Climate-driven changes in northeastern US butterfly communities

Climate warming is expected to change the distribution and abundance of many species. Range shifts have been detected in a number of European taxa for which long-term government-initiated or organized-survey data are available. In North America, well-organized long-term data needed to document such shifts are much less common. Opportunistic observations made by citizen scientist groups may be an excellent alternative to systematic surveys. From 1992 to 2010, 19,779 butterfly surveys were made by amateur naturalists in Massachusetts, a geographically small state located at the convergence of northern and southern bioclimatic zones in eastern North America. From these data, we estimated population trends for nearly all butterfly species (100 of 116 species present) using list-length analysis. Population trajectories indicate increases of many species near their northern range limits and declines in nearly all species (17 of 21) near their southern range limits. Certain life-history traits, especially overwintering stage, were strongly associated with declines. Our results suggest that a major, climate-induced shift of North American butterflies, characterized by northward expansions of warm-adapted and retreat of cold-adapted species, is underway.