Floral diversity and community structure in Pedicularis (Orobanchaceae)

A pervasive hypothesis at the interface of ecology and evolution is that biotic interactions contribute to regional biodiversity by accelerating adaptation and speciation. We investigated this question in the context of closely related, bumble bee-pollinated plants (Pedicularis spp.) in the Hengduan Mountains of south-central China, where they exhibit spectacular levels of richness, endemism, and floral diversity. Because these species co-occur frequently, flower synchronously, and share pollinators during the brief reproductive season, we predict that pollinator-mediated interactions may influence their community assembly and evolutionary diversification. If disparity in floral traits reduces competitive interactions between species, as would happen if floral isolation mitigates reproductive interference caused by heterospecific pollen flow, then species with dissimilar flowers should co-occur more often, yielding greater floral diversity at local scales than expected by chance. Moreover, if such interactions have repeatedly driven character displacement, then floral traits should exhibit homoplasy, the phylogenetic signature of labile evolution. We present evidence supporting these predictions, and find that local species richness is best explained by a model including both floral diversity and phylogenetic distance. Our results suggest that a dynamic mosaic of pollinator-mediated interactions among Pedicularis in the Hengduan region promotes ecological sorting through recurrent selection against reproductive interference, causing rapid species turnover at local scales, and accelerating the rate of floral divergence among species. Together these processes may have contributed to the remarkable accumulation of florally diverse species of Pedicularis endemic to the Hengduan Mountains biodiversity hotspot.