253ECOLOGY OF PRIMARY TERRESTRIAL CONSUMERS
BASIC CONCEPTSThere 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 WEBSThe 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 hierarchyof 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 PYRAMIDSThe concept of the food-chain permits us to equate, in terms
of ecosystem function, all organisms which occupy the sameC005_002_r03.indd 253C005_002_r03.indd 253 11/18/2005 10:20:39 AM11/18/2005 10:20:39 AM