Individual species can influence the composition of whole communities in a variety of ways. In this chapter we pay particular attention to the manner in which competition, predation and parasitism can shape communities.
The view that interspecific competition plays a central and powerful role in the shaping of communities was first fostered by the competitive exclusion principle, with its implication of a limit to the similarity of competing species, and thus a limit to the number of species that can be fitted into a particular community before niche space is fully saturated. There is no argument about whether competition sometimes affects community structure, nor does it play an overriding role in every case. Thus, other factors may prevent competition from progressing to competitive exclusion, by depressing densities or periodically reversing competitive superiority. Moreover, even when competition is intense, the species may coexist if they have aggregated distributions, with each species distributed independently of the others.
Evidence from community studies of niche differentiation for important resources in space and time is consistent with a role for competition in determining community composition. However, the documentation of mere differences among species is insufficient. The approach has been to build null models of actual communities that retain certain of the characteristics of their real counterparts (in terms of diets, feeding morphologies or distributions of coexisting species) but reassemble the components at random, excluding the consequences of competition. Comparisons of predicted and observed patterns have sometimes supported a role for competition, but by no means in all cases.
Grazing animals sometimes increase plant species richness (exploiter-mediated coexistence) by interrupting the process of competitive exclusion, thus imposing their own order on community composition. Plant coexistence is more likely to be fostered by grazers in nutrient-rich situations, and where the preferred food plants would otherwise be competitively superior to less preferred ones.
Carnivorous animals may, likewise, increase the species richness of prey. This has been recorded for rocky shore invertebrates and woodland bird communities, but not for deep-sea vent communities or in terrestrial insect and spider studies. The outcome for species richness in the face of predation again depends on a number of factors, including the pattern of dietary preference and the relative competitive status of the prey.
The incidence of a parasite, like that of other types of exploiter, may determine whether or not a host species occurs in an area; parasites can cause more subtle effects too, by influencing species that are themselves strong interactors or ecosystem engineers in terrestrial, freshwater and marine communities. Parasites are sometimes responsible for exploiter-mediated coexistence.
Communities are not necessarily structured by a single biotic process and the role of consumers in shaping community structure can be expected to be modified according to abiotic conditions. Biotic effects may often be least significant in communities where physical conditions are more severe, variable or unpredictable.
remember that some biotic interactions are positive in their effects mmk
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