Encyclopedia of Environmental Science and Engineering, Volume I and II

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

 


 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

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