Is reproduction of endemic plant species particularly pollen limited in biodiversity hotspots?

Current evidence suggests that plants in biodiversity hotspots suffer more from pollen limitation of reproduction than those in lower diversity regions, primarily due to the response of self-incompatible species. Species in biodiversity hotspots may thus be more at risk of limited reproduction and subsequent population decline. Should these species have restricted ranges (i.e. be endemics to a certain region), pollen limitation within highly diverse regions may pose an important threat to global plant biodiversity. We further dissect the global pattern by exploring whether pollen limitation of range-restricted (endemic) species is distinctive and/or relates differently to species diversity than that of widespread (non-endemic) species. To provide a preliminary test of this prediction we conducted both cross-species and comparative phylogenetic meta-analyses to determine the effect of endemism on the magnitude of pollen limitation and its relationship with regional species richness. Our data set included 287 plant species belonging to 78 families distributed world-wide. Our results revealed that endemism and self-compatibility contribute to the global association between pollen limitation and species richness. Self-incompatible species were more pollen limited than self-compatible ones, and the PICs analysis indicated that transitions to endemism were associated with transitions to self-compatibility. The relationship between pollen limitation and species richness was significant only for the self-incompatible species, and was monotonically increasing in non-endemic species but accelerating in the endemic species. Thus, self-incompatible endemic species from biodiversity hotspots are at the greatest risk of pollination failure, a previously unknown aspect suggesting this group of species as a top priority for future development of conservation strategies. In contrast, reproduction of self-compatible species appears to be unrelated to plant diversity, although we caution that current data do not account for the reproductive limitation due to the quality of pollen received. Understanding the mechanisms underlying these patterns requires further investigation into plant-plant pollinator mediated interactions and the dynamics of pollen transfer in communities differing in species diversity.