Feeding friend and foe: ample pollen mitigates the effects of pollen theft for a gynodioecious plant, Polemonium foliosissimum (Polemoniaceae).

BACKGROUND AND AIMS Most angiosperms rely on pollinators to transport pollen and effect fertilization. While some floral visitors are effective pollinators, others act as thieves, consuming pollen but effecting little pollination in return. The importance of pollen theft in male and female reproductive success has received little attention. Here, we examined if pollen consumption by flies altered pollen receipt and exacerbated pollen limitation for a bumblebee-pollinated plant, Polemonium foliosissimum (Polemoniaceae). METHODS To examine the effect of pollen-thieving flies, we took a three-pronged approach. First, we used single-visit observations to quantify pollen removal and pollen deposition by flies and bumblebees. Second, we manipulated pollen in the neighbourhood around focal plants in two years to test whether pollen reduction reduced pollen receipt. Third, we combined pollen reduction with hand-pollination to test whether pollen thieving exacerbated pollen limitation. Polemonium foliosissimum is gynodioecious in most populations in the Elk Mountains of central Colorado, USA. Thus, we also tested whether pollen theft affected hermaphrodites and females differently. RESULTS Flies removed significantly more pollen and deposited less pollen per visit than did bumblebees. Reduction of pollen in the neighbourhood around focal plants reduced pollen receipt in both years but only nearly significantly so in 2015. In 2016, plants were significantly pollen-limited; hand-pollination significantly increased seeds per fruit for both hermaphrodites and females. However, the reduction of pollen around focal plants did not exacerbate pollen limitation for either hermaphrodites or females. CONCLUSIONS Our results suggest that plants tolerate significant consumption of pollen by thieves and pollinators by producing ample pollen to feed both and fertilize available ovules. Our results demonstrate that pollen limitation in P. foliosissimum is driven by lack of effective pollinators rather than lack of pollen. Teasing out these effects elucidates the relative importance of drivers of reproductive success and thus the expected response to selection by different floral visitors.