Multiple dimensions of bird beta diversity support that mountains are higher in the tropics

Aim We examine latitudinal effects of breeding bird taxonomic, phylogenetic and functional β-diversity (Tβ, Pβ and Fβ, respectively) along elevational gradients to test predictions derived from Janzen’s (1967) classic ideas that tropical mountains represent stronger dispersal barriers than temperate mountains. Location Global Taxon Birds Methods Using a global dataset from 46 mountains, we examine latitudinal patterns of Tβ, Pβ, and Fβ, and their components: β rich and β repl. For each mountain and each dimension of diversity we calculated (a) total β-diversity, (b) the steepness of distance decay patterns, and (c) within-mountain variability in pairwise β-diversity and regressed each one of these response variables against absolute latitude. We predicted that tropical montane biotas would have (1) overall higher Tβ, Pβ, and Fβ, (2) faster distance decay patterns and (3) higher within-mountain variability in pairwise β-diversity. Furthermore, we expected the richness component ...