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Environmentally-contingent behaviour of invasive plants as drivers or passengers

Wilson, S. D. and B.D. Pinno. 2013. Oikos


Invasive plants are often regarded as drivers that actively reduce diversity and alter ecosystem processes such as succession. Alternatively, invaders may be passengers that simply colonize openings produced by anthropogenic disturbance and are present only temporarily. Here we test whether the behaviour of invasive species as drivers or passengers is contingent on disturbance and nutrient availability. We created twelve experimental environments (three levels of annual disturbance four levels of nitrogen availability) for 18 years in a grassland at the northern edge of the North American Great Plains. Out of 19 invasive species initially present, two, perennials (Bromus inermis, a grass, and Cirsium arvense, a forb) acted as drivers, maintaining or increasing dominance, maintaining low species richness, and forming an invader- dominated successional sequence. Behaviour as drivers was environmentally contingent: Bromus behaved as a driver only in less disturbed environments, and the tendency of Cirsium to behave as a driver increased significantly with both disturbance and nitrogen availability. Most invasive species (90%), however, consistently behaved as passengers, disappearing or becoming rare. Th e importance of disturbance and fertility for starting invasions is well-known, but our study shows that these factors also contribute to the behaviour of some invaders as drivers. The emergence of drivers and invader-dominated successional sequences suggests that, as rates of invasion, disturbance and eutrophication continue to increase with human activity, invasive species that act as drivers may form low-diversity communities that persist for decades.