Beyond pairwise interactions: multispecies character displacement in Mexican freshwater fish communities

Competition has long been recognized as a central force in shaping evolution, particularly through character displacement. Yet research on character displacement is biased as it has focused almost exclusively on pairs of interacting species, while ignoring multispecies interactions. Communities are seldom so simple that only pairs of species interact, and it is not clear if inferences from pairwise interactions are sufficient to explain patterns of phenotypes in nature. Here we test for character displacement in a natural system of freshwater fishes in western Mexico that contains up to four congeneric species of the genus Poeciliopsis. We analyzed body shape differences between populations with different numbers of competitors while accounting for confounding environmental variables. Surprisingly, we found evidence for convergent character displacement in populations of P. prolifica, P. viriosa and P. latidens. We also found that the convergence in body shape was not
consistently in the same direction, meaning that when three or more competitors co-occurred, we did not find more extreme body shapes compared to when there were only two competitors. Instead, when three or more competitors co-occur, body shape was intermediate to the shape found with a pair of species or no competitor present. This intermediate shape suggests that evolution in multispecies communities likely occurs in response to several competitors, rather than to simple pairwise interactions. Overall, our results suggest that competition among multiple species is more complex than simple pairwise competitive interactions.