1 INTRODUCTIONWEBS IS ‘TOP-DOWN’ (THE ABUNDANCE, BIOMASS OR DIVERSI...

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