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

Conversion of land into urban areas alters the landscape in many ways, predominantly in the proliferation of concrete and other impervious materials. This increase in impervious surfaces results in greater heat retention and also coincides with a decrease in green spaces—which help to keep air cool by evapotranspiration—leading to warmer temperatures in urban areas, a phenomenon known as the urban heat island effect. An understanding of how ectothermic animals like native bees respond to such 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, phenology (i.e., flight season), 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 traits as well as physiological ones, namely, critical thermal maxima (CTmax; the temperature above which an organism cannot function), in native bees as they respond to urbanization. We found that CTmax did not significantly change along the urbanization gradient. Furthermore, female and male bees had similar CTmax overall, but there was substantial variation across species. For example, female Melissodes bimaculatus and Ptilothrix bombiformis had 5-10% higher CTmax than males. Variation in CTmax and in traits that affect temperature regulation may help to buffer some of the negative effects of temperature change, which has implications for understanding how native bee populations are responding to increasing urbanization and how they may respond to global climate warming.