Beta diversity of orchid bees in a tropical biodiversity hotspot

A controversial issue in ecology and conservation is whether community composition is controlled by niche or dispersal assembly. We assessed the importance of climatic factors and geographic gradients on the distribution of orchid bees in a severely-fragmented and species-rich tropical forest region in Brazil. Orchid-bee males were attracted to 17 different scent baits and collected in 15 forest sites. In total we captured 11,081 bees from 40 species. Climatic variables explained twice as much of the observed variation in the bee species data set as did pure spatial variation. However, most of the climatic explained variation was spatially structured, indicating that the species and the climatic data have a similar spatial arrangement. In fact, part of the observed latitudinal changes in community composition appears to be explained by a concomitant gradient in precipitation seasonality. Similarly, reduced temperatures and a more seasonal precipitation may help to explain the relative distinctiveness of the fauna from some of the westernmost sites. The level of similarity among the sampled sites, although highly variable, decayed both as function of the climatic and geographic distances among these sites. The greatest pairwise dissimilarities in the composition of the orchid-bee fauna were observed among sites 200-300 km from each other, since in many case those sites were more dissimilar in terms of climate than those further apart. It is suggested that global warming and consequent altered climatic regimes will influence the distribution patterns of orchid bees in a region already threatened by deforestation and forest fragmentation.