In the previous chapter we began to consider how population interactions can shape communities. Our focus was on interactions between species occupying the same trophic level (interspecific competition) or between members of adjacent trophic levels. It has already become clear, however, that the structure of communities cannot be understood solely in terms of direct interactions between species. When competitors exploit living resources, the interaction between them necessarily involves further species -those whose individuals are being consumed - while a recurrent effect of predation is to alter the competitive status of prey species, leading to the persistence of species that would otherwise be competitively excluded (consumer-mediated coexistence).
In fact, the influence of a species often ramifies even further than this. The effects of a carnivore on its herbivorous prey may also be felt by any plant population upon which the herbivore feeds, by other predators and parasites of the herbivore, by other consumers of the plant, by competitors of the herbivore and of the plant, and by the myriad of species linked even more remotely in the food web. This chapter is about food webs. In essence, 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 ecosystem ecology. Thus, we will focus both on the population dynamics of interacting species in the community (species present, connections between them in the web, and interaction strengths) and on the consequences of these species interactions for ecosystem processes such as productivity and nutrient flux.
First, we consider the incidental effects - repercussions further away in the food web - when one species affects the abundance of another (Section 20.2). We examine indirect, 'unexpected' effects in general (Section 20.2.1) and then specifically the effects of 'trophic cascades' (Sections 20.2.3 and 20.2.4). This leads naturally to the question of when and where the control of food webs is 'top-down' (the abundance, biomass or diversity at lower trophic levels depends on the effects of consumers, as in a trophic cascade) or 'bottom-up' (a dependence of community structure on factors acting from lower trophic levels, such as nutrient concentration and prey availability) (Section 20.2.5). We then pay special attention to the properties and effects of'keystone' species - those with particularly profound and far-reaching consequences elsewhere in the food web (Section 20.2.6).
Second, we consider interrelationships between food web structure and stability (Sections 20.3 and 20.4). Ecologists are interested in community stability for two reasons. The first is practical - and pressing. The stability of a community measures its sensitivity to disturbance, and natural and agricultural communities are being disturbed at an ever-increasing rate. It is essential to know how communities react to such disturbances and how they are likely to respond in the future. The second reason is less practical but more fundamental. The communities we actually see are, inevitably, those that have persisted. Persistent communities are likely to possess properties conferring stability. The most fundamental question in community ecology is: 'Why are communities the way they are?' Part of the answer is therefore likely to be: 'Because they possess certain stabilizing properties'.
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