Native plant diversity and introduced earthworms have contrasting effects on the success of invasive plants

Background/Question/Methods

Theoretical predictions and empirical studies suggest that species diversity is an important driver of community invasibility. Through trait-based processes, communities with high resident species diversity occupy a wider range of ecological niches and are more productive, reducing the opportunities for invasion. Other biotic factors, e.g., invasive earthworms in terrestrial ecosystems, can also affect invasibility. Earthworms are known to reduce leaf litter and increase the likelihood that previously earthworm-free forest stands will be invaded by introduced plants. Previous studies have documented independent effects of plant diversity and earthworms on invasive species success but it remains unexplored how the two will interact to affect plant invasion. We investigated the following questions:

(1) Does increased resident plant functional diversity reduce the success of invaders?

(2) Does the presence of earthworms increase the success of invaders?

(3) Do earthworms and resident species diversity interactively affect the success of invaders?

In the greenhouse, we simultaneously manipulated resident species diversity (one species/one functional group, two species/one functional group, two species/two functional groups, six species/three functional groups) and earthworm presence (one anecic species) to investigate independent and interactive effects of these two biotic variables on the success of invasive taxa planted into previously established microcosms.

Results/Conclusions

Higher diversity of resident species was associated with lower biomass of invasives due to selection (p=0.01), not complementarity effects (p=0.06). The presence of earthworms increased the biomass of invasive species across all levels of resident species diversity (R² = 0.37, p<0.001). The best-fit general linear model predicting invasive species biomass included resident species biomass and soil moisture together with the presence of earthworms (R²= 0.62). Path analysis showed that resident species diversity had an indirect negative affect on invasive species by increasing total resident species biomass and decreasing soil moisture. Also, earthworms had a strong direct positive effect on invasive species biomass and a weaker, indirect negative effect via decreased soil moisture. Earthworms also weakened the positive correlation between resident species functional diversity and productivity across all levels of diversity.

The results suggest the presence of earthworms in previously earthworm free sites may undermine management efforts that attempt to maximize resident species diversity in order to reduce the susceptibility of plant communities to invasive species. Furthermore, earthworms may exacerbate the effects of reduced resident species diversity caused by climate change, habitat fragmentation, high density of white-tailed deer, and large-scale changes in disturbance regimes.