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

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 rangela

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 decomposconsum-ers, 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

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 numeherbivo-rous 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, dependstand-ing 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 prorepro-duction 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

 uuctionRespiraton 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

estimate the effi ciency with which these processes are carried

out, as for example

assimilation efficiency calories assimilated

calories ingeste



d growth efficiency calories in growth

calories ingested yield



eefficiency calories 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 population

calories of available plaant food

measures their food-chain effi ciency, while the ratio

calories of herbivores consumed by carnivores

calories 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 (butter-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),

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 producpropor-tion 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, 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

techniques by Umbreit et al., 1957) Much of this work has

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

(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 producconstruc-tion 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

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

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FRANCIS C EVANS

University of Michigan

ECOLOGY RADIATION: see RADIATION ECOLOGY