Food resources and vegetation structure mediate climatic effects on species richness of birds

Aim: Climate is widely recognized as a major predictor of species richness patterns along large-scale environmental gradients. Nevertheless, the mechanisms by which climate influences species richness are still a matter of debate. We disentangle whether climate influences species richness of birds directly via physiological limitations or indirectly via vegetation structure or the availability of food resources. Location: Mount Kilimanjaro, Tanzania. Methods: We recorded bird species richness along an elevational gradient from 870 to 4550m a.s.l. We quantified local climatic conditions, vegetation structure and the availability of food resources, and applied path analysis to disentangle their direct and indirect effects on species richness of all birds, frugivores and insectivores. Results: Overall, we recorded 2945 individuals from 114 bird species. Species richness of all birds was closely correlated with temperature, vegetation structure and invertebrate biomass and both direct and indirect (via vegetation structure and availability of food resources) climatic effects were important for the diversity of the whole, trophically heterogeneous bird community. The species richness of insectivorous birds was linked to vegetation structure and invertebrate biomass, while the richness of frugivores was strongly associated with fruit abundance. Climatic factors influenced bird species richness of both avian feeding guilds exclusively indirectly via vegetation structure and availability of food resources. Main conclusions: We reveal the importance of trophic interactions for generating species richness patterns along large-scale environmental gradients. Our results challenge the general assumption that temperature and water availability influence species richness mostly directly, and underscore the importance of vegetation structure and the availability of food resources as principal mediators of climatic effects on species richness patterns on macroecological scales. copy; 2014 John Wiley & Sons Ltd.