Rising temperatures and variable precipitation will continue to accentuate aridity globally. The direct effects of drought on plants is well known but surprisingly little information exists concerning how such change affects floral resources and, in turn, visitation by pollinators. Here we experimentally examine how drought affects plant-pollinator interactions under field conditions. Using bee-pollinated squash (Cucurbita pepo), we manipulated precipitation received by plants by irrigating them at two different levels, either control or drought (30% control). For replicate plants in the two experimental groups, we bagged flowers to ensure that plants received pollen only from plants within the same experimental group. Interspersed among bee-pollinated plants were hand-pollinated plants that experienced the same irrigation conditions. We measured the daily production of floral advertisements (i.e. flower size, nectar production, and pollen production) and monitored bee visitation and pollination by both specialist (Peponapis pruinosa) and generalist (Apis mellifera) pollinators. At the end of the season, we measured fruit and seed set for all plants. To test how drought affects the pollination mutualism, we compared response variables between hand-pollinated and bee-pollinated plants.
Results/Conclusions
Hand-pollinated Cucurbita pepo plants produced larger fruits with increasing soil moisture and these fruits were bigger on average than the fruits produced in bee-pollinated plants. Meanwhile, in bee-pollinated plants, seed mass increased with soil moisture resulting in 22% lower mean seed mass in drought plants. This trend was not seen in the hand-pollinated plants. Consequently, bee-pollinated plants overall produced 16% smaller fruits with 11% smaller seeds than hand-pollinated plants. In both hand and bee-pollinated plants, fruit sugar content decreased at lower irrigation levels resulting in 43% lower mean sugar content in drought plants. These results suggest that drought conditions may affect the plant-pollinator mutualism through novel methods that ultimately reduce plant reproductive performance.