Abstract
Plants often lose natural enemies (herbivores and pathogens) while invading new geographic regions, as predicted by the Enemy Release Hypothesis. However, a similar reduction in attack might occur at a more local scale within an invader's range as plants in marginal areas escape enemies that fail to find them or cannot maintain local populations. In this study, we test the hypothesis that isolated populations near the northern edge of an invader's range escape the enemies present in more southern populations, using the non-native monocarpic biennial, common burdock (Arctium minus), as a model species. In southern Ontario, this plant is attacked by a wide range of insect herbivores, including generalist leaf chewers as well as leaf-mining flies (Liriomyza arctii, Calycomyza flavinotum) and an abundant lepidopteran seed predator (Metzneria lappella). Surveys over an 815 km transect from temperate southern to boreal northern Ontario indicate that damage by all of these enemies declines sharply with latitude, while plants in more northern areas are slightly larger and more fecund. Critically, seed parasitism drops from more than 85 % in the south to less than 25 % in the north. These results indicate that populations of Arctium near this species' northern limit escape many of their usual natural enemies, potentially counteracting less favourable environmental conditions. Such escape from enemies near invaders' range margins may accelerate further spread, including expected migration in response to climate change.