Massive floral display affects insect visits but not pollinator-mediated pollen transfer in Rhododendron ferrugineum

Fragmentation of natural vegetation creates one of the largest threats to plant-pollinator interactions. Although fragmentation impacts on plant populations have been explored in many, mainly herbaceous, species, the response of wild mass-flowering species is poorly known. Here, we studied 28 heathland patches dominated by the mass-flowering shrub Rhododendron ferrugineum, each presenting different R. ferrugineum floral display sizes (total inflorescence number per patch) and patch isolation (median distance to the three nearest patches). We assessed the impacts of these two factors on (i) heathland patch visitor assemblage (considering R. ferrugineum versus surrounding community) and (ii) R. ferrugineum flower visitation rate and pollen transfer limitation (comparing seed set from emasculated to pollen-supplemented flowers). We found that diversity and abundance of bees visiting R. ferrugineum in heathland patches significantly decreased with decreasing R. ferrugineum floral display, while overall visitor density per patch and flower visitation rate increased. Moreover, a decrease in massive floral display and increase in patch isolation resulted in reduced visitor density in the surrounding community. Even in patches with few individuals, we found disproportionate visitor abundance in R. ferrugineum compared to the surrounding community. Finally, pollen transfer limitation in R. ferrugineum was neither affected by visitation rate nor by patch attributes. By disproportionally attracting pollinators from co-flowering species, and probably promoting geitonogamous pollen transfer, the mass-flowering trait appears adequate to compensate, in terms of conspecific pollen transfer, for the decrease in visitor diversity and abundance and in mate availability, which usually result from population fragmentation.