THE MECHANISMS OF SPECIES COEXISTENCE AS SEEN BY ECOLOGICAL THEORY

Abstract

The several mechanisms proposed by the theory to explain species coexistence are presented and discussed in this review. The existence of a superspecies, capable to dominate the environment and to exclude any other species, is a theoretical possibility. Nevertheless, what we do observe in nature, mainly in the tropics, are places with many coexisting species. It is a central issue for Community Ecology understanding which kind of features those species and places should have in order to persist. Historically, competition is the most studied ecological interaction, although other antagonistic interactions as predation, parasitism and apparent competition are important too. Here we review simple models used for mathematical analysis of coexistence, which include all of those referred interactions, especially competition. The concepts of competitive exclusion and the limiting similarity are presented, offering a framework to the discussion about coexistence in homogeneous environments. Then, it is showed that including temporal and spatial variability gives rise to new important mechanisms, which in theory can allow an indefinitely high number of species to coexist even on a unique resource. Essentially, the mechanisms here discussed depend on two common factors to work: i) niche differences among species and ii) ecological trade-offs. Niche differences consist not only in classical resource partitioning, but also in the species distinct reactions to environmental fluctuations. Ecological tradeoffs are also very important, as they make possible that each competing species be the best at some conditions, impeding the complete environment dominance by just one species.