SD10: Variation in thermal tolerance in native bees on an urban-rural gradient

Conversion of land into urban areas increases impervious surface cover, leading to greater heat retention and warmer temperatures, a phenomenon known as the urban heat island effect. Understanding how ectothermic animals like native bees respond to these novel environments is necessary given the intensification of urbanization, climate change, and concerns about declines in native bee populations. Responses to urbanization and warming temperatures involve changes in behavior, morphology, and thermal tolerances. Understanding variation in species’ responses to environmental change is important in capturing ecosystem function and stability. We studied intraspecific variation in morphological and physiological traits, namely, critical thermal maxima (CTmax; the temperature above which an organism cannot function) in native bees as they respond to urbanization. The response of CTmax to impervious surface cover varied across bee species. For example, Melissodes bimaculatus and Ptilothrix bombiformis showed a positive relationship between CTmax and impervious cover, but Lasioglossum (Dialictus) spp. showed a complex, nonlinear relationship. Within M. bimaculatus, we had enough replication to robustly test for differences between female and male bees. We found that CTmax for both sexes showed nearly the same response to impervious cover. In contrast, the CTmax of female M. bimaculatus increased throughout the flight season, but we found no similar change for males. Variation in CTmax and in traits that affect temperature regulation may help buffer some of the negative effects of temperature change, which has implications for understanding how bee populations are responding to increasing urbanization and how they may respond to global climate warming.