Insufficient pollen receipt is a predominant driver of pollen limitation and can be caused by a paucity of pollinators, or an excess of inefficient pollinators. However, mechanisms underlying insufficient pollen receipt are often unclear. We ask whether levels of pollen limitation are associated with heterogeneity in the pollinator community across the range of the protandrous herb, Campanula americana. We then measure visitation rates to male and female phases of the flower for each of the dominant pollinator classes, and assess single-visit pollination efficiency for female and male aspects of plant fitness. Finally, we combine visitation rates, sex-biased floral visitation behaviors, and single-visit efficiencies to quantify how the dominant pollinators contribute to depletion of pollen in natural populations. Together, these experiments elucidate how pollinator visitation behaviors and efficiencies drive variability in pollen limitation.
Results/Conclusions
Small bees (largely from the Halictidae), the intermediate-sized bellflower resin bee (Megachile campanulae), and bumblebees are the dominant flower-visitors. Visitation rates by these three groups explains over 60% of the variation in pollen limitation among populations. Range-wide, visitation from bumblebees and Megachile reduces pollen limitation, but more visitation from small bees exacerbates reproductive failure. Small bees and Megachile preferentially visit pollen-bearing male-phase flowers, whereas bumblebees equally visit males and females. Bumblebees deposit the most pollen on stigmas and affect the most seed production per visit, while Megachile removes the most pollen per visit to male flowers. However, high visitation rates and strong male-biased visitation of small bees leads to them depleting the most pollen from natural populations. Thus, the presence of small bees reduces the amount of pollen available for transfer by more efficient bumblebee pollinators, suggesting they may be parasitic to C. americana due to their high rates of pollen removal. These results highlight the importance of bumblebees for reproductive success in wild plants and suggest that visitation from native solitary bees may not make up for reproductive failure incurred by bumblebee declines.