How phenotypes respond to changing environments is a key open question in ecology and evolutionary biology. To forecast responses to future change, we need to understand both the mechanisms underlying responses, and the specific aspects of the environment to which phenotypes respond. Here, I approach these topics using case studies of butterfly responses to global change, focusing on an assemblage of twenty butterfly species across the state of Ohio, USA. I explore how a complex phenotype, the timing of life cycle events (phenology), responds to gradients of urban land-use change and the elevated environmental temperatures associated with urban heat islands. I further explore whether the background climate of the urbanization gradient impacts the magnitude and direction of phenological responses to urbanization.
Environmental context—the background climate of the urbanization gradient—significantly and predictably impacted the phenological response to urban warming for many butterfly species. Nearly half of the Ohio butterfly species assemblage exhibited phenological delays (potentially indicative of a stress response) in relatively extreme environments with warming from both climate and land-use change, but exhibited more typical responses of phenological advancement in environments with either warming due to background climate or warming due to land-use change alone. Mechanistically, this suggests an important role for non-additive effects of multiple sources of environmental change on plasticity in phenology across a large fraction of the butterfly assemblage. Together, these results further support the idea that quantifying relevant environmental attributes and identifying the mechanisms underlying responses are essential to forecast responses to future environmental change.