Chapter 20 Food Webs 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 or between members of adjacent trophic levels. | Chapter 20 Food Webs Introduction 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 . We examine indirect unexpected effects in general Section and then specifically the effects of trophic cascades
