Invasive plants may affect the reproductive success of native sympatric plants in various ways. One important but often overlooked mechanism is by altering the assemblages of insect pollinators that visit native plants. For example, introduced plants may attract a larger proportion of generalist pollinators potentially leading to greater receipt of heterospecific pollen and reductions in transfer of conspecific pollen. This study examines how floral neighborhood affects the taxonomic composition of insect assemblages that visit a focal native plant and the potential impacts of any shifts in this composition. I examined how the presence of the invasive plant, Brassica nigra and other heterospecific plant species affect the visitation rate and insect assemblages visiting the flowers of a California native, Phacelia parryi. Visitation was observed to P. parryi flowers as a function of the floral neighborhood in the surrounding 2 m x 2 m area. I also compared the relative effectiveness of different types of insect visitors by measuring the loads of conspecific pollen deposited on native stigmas in single visits to flowers.
Results/Conclusions
Although B. 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 (all pollinators excluding the European honeybee, A. mellifera), increased significantly with increasing abundance of other heterospecific species in the floral neighborhoods. Furthermore, per-visit pollen loads deposited by native pollinators contained twice as many P. parryi pollen grains as pollen loads deposited by A. mellifera. These results suggest that native pollinators have the potential to be superior pollen vectors for P. parryi when compared to non-native pollinators. Even if invasive plants do not have a direct effect on visitation rate to surrounding natives by competing for pollination services, they may indirectly influence reproductive success by modifying the mix of pollinators that visits them.