Encyclopedia of Environmental Science and Engineering, Volume I and II

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

dd

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 (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),

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