Relative roles of generalist vs. specialist herbivores in plant invasions

Non-native species are widely suspected to thrive by escaping their coevolved specialist enemies. However, after escaping their specialists, most introduced species gain evolutionarily novel, generalist enemies that can lessen their invasive success. The net effects of these processes, however, are rarely considered. Here, we describe experimental and synthetic analyses of the relative impacts of generalist versus specialist herbivores on exotic plant success. We conducted a whole ecosystem, manipulative field experiment testing the relative effects of generalist and specialist consumers on the invasive plant Eurasian watermilfoil Myriophyllum spicatum. Generalist Canada geese significantly suppressed the % cover and stem density of Eurasian watermilfoil early but not late in the growing season, whereas both crayfish (generalists) and weevils (specialists) had negative but non-significant effects on Eurasian milfoil. Thus, in a large-scale field experiment, generalist vertebrate herbivores had larger negative impacts on invasive plant success than did specialist insects. These results suggest that native herbivores can provide Biotic Resistance to exotic plant invasions, and that generalist vertebrates can have stronger impacts on plant invasions than invertebrates. Additionally, synthetic analyses across several species show that enemy pressure in the introduced range is often comparable to that in the native range, further suggesting that most non-native plants thrive despite facing considerable herbivory, and that invasion success may result from factors other than enemy release alone.