Species redistribution, not adaptation, drives butterfly responses to climate change

Climate change is a growing threat to global biodiversity, and so there is a pressing need to understand which traits impact species vulnerability, and whether traits can adapt to these changes. We quantified two thermal traits (thermoregulation, thermal tolerance) and functional traits (wing length, colouration, wing condition, sex) across an elevational gradient in central Europe to investigate adaptation in thermal traits or the functional traits that influence them to local climatic conditions. We found no intraspecific adaptation in thermal traits, suggesting that these are fixed within species. At low elevations, large species were better at avoiding high body temperatures relative to small species. Dark species had slower heating rates but maintained consistently higher body temperatures relative to pale species. This implies that small and dark species will be particularly vulnerable to extreme heat. We subsequently detected shifts from small dark species at high elevations to large pale species at low elevations. Finally, species with poor thermoregulatory performance or larger wings had greater upwards elevational range shifts. This demonstrates that butterflies are responding to climate change through redistribution of species rather than adaptation in place, and that thermoregulatory performance will be a key driver of responses to climate change.