The impacts of partner abundance on benefits from facultative pollination mutualism

Abstract

Partner abundance affects costs and benefits in obligate mutualisms, but its role in facultative partnerships is less clear. I investigated this topic in a small alpine pollination web in the Colorado Rocky Mountains consisting of two clovers, Trifolium dasyphyllum and T. parryi, that vary in specialization on a shared bumblebee pollinator, Bombus balteatus. I examined a) foraging choices of queen B. balteatus among the clovers and explanatory mechanisms behind observed foraging biases, b) how intraspecific and interspecific variation in pollination niche breadth impacts individual- to population-level plant responses to manipulated pollinator density, c) habitat-scale relationships between natural bumblebee colony abundance and clover reproductive rates, and d) the broader impacts of this research in the setting of an outreach program using pollinator gardens at a local high school. Results showed that architectural trait differences between the clovers leading to differences in foraging efficiency likely mediate preference of B. balteatus for T. parryi. Because of bees' preferences and a dearth of co-pollinators, T. parryi benefits more than T. dasyphyllum from increases in B. balteatus density at the individual plant level, at life stages linking individual success with population growth, at population levels, and across habitats; however benefits even for T. parryi are not unlimited. In addressing the broader impacts of this research in a K-12 setting, I found that components of the participant teachers' beliefs about using outdoor classrooms mediated the implementation and outcome of planned activities.