Up High, Hot and Dry: Individual Reproductive Output in Subalpine Bees Declines With Increasing Drought Severity

Wild bees play indispensable ecological roles in many ecosystems, yet declines in many species have been documented in recent years. These declines have been linked to numerous anthropogenic pressures, including climate change, which can influence bee populations directly (i.e., via physiological mechanisms) and indirectly (e.g., through changes to floral resource quantity and quality). Widespread changes in the distributions and abundances of many bee species have been linked to climate change at large spatial scales; yet, the local-scale processes that generate these broad-scale patterns are little known. Specifically, it is unclear how climate change influences the fitness of individual bees, despite the fact that broad-scale patterns of decline must be driven by individual-level processes. Our study examines the individual fitness responses (i.e., reproductive output) of five solitary bee species in the genus Osmia to variations in local climate variables in subalpine habitats. Over nine years, we tracked brood cell production in over 700 individual female bees, conducted repeated surveys of floral abundance (while taking into account the specific floral host taxa used by each bee species), and recorded local precipitation and temperature at seven sites. Our data revealed both direct and indirect (floral resource-mediated) negative effects of drought: the total number of brood cells produced by nesting females decreased significantly in summers characterized by greater drought severity and high temperatures—a relationship-driven partly by significant decreases in floral host-plant availability in hotter and drier years. Our study highlights how the negative consequences of climate change for bee populations can manifest at the level of individuals and the role of floral resources in mediating these relationships. Overall, our results suggest that the increasingly warm and dry conditions forecasted for our study area will have net negative effects on the demographic success of these solitary bees.