Assisted colonization in a changing climate: a test-study using two U.K. butterflies

Recent climatic change in temperate regions has been rapid and there is mounting speculation that species are failing to keep track of suitable climate, perhaps necessitating assisted colonization for some species. An inability to spread into new areas may result in large reductions in species’ ranges in the future, and threaten the survival of some species. Here we use “speciesclimate” models to predict suitable sites for introductions beyond current range margins, using two U.K. butterfly species. We introduced Melanargia galathea (marbled white) and Thymelicus sylvestris (small skipper) into two sites in northern England, ∼65 and ∼35 km beyond their then-range margins, respectively, to sites that were predicted to be climatically suitable and that appeared to contain suitable habitat for the species. Both introduced populations grew and expanded their range over 6 years (2001–2006; still thriving in 2008), suggesting the existence of a colonization lag and providing evidence that well-planned assisted colonization can be successful. We suggest that assisted colonization may be a feasible and cost-effective means of enabling certain species to track climatic change.