The Effects of Floral Neighborhood on the Reproductive Success of Phacelia parryi and Implications for Invasive Weed Control

Local flower density and diversity can affect plant reproductive success in cases where sympatric species compete for pollination services.  Moreover, when invasive plant species make up a large proportion of the local flowering community, these competitive dynamics may be exacerbated.  In this study, I investigated the effect of floral neighborhood on the reproductive success of California native plant Phacelia parryi by monitoring pollinator visitation rates and pollinator community composition.  Visitation rates by different types of insects were regressed on the flower density of Brassica nigra, a common invasive weed, and the flower density of all other heterospecific plant species.  Based on Pianka’s niche overlap index, the exotic species B. nigra shares pollinators to a high degree (>95%) with the focal native P. parryi, suggesting potential for these two species to compete for pollination services.  Although I did not find direct evidence that the presence of B. nigra has a negative effect on overall pollinator visitation rate to P. parryi, the proportion of flower visitors that were native insects increased with abundance of other heterospecific species in the floral neighborhoods (P = .0312).  These results suggest that B. nigra may have negative effects on P. parryi reproductive success in the long term through displacement of native plants, which appear to facilitate visitation.