Ant impacts on global patterns of bird elevational diversity

Abstract

AbstractAcross the world’s mountains, elevation-species richness relationships are highly variable. Here, using data on bird species elevational distributions from all 46 of the world’s major mountain ranges, bird species dietary traits, and the distribution of the low-elevation ant genusOecophylla, we show that global patterns in bird elevational diversity are likely to be affected by competition with ants.Oecophyllais an exceptionally abundant and aggressive predator of invertebrates, which preys on the same species that sympatric invertivorous bird species feed on. In mountain ranges withOecophyllapresent in the foothills, maximum species richness of invertivorous birds occurs, on average, at 960m, ∼450m higher than in mountain ranges withoutOecophylla. Further, in mountain ranges withOecophylla, species richness of invertivorous birds increases initially with with elevation to produce a mid-elevation peak in invertivore bird species richness. WhereOecophyllais absent, invertivore bird species richness generally shows monotonic declines with increasing elevation. We attribute the pattern to the following mechanism: first,Oecophyllareduces prey density for invertivorous birds; second, low invertebrate prey abundance reduces invertivorous bird density and third, lower bird density is correlated with lower bird species richness. Because invertivores dominate montane bird communities, global elevational bird diversity patterns are also driven byOecophylla. The findings emphasize how competitive interactions between distantly related taxa set geographical range limits.