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

Abstract

Abstract 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 ( The American Naturalist 101 :233–249, 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 (a) overall higher Tβ, Pβ and Fβ, (b) faster distance decay patterns and (c) higher within‐mountain variability in pairwise β‐diversity. Furthermore, we expected the richness component β rich to be more important in temperate mountains, and the replacement component β repl in tropical mountains. Results Latitude had a negative effect on total β‐diversity for all dimensions of diversity. Similarly, metrics of Tβ and Pβ mostly agree with our expectations, whereas Fβ showed contrasting results with steeper distance decay curves and higher within‐mountain variability in temperate mountains. Overall, β rich was a more important component at high elevations in the tropics and across elevations in temperate mountains, and β repl was more important in tropical low and mid‐elevations. Main Conclusions Our findings are consistent with tropical mountain assemblages containing species with narrow elevational distributions, low dispersal ability and potentially high allopatric speciation, resulting in high β‐diversity across elevations. Contrasting results for Fβ indicate high niche packing in tropical assemblages, with most changes in functional diversity among assemblages involving species redundant in trait space.