Elevational patterns of bird functional and phylogenetic structure in the central Himalaya

How communities assemble is a central and fundamental question in ecology. However, it has been mired by conflicting conclusions about whether community assembly is driven by environmental filtering, biotic interactions, and/or dispersal processes. Elevational gradients provide an ideal system for exploring the biotic and abiotic forces influencing the processes of community assembly, as these both change dramatically on mountains over short spatial distances. Here, we explored bird taxonomic, functional and phylogenetic diversity, and assessed the role of spatial (area) and environmental factors (temperature, precipitation, plant richness, habitat heterogeneity, the Normalized Difference Vegetation Index (NDVI)) in shaping bird distributions and community structure along a 3600 m elevational gradient in the central Himalayas, China. Our results showed that the three dimensions of diversity consistently showed hump-shaped patterns with similar peaks. Richness-controlled functional diversity decreased with elevation, while richness-controlled phylogenetic diversity showed a Mid Valley pattern. Mean pairwise functional distance decreased linearly with elevation, and mean pairwise phylogenetic distance was nearly constant along the elevation gradient but increased rapidly at higher elevations (above 3900-4200 m a.s.l). The functional structure of bird communities was more clustered relative to source pools (i.e. species more similar to one another) across the elevation gradient, suggesting abiotic or habitat filtering likely governed the assembly processes. However, phylogenetic structure was more clustered relative to source pools at mid-elevations and more overdispersed (i.e. species are less related) at low and high elevations. In addition, primary productivity (NDVI and/or habitat heterogeneity and/or plant richness) was a good predictor of variation for most diversity metrics. Taken together, our study demonstrated contrasting elevational patterns assessed from functional and phylogenetic measures and highlighted the necessity of considering multiple measures of biodiversity when assessing community structure.