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 beta-diversity (T beta, P beta and F beta, respectively) along elevational gradients to test predictions derived from Janzen's (Integrative and Comparative Biology 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 T beta, P beta and F beta, and their components: beta(rich) and beta(repl). For each mountain and each dimension of diversity, we calculated (a) total beta-diversity, (b) the steepness of distance decay patterns and (c) within-mountain variability in pairwise beta-diversity and regressed each one of these response variables against absolute latitude. We predicted that tropical montane biotas would have (a) overall higher T beta, P beta and F beta, (b) faster distance decay patterns and (c) higher within-mountain variability in pairwise beta-diversity. Furthermore, we expected the richness component beta(rich) to be more important in temperate mountains, and the replacement component beta(repl) in tropical mountains. Results Latitude had a negative effect on total beta-diversity for all dimensions of diversity. Similarly, metrics of T beta and P beta mostly agree with our expectations, whereas F beta showed contrasting results with steeper distance decay curves and higher within-mountain variability in temperate mountains. Overall, beta(rich) was a more important component at high elevations in the tropics and across elevations in temperate mountains, and beta(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 beta-diversity across elevations. Contrasting results for F beta indicate high niche packing in tropical assemblages, with most changes in functional diversity among assemblages involving species redundant in trait space.

Automated summary

The study found that tropical high elevations exhibit higher species diversity, with richness component being more important, while temperate mountains show a greater importance of replacement component, indicating different patterns of functional diversity.