Inter-trophic invader interactions: disturbance and seed dispersal by an introduced herbivore facilitates replacement of native forbs with introduced grasses.
Best, Rebecca*,1, 1 University of British Columbia, Vancouver, BC, Canada
ABSTRACT- Introduced herbivores can establish novel disturbance regimes, impose novel top-down interactions, and serve as novel dispersal vectors. All of these forces may in turn affect the relative success of native and introduced plant species. I used the recent expansion of giant Canada geese (Branta canadensis) into an endangered island ecosystem still relatively high in native biomass to investigate the potential for inter-trophic facilitation between introduced species. I found a clear pattern of increased dominance by palatable introduced annual grasses and reduced richness and biomass of native forbs in areas heavily disturbed by geese. I also used open and exclosed plots across this disturbance gradient to identify processes directly linking goose impacts to this pattern in plant community composition. For example, some short term changes in reproductive output resulting from altered growth habit under grazing appear to be consistent with this pattern. In addition, results from a greenhouse experiment investigating the interaction between competitor identity and nutrient addition suggest that goose nutrient cycling may enhance the competitive advantage of introduced annual grasses relative to other functional groups in the community. Finally, the results of germination trials show an extension of the positive feedback cycles associated with grazing, nutrient cycling, and forage biomass. Not only can nutrients in goose feces benefit their preferred forage species, Poa annua; geese can also transport Poa seeds in their guts. Because this introduced grass is commonly found in cultivated areas where geese feed before flying out to the islets to nest, geese likely represent a significant vector for its colonization of the islets. In total, grazing, nutrient cycling, and dispersal by introduced geese all appear to facilitate the invasion success of introduced grasses in this system.
Key words: invasion, islands, grasses
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