Biotic interactions and invasions by non-native plants

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Biological invasions are a multi-billion dollar problem worldwide, and one of the most serious threats to biodiversity, but predicting them has proven notoriously difficult. Ecological theory and recent empirical studies suggest interactions between plants and their natural enemies (herbivores and pathogens) may play a key role in many biological invasions. The Enemy Release Hypothesis (ERH) proposes that non-native plants escape their natural enemies when colonizing new regions, providing a critical advantage over native plants. The Evolution of Increased Competitive Ability Hypothesis (EICAH) suggests exotic plants therefore can reallocate resources from defence to growth and reproduction, increasing their ability to displace natives. The Biotic Resistance Hypothesis (BRH) proposes that the invasiveness of exotic plants may be reduced by competition or predation by species native to the invaded area. We are testing these hypotheses with field experiments on a broad set of plants in both their native and non-native regions. As well, we are using ragweed (Ambrosia artemisiifolia), a serious crop weed and allergen native in Canada but invasive in Europe, as a model for intensive study. Predictions include: P1: Exotic plants suffer less damage by natural enemies in invaded areas than in native areas (ERH) P2: Exotic plants suffer less damage by natural enemies than do natives (ERH) P3: Negative interactions with enemies and competitors will be greater for exotics more closely related to natives (ERH, BRH), and will increase (for enemies - ERH) or decrease (for competitors - EICAH) with time in new regions P4: Lower impacts of enemies and competitors result in greater fitness, population growth, and invasiveness of exotics (ERH, BRH, EICAH). Few such experimental tests of invasion theory yet have been undertaken, and fewer have focussed on Canadian ecosystems. Results may suggest strategies for management of this serious problem.