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

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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. Despite the relatively small quantities of

© 2006 by Taylor & Francis Group, LLC
ECOLOGY OF PRIMARY TERRESTRIAL CONSUMERS 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.
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FRANCIS C. EVANS
University of Michigan
ECOLOGY RADIATION: see RADIATION ECOLOGY
© 2006 by Taylor & Francis Group, LLC