Resident plant diversity and introduced earthworms have contrasting effects on the success of invasive plants
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Theoretical predictions and empirical studies suggest that resident species diversity is an important driver of community invasibility. Through trait-based processes, plants in communities with high resident species diversity occupy a wider range of ecological niches and are more productive than low diversity communities, potentially reducing the opportunities for invasion through niche preemption. In terrestrial plant communities, biotic ecosystem engineers such as earthworms can also affect invasibility by reducing leaf litter stocks and influencing soil conditions. In a greenhouse experiment, we simultaneously manipulated resident species diversity and earthworm presence to investigate independent and interactive effects of these two variables on the success of several invasive plants. Higher diversity of resident species was associated with lower biomass of invasives, with the effect mediated through resident species biomass. The presence of earthworms had a strong positive effect on the biomass of invasive species across all levels of resident species diversity and a weaker indirect negative effect via decreased soil moisture. Earthworms also weakened the positive correlation between resident species diversity and productivity. We did not observe any interactive effects of resident species biomass and earthworms on invasive species success. Partitioning the net biodiversity effect indicated that selection effects increased with resident species diversity whereas complementarity effects did not. Results suggest that managing for diverse forest communities may decrease the susceptibility of these communities to invasions. However, the presence of introduced earthworms in previously earthworm-free sites may undermine these efforts. Furthermore, future studies of plant community invasibility should account for the effects of introduced earthworms.
KeywordsBiodiversity–ecosystem function Functional diversity Invasibility Lumbricus terrestris Rhamnus cathartica
We thank Cindy Buschena and Susan Barrott for their help during the experiment and Forest Isbell for assistance with data analysis. Also, we thank the two anonymous reviewers whose comments helped improve the manuscript. In addition, we thank Warner Nature Center for allowing us to collect soil and leaf litter for the experiment. For assistance during the experiment we thank the undergraduate student workers in the Reich lab. This project was supported by a grant from the Legislative Citizen Commission on Minnesota Resources (M.L. 2010, Chp. 362, Sect. 2, Subd. 6c “Healthy Forests to Resist Invasion,” to PBR). Support was also provided by the Integrative Graduate Education and Research Traineeship: Risk Analysis for Introduced Species and Genotypes (NSF DGE-0653827); University of Minnesota Graduate School; the Wilderness Research Foundation; and the Dayton Fund of the Bell Museum of Natural History. NE gratefully acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG; Ei 862/1, Ei 862/2).
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