Anthropogenic activities are rapidly changing the environment, and species that do not respond face a higher risk of extinction. Species may respond to environmental changes by modifying their behaviors, shifting their distributions, or changing their morphology. Recent morphological responses are often measured by changes in body size. Changes in size are often attributed to climate change, but may instead be due to changes in available resources associated with climate change. The intertwined effects of temperature and resource abundance can be uncoupled in populations of the small carpenter bee Ceratina calcarata, as agricultural land-use has reduced the abundance of wildflower resources available to this bee independent of climate change. We studied how the morphology of this bee has changed over the past 118 years (1902 – 2019) in relation to climate change and past 45 year (1974 – 2019) in relation to land-use. Over this time, summer temperatures increased.We found that male and female size decreased with increasing temperature. Male size also decreased with agricultural expansion. Female size, however, increased with agricultural expansion. These results suggest that rising temperatures correlate with a decrease in female body size, while, opposite to predicted, decreasing local resources may select for increased female body size. These opposing pressures act concurrently and may result in bee extirpation from agricultural habitats if selection for large sizes are unsustainable as temperatures continue to increase. Furthermore, this study emphasizes the need to consider multiple environmental stressors when examining the effects of climate change due to their interactions.
