Functional community assembly and turnover along elevation and latitude

The drivers of community coexistence are known to vary with environment, but their consistency across latitudes and scales, and resulting conservation implications, remain little understood. Here, we combine functional and phylogenetic evidence along elevations to document strong biotic constraints on coexistence in avian communities in both benign (tropical low elevations) and severely harsh (temperate/polar highlands) environments. Assemblages in both are marked by high assemblage functional uniqueness, whereas in tropical highlands and temperate/polar low elevations there is strong functionally redundancy and pronounced environmental constraints. Only in harsh environments is phylogeny an effective surrogate for functional assemblage structure, reflecting nuanced shifts in the position, shape, and composition of measured multivariate trait space along gradients. Independent of scale and latitude, high elevation assemblages emerge as exceptionally susceptible to functional change.