20.1 Introduction
webs is ‘top-down’ (the abundance, biomass or diversity at lower
trophic levels depends on the effects of consumers, as in a trophic
In the previous chapter we began to consider how population inter-
cascade) or ‘bottom-up’ (a dependence of community structure
actions can shape communities. Our focus was on interactions
on factors acting from lower trophic levels, such as nutrient con-
centration and prey availability) (Section 20.2.5). We then pay
between species occupying the same trophic level (interspecific
special attention to the properties and effects of ‘keystone’ species
competition) or between members of adjacent trophic levels. It
has already become clear, however, that the structure of commun-
– those with particularly profound and far-reaching consequences
elsewhere in the food web (Section 20.2.6).
ities cannot be understood solely in terms of direct interactions
Second, we consider interrelationships between food web struc-
between species. When competitors exploit living resources, the
ture and stability (Sections 20.3 and 20.4). Ecologists are interested
interaction between them necessarily involves further species –
in community stability for two reasons. The first is practical – and
those whose individuals are being consumed – while a recurrent
effect of predation is to alter the competitive status of prey
pressing. The stability of a community measures its sensitivity to
disturbance, and natural and agricultural communities are being
species, leading to the persistence of species that would otherwise
be competitively excluded (consumer-mediated coexistence).
disturbed at an ever-increasing rate. It is essential to know how
In fact, the influence of a species often ramifies even further
communities react to such disturbances and how they are likely
to respond in the future. The second reason is less practical but
than this. The effects of a carnivore on its herbivorous prey may
more fundamental. The communities we actually see are, inevit-
also be felt by any plant population upon which the herbivore
ably, those that have persisted. Persistent communities are likely
feeds, by other predators and parasites of the herbivore, by other
to possess properties conferring stability. The most fundamental
consumers of the plant, by competitors of the herbivore and of the
plant, and by the myriad of species linked even more remotely
question in community ecology is: ‘Why are communities the way
in the food web. This chapter is about food webs. In essence,
they are?’ Part of the answer is therefore likely to be: ‘Because
they possess certain stabilizing properties’.
we are shifting the focus to systems usually with at least three
trophic levels and ‘many’ (at least more than two) species.
The study of food webs lies at the interface of community and
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