Ecosystem functions are necessary for human well-being, but how well these functions persist with declines in biodiversity is not fully known. Biodiversity research has used a wide range of approaches to address this question, including manipulations of species richness, species removal experiments, and comparisons of natural communities that differ in richness. A different approach is to extrinsically identify declining species and combine this information with surveys of ecosystem function to understand how much function declining species provide. We use this approach here to identify the effects of pollinator declines on pollination function. We sampled bees off three plant species (Phacelia tanacetifolia, Monarda fistulosa, and Polemonium reptans) at 25 sites in central New Jersey. We also measured how much pollen is provided by each bee species, and the extent of surplus pollination (potential seed set that cannot be realized by plants due to limiting factors unrelated to pollination). Declining species were defined using an existing dataset of 29,993 specimens of 446 species collected across the northeastern US over 140 years. We asked two questions: (1) How much pollination is provided by declining bee species, and (2) do plants receive enough surplus pollination to maintain seed set without declining bee species?
Results/Conclusions
We collected 8,840 individuals of 124 bee species and measured the number of pollen grains deposited to 3,012 stigmas. Although there are 104 regionally declining species in our study area, we collected only 21 of these species in our study, and none was among the 2-3 species that strongly dominated pollination function for each plant. Thus, declining species accounted for only a small percent of pollen deposition (P. tanacetifolia 5.4%, M. fistulosa 2.6%, P. reptans 15.3%). Declines in pollen deposition can impact fitness (if pollen limitation reduces seed set) or be resisted (if plants are not pollen limited). Two plant species, P. tanacetifolia and M. fistulosa, were not pollen-limited, such that even the loss of all declining species would not affect their fitness. However, P. reptans was often pollen limited, such that 60% of the pollen grains currently being deposited by declining bee species are contributing to plant fitness (and thus their loss would reduce plant fitness). These results show that declining bee species make modest contributions to pollination, and that their loss would be buffered by non-declining species.