The ecology of invasions in a Minnesota grassland: Characteristics of invasive species and invaded communities and the effects of global change

Biological invasions are causing dramatic changes in the distribution and abundance of species around the world. Invasions are one of the leading threats to conservation of native species, yet our ability to predict which species will be aggressive invaders and where they will invade is unreliable at best. Using both experiment and observation, I examine the characteristics of invasive species and invaded communities in an effort to improve predictive ability for invasions.; One particularly aggressive invader in much of North America, Hieracium aurantiacum, has invaded areas occupied by a rare, native congener H. longipilum. Using these species as a model system, I compare the traits of the native and invasive congeners, testing for differences in competitive ability under ambient and enhanced nitrogen conditions (greenhouse competition experiments), differences in response to herbivores and pathogens (field exclosure experiments), and differences in morphology (field observations) that might contribute to the success of the invasive species. The results indicate that H. aurantiacum is not a superior competitor to H. longipilum, but its success in invasion may be explained partly by enemy escape through invasion of a new range and partly by its unique morphological traits.; Biological invasion requires not only a successful invader but also a susceptible community. I report the results of an observational study, in which I examined the characteristics of the plant neighborhoods where invaders were successfully established. Invaders achieved greater sizes in neighborhoods with less crowding (fewer neighbors of smaller size) and lower species diversity.; Global change may further exacerbate the problem of biological invasions by creating conditions that are favorable for invasive plants. I report the results of a study that examined the effects of co-occurring changes in atmospheric carbon dioxide, nitrogen deposition, and species richness. This experiment indicated that different invaders may have distinct responses to global change factors, with some species benefiting from global change and others showing a negative response.