Plant invasions have the potential to disrupt community dynamics and impair essential ecosystem services, including the pollination of native flowers. Invaders most likely to invoke pollinator competition are characterized by conspicuous, resource-rich blooms and long flowering periods, while the natives most likely to be impacted are obligate out-crossers that are sensitive to disturbance or locally rare. We investigated the effects of a widespread showy invader of the Pacific Northwest, Rubus armeniacus (Himalayan blackberry), on an imperiled endemic wildflower, Sidalcea hendersonii. We observed pollinators, quantified pollen deposition and conducted pollen-supplementation experiments on S. hendersonii plants growing at three distances (1 m, 15 m and 50 m) from well-established blackberry patches in five adjacent field plots. Individual R. armeniacus flowers received more than three times as many total visits as S. hendersonii inflorescences; however, there was minimal overlap between the pollinator assemblages visiting the two species. R. armeniacus pollen was present on 67% of S. hendersonii stigmas; however, there was no significant relationship between distance and invasive pollen deposition or distance and natural seed set. Pollen-supplementation experiments revealed that S. hendersonii was moderately pollen-limited at all distances, and reproductive output was actually higher at the distance treatment closest to the invader, suggesting a positive proximity effect unrelated to pollination. Thus, although inconstant pollinator foraging and prodigious heterospecific pollen deposition suggest R. armeniacus influences pollinator interactions, it does not appear that the invader directly limits the reproductive success of S. hendersonii.
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Vol. 89 • No. 1