Aim Elevational gradients distributed across the globe are a powerful test system for understanding biodiversity. Here I use a comprehensive set of bird elevational gradients to test the main drivers of diversity, including sampling, area, mid-domain effect, temperature, temperature and water availability, and hypotheses of evolutionary history.
Location Seventy-eight elevational gradients of bird diversity from mountains in both hemispheres spanning 24.5° S to 48.2° N, including gradients from various climates, biogeographical regions and habitat types.
Methods Data on bird elevational diversity were taken from the literature. Of the 150 datasets found or compiled, only those with a high, unbiased sampling effort were used in analyses. Datasets sampled all birds, all breeding birds or all forest birds; a few studies detailed seasonal, elevational shifts. Eighteen predictions of diversity theory were tested, including three sets of interactions.
Results Birds display four distinct diversity patterns in nearly equal frequency on mountains: decreasing diversity, low-elevation plateaus, low-elevation plateaus with mid-peaks, and unimodal mid-elevational peaks. Bird elevational diversity strongly supports current climate as the main driver of diversity, particularly combined trends in temperature and water availability. Bird diversity on humid mountains is either decreasing or shows a low-elevation plateau in diversity, while on dry mountains it is unimodal or a broad, low-elevation plateau usually with a mid-elevation maximum. The predictions of sampling, area and mid-domain effect were not consistently supported globally. The only evolutionary hypothesis with preliminary support was niche conservatism.
Main conclusions Both water and temperature variables are needed to comprehensively predict elevational diversity patterns for birds. This result is consistent for breeding and forest birds, for both hemispheres, and for local- or regional-scale montane gradients. More analyses are needed to discern whether the mechanism underlying these relationships is ecological, based on direct physiological limitations or indirect food resource limitations, or historical, based on phylogenetic niche conservation or other evolutionary trends related to climate. The species–area and mid-domain effects are not supported as primary drivers of elevational diversity in birds.