LB 32-268 - Can butterfly thermal physiological traits predict their vulnerability to climate change?

Insects are declining globally, and climate change is among the threats they are facing. Temperatures have increased globally, and future climate scenarios expect them to continue increasing. Therefore, being able to predict species’ vulnerability to climate change is of critical importance when developing conservation plans. Species’ physiological tolerances of temperature could help us predict how they might respond to warmer climatic conditions. Moreover, understanding if closely related species share similar traits could help us infer the climate vulnerabilities of unmeasured species, including rare, endangered, or difficult-to-study species. Here, we test the hypothesis that physiological traits can predict species vulnerability to climate change. We used a long-term monitoring dataset of butterflies across the state of Ohio, USA to examine the relationship between species heat tolerance and their population size trends over the last couple decades. We were able to measure the critical thermal maximum (CTmax; determined by loss of movement) for 17 butterfly species in Ohio and compare this with estimates of their temporal trends in population size. Our prediction was that butterflies with greater heat tolerance would be more buffered against climate change than less tolerant species and thus exhibit small to no declines in population size over time. Interestingly, we found evidence of very low phylogenetic signal in CTmax (λ ~ 0, p = 1), contrasting with many other studies of ectotherm heat tolerance that show high phylogenetic signal in this trait. As a further surprise, we found a significant negative relationship between CTmax and temporal trends in population size (β= -0.0147 ± 0.00508, t = -2.9, p = 0.011), indicating that the most tolerant species are those showing the greatest rate of decline in population size over time. Together, these results suggest that CTmax might be a poor predictor of butterfly species vulnerability to climate change. However, given evidence of other compensatory responses to recent climate change among these particular butterfly populations, i.e. phenological and geographic range shifts, this might suggest a refinement of our approach is necessary when analyzing the relationship between physiological traits and changes in population size.