Climate and habitat availability determine 20th century changes in a butterfly's range margin

Evidence of anthropogenic global climate change is accumulating, but its po tential consequences for insect distributions have received little attentio n. We use a 'climate response surface' model to investigate distribution ch anges at the northern margin of the speckled wood butterfly Para, ge aegeri a. We relate its current European distribution to a combination of three bi oclimatic variables. We document that P. aegeria has expanded its northern margin substantially since 1940, that changes in this species) distribution over the past 100 years are likely to have been due to climate change, and that I? aegeria will have the potential to shift its range margin substant ially northwards under predicted future climate change. At current rates of expansion, this species could potentially colonize all newly available cli matically suitable habitat in the UK over the nest 50 years or more. Howeve r, fragmentation of habitats can affect colonization, and Mle show that ava ilability of habitat may be constraining range expansion of this species at its northern margin in the UK. These lag effects may be even more pronounc ed in less-mobile species inhabiting more fragmented landscapes, and highli ght how habitat distribution will be crucial in predicting species' respons es to future climate change.