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

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 pbkp_redictions 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: βrich and βrepl. 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 pbkp_redicted 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 βrich to be more important in temperate mountains, and the replacement component βrepl 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, βrich was a more important component at high elevations in the tropics and across elevations in temperate mountains, and βrepl 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.