Local and landscape management of an expanding range margin under climate change.
There is a pressing need to understand how to facilitate species' range shifts under climate change. However, few empirical studies are available to inform decision-making, particularly at fine spatial and temporal resolutions. We present a case study of a thermally constrained habitat specialist, the silver-spotted skipper butterfly Hesperia comma, at its expanding range margin in south-eastern Britain. Using data from 724 habitat patches over a 9-year interval (2000-2009), we examined local and landscape determinants of colonization, survival and population density. We then predicted probabilities of colonization and survival for habitat patches under the observed 1982 and 2009 distributions to investigate how the factors limiting range expansions change through space and time. Between 2000 and 2009, Hesperia comma continued to expand its range in Britain, but the 67 recorded colonization events were offset by 48 local extinctions. Extinctions were strongly linked to climate, occurring predominantly in cooler regions and on shaded north-facing slopes. Population density and probability of survival were closely related to conditions within a site, whereas probability of colonization was largely determined by functional connectivity. Survival probability was also influenced by connectivity, suggesting that immigration helped to support extinction-prone populations (a 'rescue effect'). Patch occupancy beyond the range margin was primarily constrained by colonization, but close to the expanding front, population survival became the key limiting factor. This pattern was conserved during range expansion, altering management priorities at individual sites. Synthesis and applications. Previous studies on facilitating range shifts have stressed the need to increase landscape-scale connectivity to remove constraints on colonization, and our data substantiate this advice. However, we show that enhancing population survival can also help to facilitate range expansions, because populations at leading range edges face high extinction risk. Population survival can be improved directly through local management actions, such as enlarging patch size and increasing habitat quality, or indirectly by improving connectivity. Thus, local management can secure vulnerable populations at the range edge and provide larger and more stable migrant sources for future expansion and deserves consideration when facilitating range shifts under climate change.