Understanding drivers of plant invasions is essential to predict and successfully manage invasions. Across forests in North America, increased white-tailed deer (Odocoileus virginianus) abundance and non-native earthworms may facilitate non-native plant invasions. While each agent may exert independent effects, their impacts are often compounded by the presence of other stressors. Using a network of twelve forested sites that differ in earthworm density, we evaluated deer exclusion effects (30 x 30 m fenced plot; with an adjacent similar sized unfenced control plot) on abundance of non-native earthworms, and on abundance of three non-native plant species: Alliaria petiolata, Berberis thunbergii and Microstegium vimineum. In addition, through a combination of seeding, seedling transplant and mark-recapture experiments at the same study sites, we quantified the single and combined effects of each factor on populations of four native plant species.
Results/Conclusions
Deer exclusion, through fencing, resulted in decreased earthworm abundance and in decreased frequency or growth of the three target invasive plant species. At our sites deer presence, invasive plants and earthworms were positively associated, such that sites dominated by non-native vegetation had less acidic soils and higher earthworm abundance compared to sites dominated by native vegetation. Strong associations among these three stressors generate interactive and unpredictable effects on native plant demography. Deer impacts on native plants were consistently negative and were significantly stronger than effects of other stressors. Invasive plant species effects were non-significant or positive (increased seedling emergence and growth under M. vimineum). Furthermore, at our sites, B. thunbergiiring growth decreased over time, in open relative to fenced plots. In this system, deer emerge as the driving force benefiting non-native earthworms and non-native plants while negatively affecting native plant populations. Therefore, reducing deer density at the landscape level is necessary to both decrease non-native plant and earthworm abundance and to protect native flora. Overall, results show invasive plant species cannot be understood in isolation from other confounding factors. Management activities should avoid focusing on non-native plants, unless a clear link to poor performance of native species has been established.