The Role of Predation in Shaping and Maintaining the Structure of Communities

Abstract

Although ecological theorists have shown that frequency-dependent prey selection may permit the coexistence of quite similar competitors, there are compelling reasons to suggest an alternative explanation for the relationship between predation and prey species diversity. Predators which respond to the distribution of prey abundance may maintain prey populations below resource limitation, where optimal utilization patterns are discriminate. More species with discriminate resource utilization patterns can coexist than can species with indiscriminate ones. Two observations, one local and ecological and the other global and evolutionary, are explicable in terms of this simple hypothesis concerning the role of predation in shaping and maintaining the structure of communities. Colonizing consumers shape the structure of communities by selecting producers which are using or otherwise would use at least one of their resources indiscriminately. By reducing producer population sizes or growth rates, and/or by increasing rates of resource regeneration, consumers may increase the abundance of producer resources, enabling producers to use them discriminately. This changes the distribution of available resource states qualitatively and quantitatively. Consequently, the population sizes of less abundant or slower growing producer species may increase and/or additional species may become established. The disproportionate consumption of abundant or rapidly growing producer species and the increased abundance of already established ones both contribute to increasingly even producer species abundance distributions. Consequent reduction in the probability of local extinction and successful colonization by additional species both increase the number of producer species. The change in the distribution of producer species abundance may result in an apparent increase in the number of species as well. All of these changes increase species diversity. Among the attributes of producers affecting their selection by consumers are those which determine or reflect producer competitive success, population growth rate and frequency, respectively. Thus, predation and competition are balanced at steady-state. Should a guild of competing producers be perturbed so that the population growth rates of its members change, their relative values to consumers would change as well. Accordingly, they might respond by consuming disproportionately those producers whose growth rates changed most. Otherwise, producer frequencies would change and consumers would respond by selecting disproportionately those whose frequencies were most altered. Thus, the same sort of predatory behavior which shapes the structure of communities also maintains it. In variable environments episodes of recolonization and succession follow severe perturbations, between which milder perturbations are weathered successfully. While phenotypic plasticity certainly appears adaptive in such environments, others are stable. Moreover, it is possible to order environments along a continuum of variability (Sanders 1968), one analogue of which is latitude. Should consumers maintain producer populations below resource limitation, narrow producer resource utilization patterns would evolve in stable environments. Since greater numbers of such species can coexist, this may contribute to the inverse relationship between latitude and the number of species. Although the more diverse communities characterizing low latitudes seem well adapted to naturally occurring environmental variation, they are fragile compared with the less diverse communities of higher latitudes (Futuyma 1973). This is not to say, however, that there is no relationship between species diversity and community stability (e.g., Horn 1974; Goodman 1975), but rather that comparing the effects of similar perturbations on temperate and tropical ecosystems is simply inappropriate. Indeed, this hypothesis about the role of predation in shaping and maintaining the structure of communities suggests that there is such a relationship. That is, a consumer behavioral pattern which seems adaptive is implicated in both, although no causal relationship between species diversity per se and community stability is suggested. More generally, however, this simple hypothesis provides for a relationship between the structural attribute complexity and the functional attribute stability suggested long ago (Odum 1953).