Encyclopedia of Environmental Science and Engineering, Volume I and II

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

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

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

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