Novel community interactions following species' range expansions

Global change is causing the reorganization of earth's biota. For example, species are being moved around the globe through trade and traffic, and species are shifting their ranges in response to climate change. Interacting species will not move in concert, however, resulting in altered and novel community interactions. These rapidly-changing interactions could have large effects on biodiversity. The aims of my dissertation are to uncover the effects of range-expanding species on their recipient communities and to examine the responses of range-expanding species to altered community interactions. I studied the recent range-expansion of a phytophagous insect in the context of its community. I found that this species was undergoing increased demographic success in its introduced range. As a result, this species negatively affected the fitness of a threatened, native butterfly by reducing the foliar nutrients of their shared host plant. I then tested a leading hypothesis to explain this species demographic success. The enemy release hypothesis (ERH) predicts that range-expanding species will lose enemies in their new range and will be released from enemy control. This is the first experimental test of the ERH for a range-expanding animal species, and despite popular expectations, I found that enemy release fails in explaining this species success. I also evaluated the ERH as a general mechanism of invasion success in a variety of ecosystems and taxonomic groups. I conducted a meta-analysis of an important, overlooked assumption of the ERH, that enemies negatively affect species in their native range. I found systematic differences in native enemy effects among prey types. These results suggest that the assumption of native enemy effects should not be overlooked and that enemy release is not an equally likely explanation for the success of all invasive species. Overall, my dissertation contributes to our understanding of the community context of range expansions. It highlights that community interactions need to be incorporated into theories, predictions, and empirical studies when assessing the effects of global change on biodiversity. Also, I show that species that move over short distances, such as those undergoing climate-driven range expansions, can undergo demographic release and impact recipient communities.