Functional and phylogenetic diversity and assemblage structure of frugivorous birds along an elevational gradient in the tropical Andes

Tropical mountains are hotspots of biodiversity, but the factors that generate this high diversity remain poorly understood. To identify possible mechanisms that influence avian species assemblages in the tropical Andes, we studied the functional and phylogenetic diversity and the structure of species assemblages of an avian feeding guild. We analysed how functional and phylogenetic diversity, structure and composition of frugivorous bird assemblages changed along a 3300 m elevational transect from the lowlands to the tree line with a novel combination of functional and phylogenetic approaches, and used null models to infer possible drivers of the observed patterns. Species richness, functional richness and phylogenetic diversity decreased almost monotonically with increasing elevation, but assemblage structure and composition changed abruptly in the Andean foothills at around 1200 m. In the lowland assemblages, species were functionally and phylogenetically less similar than expected from null models, whereas species in the highland assemblages were functionally and phylogenetically more similar than expected by chance, suggesting an abrupt reduction in the number of functionally and phylogenetically distinct species in the transition from lowlands to the highlands. Nevertheless, the functional and phylogenetic evenness of the assemblages, i.e. the regularity of the spacing of species in functional trait space and phylogeny, remained constant along the gradient, which suggests that the mechanisms that influence the co-occurrence of species within the assemblages are similar in lowlands and highlands. The observed differences between lowland and highland assemblages imply sharp distributional limits for frugivorous bird species in the Andean foothills, probably caused by environmental factors other than climate, e.g. changes in habitat types or topography, or independent species radiations in lowlands and highlands. These strong distributional limits may hinder uphill range shifts of frugivorous bird species, and the plant species they disperse, in the tropical Andes as a response to climate change.