Plant-pollinator mutualisms in the face of environmental change: an experimental test with Cucurbita and its bee pollinators

Plant-pollinator mutualisms provide essential services to both wild and cultivated ecosystems across the globe. Yet these important interactions face many environmental threats that could impact the ability of pollinators to effectively mediate reproduction in plant hosts. Climate change is one such threat. Past studies have investigated how abiotic stress can lead to mismatches in phenology and distribution in plant-pollinator relationships, but less research has focused on the effects of altered environmental conditions on plant floral traits. This dissertation centers on the impacts that warming and drought stress have on the production of floral resources, how bee pollinators respond to those changes, and how plant reproduction is ultimately affected. For this work, we utilized the Cucurbita system, including both cultivated squash (Cucurbita pepo) and free-living gourd (C. foetidissima), which is pollinated by generalist honey bees (Apis mellifera) as well as specialist squash bees (Eucera). In Chapter 1, we examined how the combined effects of warming and drought altered the C. pepo system and found that water stress increased pollen limitation in bee-pollinated plants due to (i) decreased pollen competition caused by low-levels of stigmatic pollen deposition, (ii) reduced viability of pollen produced by plants grown under low soil moisture conditions, and (iii) a reduced capacity of self-pollinated fruits to increase seed set in response to increasing soil moisture. In Chapter 2, we investigated how generalist and specialist bees responded when given a choice between C. pepo plants grown at varying soil moistures, and discovered that only generalist honey bees increased visitation with plant soil moisture, thereby increasing deposition of pollen from well-watered plants and increasing seed set. In Chapter 3, we focused on comparing the effectiveness of honey bees and squash bees as pollinators of both C. pepo and C. foetidissima and assessed that, overall, squash bees removed more pollen, deposited more pollen, and contributed more to fruit set and seed set on both Cucurbita species. The results of this dissertation may be applied to other plant-pollinator systems and reveal that the contributions of generalist and specialist pollinators to pollination services may be impacted in an altered climate