Exchanges of enemies between native and non-native plants

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Escape from native-range natural enemies (herbivores and pathogens) often is argued to increase the invasiveness of non-native plants, as described by the Enemy Release Hypothesis. However, once in a new region, exotic invaders acquire new enemies recruited from invaded-range species. As well, enemies from the invader's original range (including biocontrol agents) may be accidentally or deliberately co-introduced; these species also may attack local natives. As a result, few invaders are free of natural enemies, many of which are shared with co-occurring native plants.***Shared enemies can have significant impacts on invaders, reducing their success through biotic resistance. As well, shared enemies may participate in feedbacks on natives through spillover (spread of introduced enemies from exotic hosts to natives) and spillback (spread of native enemies to exotics followed by spread back onto their original hosts); these feedbacks constitute an important but underexplored component of invaders' ecological impacts. However, not all invasive populations may be equally affected by such interactions. First, the probability of exchange is likely to be lower between more distantly-related natives and exotics. Second, plants that are spatially isolated from sources of enemies may experience reduced exchange. Third, populations of enemies may vary regionally, resulting in geographic trends in exchange. Fourth, even if enemies are shared, tolerance to damage may reduce their effects in favourable habitats. Finally, impacts such as herbivory are likely to be most critical for small populations of invaders; this is the stage at which most unsuccessful invasions fail.***Here, I propose to investigate key factors that affect the exchange of enemies between natives and exotics, and the consequences for the performance of both. We will experimentally test the importance of phylogeny, spatial isolation, and geographic variation for shared enemies, will evaluate the role of tolerance in mitigating these impacts, and will investigate factors affecting the success of small populations. By illuminating the complex web of interactions surrounding invasive plants, this novel research will clarify the importance of shared enemies to the spread of exotics and their impacts on co-occurring natives, and will contribute to my long-term objective of understanding interactions between invasive plants and their natural enemies.**