Animal-pollinated plants depend on approximately sequential pollinator visits to conspecifics for reproduction because pollen collected at one plant individual is generally deposited within a small number of subsequent plant visits. Therefore, in co-flowering plant communities, the reproductive success of a focal plant species is strongly influenced by the proportion of visits to it within individual pollinator foraging bouts. However, which plants a bee chooses to visit within a foraging bout is not well described in natural systems and at large spatial scales relevant to known pollinator foraging ranges. We describe the effect of relative densities of multiple plant species on bee visitation to a focal species within foraging bouts. Our study centered on a focal plant, Astragalus scaphoides (Bitterroot Milkvetch), which is endemic to the northern Rocky Mountains, and three co-flowering bee-attractive plants. We followed foraging bees and mapped their foraging bouts within this natural, multi-species plant community. We used quasibinomial GLMs to quantify the relationship between visits to A. scaphoides within foraging bouts and the relative densities of each plant species.
Results/Conclusions
We found that both conspecific and heterospecific floral density influenced the proportion of visits to A. scaphoides during foraging bouts. Visitation to A. scaphoides was significantly positively associated with A. scaphoides density and significantly negatively associated with co-flowering Castilleja spp. density. However, the effects of density on bee visitation to A. scaphoides varied among two abundant bee groups, the social genus Bombus and the solitary genera Anthophora and Eucera. Bombus showed a significant overall preference for A. scaphoides across the range of A. scaphoides densities in our study. Anthophora and Eucera showed no overall preference or lack of preference for A. scaphoides, but the relationship between visits to A. scaphoides and its density varied significantly over the range of A. scaphoides densities in our study. Anthophora and Eucera visit A. scaphoides more than expected when it is rare, and as expected when it is common. This result is consistent with resource-mixing behaviors, i.e. collection of pollen and/or nectar from different species to balance nutritional requirements. Our results demonstrate facilitative effects of conspecific density on pollinator visitation and competitive effects of heterospecific density, and suggest that bee responses to density vary predictably across bee taxonomic groups.