Co-Flowering Plant Densities Affect Bee Visitation to a Focal Plant Species, but Bee Genera Differ in Their Response

The relative densities of co-flowering plant species can affect pollinator foraging behavior, i.e., bee choice of which plants to visit in a foraging path. Because animal-pollinated plants depend on sequential pollinator visits to conspecifics for successful reproduction, understanding the relationship between relative densities of co-flowering plants and bee visits within a foraging path is necessary to determine whether co-flowering species facilitate or compete for bee visitation. We examined the role of relative co-flowering plant species densities in determining bee visits to two focal plant species (Astragalus scaphoides and Penstemon albertinus) within foraging paths by following individual bees in natural multispecies co-flowering plant communities. We found that conspecific and heterospecific plant density influenced the proportion of visits to the focal plant species during foraging bouts, but effects of density differed among bee groups. Considering all bees, Lupinus spp. facilitated visits to A. scaphoides while Castilleja spp. competed with A. scaphoides for visits. When three bee groups, Bombus, Anthophora/Eucera, and “small bees,” were modeled separately, Bombus consistently visited A. scaphoides more than expected regardless of its density and P. albertinus less than expected at intermediate densities. Anthophora/Eucera exhibited a nonlinear response to A. scaphoides floral density but visited P. albertinus in direct proportion to its density. “Small bees” visited both plants in direct proportion to their densities. Our results highlight that bee groups should be treated separately in models of pollinator visitation behavior, and that the composition and density of the co-flowering plant community may impact reproductive success of focal plant species.