COOCCURRENCE OF MORPHOLOGICALLY SIMILAR SPECIES OF STREAM FISHES

A well-known model of community assembly predicts that only certain co mbinations of species can coexist, and those that do differ in critica l aspects of morphology or size that allow them to exploit different r esources, 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 asp ect of this model by investigating whether very similar species of str eam 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 possibl e pairs of cyprinid (minnow) species; and estimated their morphologica l similarity and phylogenetic relatedness. Only sympatric or parapatri c pairs were analyzed to allow for the possibility of ecological inter actions and to make the analysis independent of the size of the area s ampled, Randomization tests demonstrated a significantly low degree of co-occurrence between morphologically very similar pairs compared wit h less similar pairs of species. However, there was no significant dif ference in co-occurrence between the species most related phylogenetic ally and more distantly related species. Of six alternative hypotheses , interspecific competition best explained the pattern.