Ecological Character Displacement

Abstract

A theoretical study is made of the conditions under which competition mediated by a quantitative character in each of two species will result in the convergence or divergence of the mean values of the character. Both within— and between—species competition are considered and a general model of the genetical basis of the character is used. Both the genetical and ecological parts of the model are based on the biological assumptions that are made in discussions of ecological character displacement, which include the assumptions that there are no constraints on the evolution of the character in either species and that individuals in each species with the same value of the character use the limiting resources in the same way. One of the main results is that the usual biological assumptions do not necessarily lead to permanent and significant displacement (i.e., d/w > 1). While some previous models have led to different conclusions, it is shown that those results are due to other, implicit, assumptions in those models. In the present model, two types of conditions lead to significant character displacement under biologically reasonable assumptions. One type of condition, which would apply to species that are roughly equally abundant at equilibrium, is that there be some differences in the set of limiting resources of the two species. In that case, the equilibrium displacement would be much larger than the differences between the resource spectra. Small and possibly unmeasurable differences between species could lead to significant displacements. The other type of resource distribution that leads to significant displacements is a skewed distribution. This requires no differences between the resource spectra for the two species but it leads to an equilibrium in which one of the species is much more abundant than the other. One consequence of these results is that the relative abundances of two species thought to have undergone character displacement are important to understand the possible mechanism leading to that displacement.