Interactions between non-native plants and their natural enemies in core and marginal populations

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Non-native plants are a serious environmental and economic problem in southern Canada; however, few extend to northern latitudes. The northern range limits of non-natives may be set directly by cold climate, but also may be influenced both positively and negatively by interactions with natural enemies (herbivores and pathogens). Northern populations may gain an advantage by escaping enemies that are themselves unable to disperse to or survive in northern sites; on the other hand, slow-growing northern plants may have trouble replacing tissues lost to any enemies that they fail to escape, reducing their success. In this research, I will investigate whether interactions with natural enemies help to set limits to the distribution of non-native plants; I also will compare these results with effects on native species, to see if natives and exotics are limited by different factors. I will establish field experiments and surveys at Ontario sites along a latitudinal gradient from the Great Lakes to the Hudson Bay Lowlands. At each location, I will measure the effects of damage by herbivores and pathogens, to test five hypotheses: H1: Non-native plants are unable to survive unassisted beyond current northern limits; H2: Marginal populations of non-native plants are less attacked by natural enemies than interior populations; H3: Pathogens of exotics are less prominent in soils of marginal areas than soils of interior areas; H4: Tolerance of herbivory is reduced in marginal vs. interior populations of exotics; H5: Net effects of competition and herbivory on performance differ for marginal and interior populations of exotics. Results will determine if we can improve our ability to predict and manage further spread of exotics with information about their enemies and competitors. As well, understanding processes in marginal populations will help to predict likely responses of invasive species to future climate change.