Supplemental water modulates pollinator visitation on resource-rich plant species
Urban green spaces provide important floral resources for pollinators, and supplemental watering can improve the abundance and quality of these resources. However, increasing frequency and intensity of drought under climate change create tradeoffs between ecological benefits and water conservation. Manipulating the timing and duration of irrigation may offer a way of enhancing floral traits that support pollinator communities while moderating water use. Focusing on two resource-rich native plant species, Phacelia californica and Helenium puberulum, we examined how the addition of supplemental water in spring and summer affects floral traits and pollinator visitation. We examined mechanisms that may govern the effects of supplemental watering on pollinator visitation, namely the number of flowers, nectar volume, nectar sucrose concentration, and corolla length. Overall, plant individuals receiving more water were visited by more pollinators. In both plant species, this was apparently mediated by floral abundance; individual plants with more flowers received more visitors, though effects of watering varied slightly between the two plant species and the insect visitor taxa. Water additions in spring increased pollinator visitation per plant on P. californica, but visitation rates per flower did not differ among treatments. Responses among pollinator groups varied, with effects driven primarily by sweat bees (family: Halictidae) and bumble bees (Bombus species), while honeybee visitation was consistent across treatments. Our results suggest that insects prefer to visit supplementally watered P. californica plants. Because honeybee visitation did not vary by treatment, supplemental water may disproportionately benefit native pollinators, including bumblebees, which are imperiled. Similar to P. californica, Helenium puberulum plants watered in spring and summer had the most flowers and the most floral visitors per plant and per flower. Spring-watering H. puberulum did not have a significant effect on flower production but did increase insect visitation per flower. Overall, our results emphasize that irrigation and other avenues of water addition are key drivers of flower production and insect visitation, such that homeowners who wish to conserve water must weigh effects on pollinators with the need to conserve water in arid climates.