Floral neighborhood influences pollinator assemblages and effective pollination in a native plant.

Pollinators represent an important intermediary by which different plant species can influence each other's reproductive fitness. Floral neighbors can modify the quantity of pollinator visits to a focal species but may also influence the composition of visitor assemblages that plants receive leading to potential changes in the average effectiveness of floral visits. We explored how the heterospecific floral neighborhood (abundance of native and non-native heterospecific plants within 2 m × 2 m) affects pollinator visitation and composition of pollinator assemblages for a native plant, Phacelia parryi. The relative effectiveness of different insect visitors was also assessed to interpret the potential effects on plant fitness of shifts in pollinator assemblage composition. Although the common non-native Brassica nigra did not have a significant effect on overall pollinator visitation rate to P. parryi, the proportion of flower visits that were made by native pollinators increased with increasing abundance of heterospecific plant species in the floral neighborhood other than B. nigra. Furthermore, native pollinators deposited twice as many P. parryi pollen grains per visit as did the nonnative Apis mellifera, and visits by native bees also resulted in more seeds than visits by A. mellifera. These results indicate that the floral neighborhood can influence the composition of pollinator assemblages that visit a native plant and that changes in local flower communities have the potential to affect plant reproductive success through shifts in these assemblages towards less effective pollinators.