Co-Occurrence of Morphologically Similar Species of Stream Fishes

Abstract

A well-known model of community assembly predicts that only certain combinations of species can coexist, and those that do differ in critical aspects of morphology or size that allow them to exploit different resources. The model has been controversial, because it has been nearly impossible to confirm experimentally and because many of the reported patterns are no different than expected by chance. I addressed an aspect of this model by investigating whether very similar species of stream fishes co-occurred less frequently than more dissimilar species. I sampled 219 assemblages from the Red River basin in Texas, Oklahoma, and Arkansas; calculated an index of co-occurrence between all possible pairs of cyprinid (minnow) species; and estimated their morphological similarity and phylogenetic relatedness. Only sympatric or parapatric pairs were analyzed to allow for the possibility of ecological interactions and to make the analysis independent of the size of the area sampled. Randomization tests demonstrated a significantly low degree of co-occurrence between morphologically very similar pairs compared with less similar pairs of species. However, there was no significant difference in co-occurrence between the species most related phylogenetically and more distantly related species. Of six alternative hypotheses, interspecific competition best explained the pattern.