Do Pathogens and Herbivores Regulate Plant Populations in Their Native and Introduced Ranges? A Test of the Enemy Release Hypothesis

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Invasive plants (aggressive, non-native weeds) cause several billion dollars per year in losses to agriculture and rangelands in the United States alone. Invasions also threaten the conservation of many native species and are a threat to biodiversity. This project will test one of the major hypotheses for why some plants are invasive: The enemy release hypothesis (ERH). The ERH states that when a plant is introduced to a new habitat, it grows better than native plants do because its enemies (herbivores and pathogens) are not present in the new habitat. A key untested assumption of the ERH is that herbivores and pathogens decrease population size in the native range but not in the invasive range. Enemy removal experiments and field surveys will be used to test whether enemies actually regulate plant populations for six grass species where they are native (Switzerland) and where they are invasive (Oregon). This project will provide financial support for students and will be integrated into two courses taught by the PI. Public outreach will be furthered through the training of volunteers and through public lectures. Work on invasive species is of great interest to federal agencies and conservation groups. We have been working with staff from the following: Finley National Wildlife Refuge, the Nature Conservancy, Lane County Parks, Army Corps of Engineers, BLM and the West Eugene Wetland Partners. Results will be disseminated to these agencies as well as in peer reviewed publications.