Biological invasions threaten native biodiversity and invasions that involve reproductive interference, such as hybridization, can be especially detrimental to the persistence of native species. One general objective of my dissertation research was to determine if the decline of a North American vine, Celastrus scandens, in the eastern portion of its native range is related to reproductive interference from an introduced congener, C. orbiculatus. The second general objective was to examine how anthropogenic factors, such as commerce and alteration of biogeochemical cycles, contribute to invasion by C. orbiculatus. I used genetic markers and field observation to survey individuals across the USA to determine the prevalence of wild hybrids. I also collected seeds from both species and tested the hybrid status of resulting seedlings. Hybrids were widespread in the wild, but only 8% of non-native genotypes surveyed were hybrids. Unidirectional pollen flow was evident, as all 20 hybrids tested had maternally inherited markers indicative of C. scandens. Additionally, 51% of C. scandens seeds were hybrids, compared to 1.6% of C. orbiculatus seeds. Hybridization rate in C. scandens was negatively associated with distance to the nearest staminate C. orbiculatus. I used genetic markers to test the species identity of commercially available plants marketed as C. scandens and found that the majority of samples and named varieties obtained were actually C. orbiculatus. Finally, I used experimental enrichment trials to simulate the elevated atmospheric carbon dioxide and increased nitrogen deposition expected in the future. Biomass allocation in both species responded to carbon dioxide and competition treatments, but only C. orbiculatus responded to increased nitrogen. The results suggest increased phenotypic plasticity in C. orbiculatus, and that the invasive vine may be better able to cope with changes to biogeochemical cycles of the future. Overall, the work in this dissertation reveals factors involved in the decline of C. scandens and successful invasion of C. orbiculatus. Human commerce, changing biogeochemical cycles, and especially reproductive interference likely played a role in the decline of the native vine and spread of the invasive congener in the past, and their influence will likely increase in the future
