We can also consider the relationship between the model in Figure 21.1 and two important kinds of species interactions described in previous chapters - interspecific competition and predation (see especially Chapter 19). If a community is dominated by interspecific competition, the resources are likely to be fully exploited. Species richness will then depend on the range of available resources, the extent to which species are specialists and the permitted extent of niche overlap (see Figure 21.1a-c).
Predation, on the other hand, is capable of exerting contrasting effects. the role of predation First, we know that predators can exclude certain prey species; in the absence of these species the community may then be less than fully saturated, in the sense that some available resources may be unexploited (see Figure 21.1d). In this way, predation may reduce species richness. Second, though, predation may tend to keep species below their carrying capacities for much of the time, reducing the intensity and importance of direct interspecific competition for resources. This may then permit much more niche overlap and a greater richness of species than in a community dominated by competition (see Figure 21.1c). Finally, predation may generate richness patterns similar to those produced by competition when prey species compete for 'enemy-free space' (see Chapter 8). Such 'apparent competition' means that invasion and the stable coexistence of prey are favored by prey being sufficiently different from other prey species already present. In other words, there may be a limit to the similarity of prey that can coexist (equivalent to the presumed limits to similarity of coexisting competitors).
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