRIMARY TERRESTRIAL CONSUMERS 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 Egestion Assimilation Prod ϭϩ ϩ ϭ uuction Respiratonϩ 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 © 2006 by Taylor & Francis Group, LLC ECOLOGY OF PRIMARY TERRESTRIAL CONSUMERS 255 estimate the effi ciency with which these processes are carried out, as for example assimilation efficiency calories assimilated calories ingeste ϭ dd growth efficiency calories in growth calories ingested yield ϭ eefficiency calories to man (or other exploiter) calories ingest ϭ 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 population calories of available pl aant food measures their food-chain effi ciency, while the ratio calories of herbivores consumed by carnivores calories of plant foo dd 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), © 2006 by Taylor & Francis Group, LLC 256 ECOLOGY OF PRIMARY TERRESTRIAL CONSUMERS 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 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 References Bush-clover stand (S.C.) 0.4–1.4 Menhinick, 1967 Festuca grassland (Tenn.) 9.6 Van Hook, 1971 Spartina salt marsh (Ga.) 7.0–9.2 Teal, 1962 mesophytic woodland (Tenn.) 1.5 Reichle and Crossley, 1967 Mature deciduous forest (southern Canada) 1.5–2.5 Bray, 1964 © 2006 by Taylor & Francis Group, LLC techniques by Umbreit et al., 1957). Much of this work has ECOLOGY OF PRIMARY TERRESTRIAL CONSUMERS 257 sheep, 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 © 2006 by Taylor & Francis Group, LLC 258 ECOLOGY OF PRIMARY TERRESTRIAL CONSUMERS (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. 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