Drivers of pollen limitation: macroecological interactions between breeding system, rarity, and diversity

Background: Plant species in biodiversity hotspots suffer more from pollen limitation than those in lower diversity regions, though this pattern is largely restricted to self-incompatible species. It is unknown whether higher pollen limitation is due to increased pollinator sharing or declines in pollinator abundance. Aims: Macroecological examinations of pollen limitation have been challenged by statistical confounds of phylogenetic non-independence and interrelationships between variables. Here, we perform phylogenetically corrected analyses of pollen limitation, examining an ensemble dataset of endemicity, abundance, species diversity, breeding system, floral symmetry, and pollinator richness. Methods: We apply model selection and path analysis to a large dataset of published studies of pollen limitation on 275 plant species distributed worldwide. Results: Plant diversity and breeding system were included in the best model. Even the best model explained only 13% of the among-species variation in pollen limitation, indicating a stochastic component in pollen receipt. Pollinator richness remained a consistent determinant of pollen limitation, influenced by floral symmetry and, to a lesser extent, plant diversity. Conclusions: Our results suggest that many traits examined thus far explain relatively little of the variation in pollen limitation, partly because their effects are subsumed by the roles played by breeding system and plant diversity.