Invasive species can influence plant-pollinator interactions, and their impact could vary in time and space. Invasive plant species can influence pollinator visitation rates, pollinator community composition, interspecific pollen transfer and seed production of native plants. These ecological effects could also influence the evolution of plant-pollinator interactions. We examined whether the presence of the invasive Linaria vulgaris influences pollinator visitation rate, pollinator community composition and pollinator-mediated selection in the native Penstemon strictus. We tested these effects on two scales: small spatial manipulations within a year, and comparisons across years that varied naturally in phenological overlap with Linaria. In each year, we set up at least five blocks, with each block containing two 3x3m plots, with and without Linaria. Five potted or cut inflorescences of Penstemon were placed into each plot. Data were collected from 2015 to 2017, with more expected in 2018. Pollinators were observed for three hours at each block to determine pollinator visitation rates and pollinator composition. Seed production was calculated from all flowers open during the pollinator observations. To determine pollinator-mediated selection, a wide array of floral traits were measured. Two proxies for fitness were used: pollinator visitation rate and seed production.
Results/Conclusions
The two plant species overlapped in flowering during 2015 and 2017 but not at all in 2016. In the years of flowering overlap, presence of Linaria had no detectable effect on pollinator visitation rate to Penstemon strictus. Pollinator visit rate varied greatly across years, being two times higher in 2016, the year of non-Linaria flowering overlap. Pollinator community composition also varied across years; bumblebees were most frequent in the year of non-overlap, and solitary bees were more frequent in 2017 than in 2015. Only in the year of non-overlap did we detect directional selection for larger Penstemon flowers. Variation in flowering overlap could be due to differences in phenological responses to precipitation. Our results suggest that Linaria could be affecting pollination and pollinator-mediated selection in Penstemon at a scale larger than our experimental manipulation. Yearly environmental variation can influence the impact that invasive plant species have on pollination and selection of traits in native plant species.