Ecological and evolutionary processes at expanding range margins

Many animals are regarded as relatively sedentary and specialized in margin al parts of their geographical distributions(1,2). They are expected to be slow at colonizing new habitats. Despite this, the cool margins of many spe cies' distributions have expanded rapidly in association with recent climat e warming(3-10). We examined four insect species that have expanded their g eographical ranges in Britain over the past 20 years. Here we report that t wo butterfly species have increased the variety of habitat types that they can colonize, and that two bush cricket species show increased fractions of longer-winged (dispersive) individuals in recently founded populations. Bo th ecological and evolutionary processes are probably responsible for these changes. Increased habitat breadth and dispersal tendencies have resulted in about 3- to 15-fold increases in expansion rates, allowing these insects to cross habitat disjunctions that would have represented major or complet e barriers to dispersal before the expansions started. The emergence of dis persive phenotypes will increase the speed at which species invade new envi ronments, and probably underlies the responses of many species to both past (11) and future climate change.