Mixed support for the idea that lower elevation animals are better competitors than their upper elevation relatives

What factors set species’ range edges? One general hypothesis, often attributed to Darwin and MacArthur, is that interspecific competition prevents species from inhabiting the warmest portions along geographic gradients (i.e., low latitudes or low elevations). A prediction arising from the Darwin-MacArthur hypothesis is that lower elevation species are better competitors than are related upper elevation species. I tested this prediction by conducting a meta-analysis of studies that have measured behavioral competition between related species along elevational gradients. I found that (1) interspecific aggression appears to be a reliable indicator of interspecific competition; (2) as predicted, lower elevation species showed stronger interspecific aggression, but only for tropical species-pairs tested with playback experiments (nearly all songbirds); (3) for a broader range of taxa where authors directly observed behavioral interactions, upper elevation species showed stronger interspecific aggression; and (4) in general, larger species showed greater interspecific aggression. One potential explanation for why upper elevation species often show more interspecific aggression is that they tend to be larger (Bergmann’s rule; larger body sizes in colder environments). Supporting this possibility, tropical species tested with playback experiments, which do not follow Bergmann’s rule, were the only group that matched predictions arising from the Darwin-MacArthur hypothesis. Hence, available evidence does not consistently support the longstanding hypothesis that relative range position predicts the outcome of interspecific competition. Instead, body size is a better predictor of behavioral competition. Last, I consider these results in the context of the hypothesis that behavioral interactions may impact rates of upslope range shifts associated with recent warming.