Similarity and local co-existence of species in regional biotas

Abstract

The notion of ‘community-wide character displacement’ hypothesizes that locally co-existing sets of competing species should be less similar than expected when compared to random expectations from a broader regional species pool. Here I use a mechanistic approach to the niche concept to show how this expectation is dependent on the types of traits involved. I investigate how two different niche components, those that relate to species' requirements (or responses to environmental factors) versus those that relate to species' impacts (or effects on environmental factors), affect predictions about the similarity of locally co-existing species. In contrast with more conventional approaches that focus on species impacts, I focus on species responses to conclude that locally co-existing species should be more similar in such traits than expected on the basis of random assortment from a larger equilibrium regional biota. In addition, I explore the evolutionary implications of exceptions that might favour the co-existence of species with dissimilar traits (especially those that determine species' impacts on the environment) and conclude that these implications differ when species compete for shared resources, interact via shared predators, or interact via both mechanisms. The analysis developed in this paper shows that the co-existence of species that are more similar than expected by chance is not incompatible with the notion of strongly interacting species in saturated local communities near equilibrium.