Plant-pollinator interactions and environmental change: effects of experimental changes in phenology and water availability on a montane wildflower

With an estimated 87% of angiosperms depending on animal-mediated pollination, plant-pollinator mutualisms play a key role in maintaining ecosystems and supporting global crop production. Climate-driven changes in flowering phenology and water availability, however, may disrupt these ecologically and economically important relationships. This dissertation investigates the mechanisms through which altered water availability and flowering phenology impact pollination and plant reproductive success of Mertensia ciliata (Boraginaceae).First, I tested the hypothesis that impacts of climate on plant-pollinator interactions operate through changes in water availability, and specifically that such effects occur through alteration of floral attractants. Through a multi-year water manipulation experiment, I found that changes in water availability can impact pollinator visitation through pollinator responses to differences in floral attractants, and that the effects of water on visitation can be non-linear. Seed set, however, increased linearly with water (Chapter 1). By manipulating the onset of flowering, I tested the hypothesis that phenological shifts alter pollination and seed set through changes in the frequency and composition of pollinator visitors. I found that despite a five-fold decrease in pollinator visitation over four weeks, there was no significant difference in conspecific pollen receipt or seed set among phenology treatments. Measurements of single-visit pollinator effectiveness revealed that, on a per-visit basis, each bumblebee transferred more conspecific pollen than did a solitary bee or a fly. Thus, while the total pollinator visitation rate declined over the season, because the proportion of visits by more effective worker bumblebees increased, differences in flowering phenology had no effect on seed set (Chapter 2). Because changes in phenology and water availability are often co-occurring responses to environmental change, I hypothesized that they might have interactive effects on pollination and seed set. Through a factorial experimental manipulation, I found that changes in water interacted with differences in phenology in their effects on pollinator taxonomic composition, but their effects on pollinator visitation and seed set were additive (Chapter 3). Taken together, these results highlight the importance of determining the separate and potential interactive effects that co-occurring ecological responses to environmental change may have on pollination and plant reproductive success