Urbanization can reduce pollinator biodiversity and therefore impact the quantity and quality of plant-pollinator interactions. To mitigate losses of pollination services, cities have implemented urban garden initiatives to preserve pollinator biodiversity. However, there are several effects of human-mediated plant introductions on the surrounding landscape that remain unstudied, especially the impact of introduced plant species on naturally occurring plants (i.e. wild remnant species). Thus, characterizing the degree to which plants within urban areas interact via shared pollinators is a first step in understanding how introduced species impact wild remnant species. We hypothesized that introduced species and wild remnant species negatively interact pre-pollination via competition for pollinator visitation and post-pollination through the transfer of heterospecific pollen. We recorded pollinator visitation rates and pollinator pool composition of two wild remnant native plants (Impatiens pallida, Impatiens capensis) and one introduced common garden species (Heliopsis helianthoides) in varying natural plots (I. pallida only, I. capensis only, I. pallida/I. capensis, I. pallida/H. helianthoides, I. pallida/I. capensis/H. helianthoides) in a large park in Pittsburgh, PA. We also collected stigmas of all focal species and evaluated the quantity and quality of pollen receipt by scoring the amount of conspecific and heterospecific pollen grains.
In terms of pre-pollination interactions, we found that average pollinator visitation rates to both wild remnant species (Impatiens spp.) in natural plots with introduced H. helianthoides were significantly lower relative to natural plots without this species, suggesting competition for pollination services. The pollinator community composition of wild remnant plants also contained significantly more pollinator functional groups when H. helianthoides was present. In terms of post-pollination interactions, the presence of H. helianthoides pollen on Impatiens spp. stigmas was very low, perhaps because the small bees recruited to the site by the presence of H. helianthoides were not as effective at pollinating either Impatiens when visiting them. Future studies will directly assess the fitness consequences of heterospecific pollen receipt by both Impatiens via controlled hand pollinations between all focal species, as well as explore other effects of introduced species on wild remnant species (i.e. viral and bacterial pathogen transmission), which could inform comprehensive management practices that protect all plant species and maintain ecosystem services within a community, especially in the face of an ever-changing urban landscape.