Supplemental Material - 1000 randomly-chosen candidate topologies for the Canadian butterfly phylogeny

Climate change is driving rapid and accelerating shifts in range limits, both poleward expansions and equatorward contractions. However, many species are falling behind the pace of change in their dispersal into newly suitable habitats and now show “climate debts”, lags between predicted and observed range expansions under changing climates. Failure to track changing climates may be due to interspecific interactions such as particular food availability for specialists, abiotic barriers such as mountain ranges, or intrinsic traits such as dispersal limitation. A trait-based analysis of climate change performance would help identify causes of climate debt.To understand the correlates of climate debt within a large clade of organisms we use historical and modern observations of butterflies from western Canada as a case study to construct and project individual climate-based environmental niche models. By comparing projected distributions based on historical records to observed modern distributions we are able to construct estimates of climate debt and evaluate the effect of dispersal ability, diet breadth and a proxy for range size on these species' measured climate debt.High levels of climate debt are accumulating within the butterflies of Western Canada, independently of dispersal ability, diet breadth and phylogeny. Range size emerges as the only variable that significantly reduces climate debt, suggesting that more narrowly-ranged species may be at risk of being squeezed out by both a reduction of suitable habitat in their current range and the failure to colonize newly available habitat. These findings underscore the need to investigate potential landscape-level determinants of climate debt that may be limiting range expansions in this group.