Climate change and habitat associations at species' range boundaries

Species are more restricted in their habitat associations at their leading-edge range margins where climatic conditions are marginal. Hence they are predicted to broaden their associations in these locations as the climate warms, potentially increasing habitat availability and rates of range expansion. I analysed long-term distribution records (collected by volunteers) and abundance data (UK Butterfly Monitoring Scheme transect data) to investigate how the habitat and host plant associations of two butterfly species that reach their leading-edge range margins in Britain have changed over 40 years of climate warming. The speckled wood (Pararge aegeria) is primarily associated with woodland but its habitat associations vary spatially and temporally. I found that this species has a weaker association with woodland in warmer parts of Britain, particularly in regions with warm and wet summers. Over time, its occurrence outside of woodland has increased most where summer and winter temperatures and summer rainfall have increased the most. Field experiments showed that larval performance is poorer in open (grassland) than closed (woodland) habitats, associated with microclimatic differences between habitats. Thus I conclude that slower population growth rates outside woodland play an important role in driving the observed variation in habitat associations. The brown argus (Aricia agestis) was previously restricted to using rockrose (Helianthemum nummularium) as its larval host plant in Britain, which grows in locations with warm microclimates. I have shown that warmer summers have allowed it to increase its use of Geraniaceae host species, which occur in cooler locations. Geraniaceae species are widespread and so habitat availability has increased substantially for the butterfly, leading to extremely rapid range expansion in this species. Species are broadening their habitat and host plant associations at their leading-edge range margins in response to climate change, resulting in substantial increases in rates of range expansion.