Invasive species impose severe biological and economic costs worldwide. In forested ecosystems, invasive species supplant native species, resulting in decreased biodiversity. Furthermore, human-mediated disturbances may stress native forest species and benefit invasive species that thrive in disturbed habitats. Anthropogenic changes to the landscape have caused unprecedented increases in white-tailed deer (Odocoileus virginianus) numbers, which has resulted in the degradation of understory native plant communities. We suggest that deer-mediated disturbance to understory communities facilitates the success of invaders in forests. Many North American forests experience both exotic plant invasion and deer overabundance, but the two problems have never been empirically linked. In this paper, we quantify deer effects on native and exotic understory herbs in a western Pennsylvania forest. We show that the percentage of bare ground and the relative abundances of the invasive herbs, Alliaria petiolata and Microstegium vimineum are significantly greater, while native plants are significantly smaller with a lower proportion flowering in deer access vs. deer exclusion plots. Our data support the idea that invader success is due in part to both preferential foraging by deer on native herbs that reduces their size and flowering status and the creation of open patches when deer are present. We conclude that deer overabundance facilitates the success of invasive species in forested ecosystems and that management will benefit by considering these factors jointly.
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