Non-native species pose serious threats to native communities and are a top cause for species endangerment worldwide. We studied the effects of two non-native and invasive plant species (Asian Stilt grass (Microstegium vimineum) and garlic mustard (Alliaria petiolata)) and exotic earthworm communities on understory forest diversity in Southwestern Virginia. We tested the competing hypotheses of The Rich Get Richer and Diversity Resistance on forest understory and earthworm communities. And we looked for evidence of exotic species facilitation (i.e., the Invasion Meltdown Hypothesis). We surveyed plant and earthworm communities in eight forests in 2007 - 2009. These sites ranged from small suburban fragments (<10 ha) to large nature reserves (5,100 ha).
Results/Conclusions
After three years of sampling, earthworm communities in our study sites appear to be comprised solely of exotic species. We found positive correlations between exotic earthworm richness and abundances and midstory and canopy tree richness. Further, exotic plant species richness was greater within areas of dense M. vimineum and A. petiolata populations compared to areas without these invasive plant species (Paired T-tests, P < 0.01; N= 12). Lastly, we found a negative correlation between native plant species richness and invasive plant species abundance (linear regression; r2 = 18.3, P < 0.05, N = 24). Taken together, our results suggest that invasive plants and exotic earthworms may facilitate each other similar to the Invasion Meltdown mechanism and our sites seem to exhibit Biotic Resistance where high native species richness inhibits invasions.