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 CONSUMERSAlthough 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 thatman 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 REMARKSThis 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 ofC005_002_r03.indd 258C005_002_r03.indd 258 11/18/2005 10:20:40 AM11/18/2005 10:20:40 AM