Convergence, divergence, and the effect of congeners on body size ratios in stream fishes

Abstract Using collections from the years 1892–1999, I determined maximum standard length within each of 1030 populations of riffle-inhabiting darters ( Etheostoma spp.) representing five species from 788 sites in Illinois. Each site contained one to four riffle-inhabiting species of Etheostoma . Based on maximum-sized individuals in each collection, I calculated a ratio of standard lengths for all sympatric species pairs. Null models were developed using random pairings of body size measurements from sites with only one species of riffle Etheostoma to test whether body size ratios of sympatric species pairs are larger, indicating divergence, or smaller, indicating convergence, than ratios generated from allopatric populations. Results suggest that two of nine species pairs tend to converge in body size when sympatric. This suggests that convergence, in some cases, may facilitate the persistence of similar species in diverse communities. When variation in sympatric congener number is accounted for, the E. caeruleum : E. spectabile interaction results in significantly increasing size ratios as congener number increases. Etheostoma caeruleum and E. spectabile are the most ecologically similar and evolutionarily closely related species in the dataset. The divergence within this pair, associated with sympatric congener number, suggests that the degree of divergent character displacement may be indirectly enhanced by community structure.