Because forests in the northern temperate region of the United States lack native earthworms, the invasions of European and Asian earthworms pose many questions about their effects on native understory vegetation. Earthworms’ ability to increase leaf litter decay, alter nutrient cycling by mixing the organic layer with mineral soil, and decrease plant species richness leads to concern for an Asian earthworm genus, Amynthas, that was found in a research arboretum in southern Wisconsin in 2013. Amynthas earthworms live in the upper layer of the forest floor, have high dietary flexibility, reduce soil microbial diversity, and create a characteristic granular soil texture, suggesting they may have detrimental effects to understory plants and seedling establishment. In order to understand where Amynthas and other European earthworm species are located in the arboretum and how understory plants may be changing, earthworm and vegetation surveys of the herbaceous layer were performed.
Results/Conclusions
We found a distinct region of Amynthas in 31 of 82 plots through the center of two sugar maple forests surveyed in the Arboretum, stretching north from the potential introductory mulch sites. Plots also contained earthworm species Lumbricus terrestris, L. rubellus, and Apporectodea sp. Leaf litter depth, herbaceous species richness, and tree and shrub seedling number were not significantly different (p > 0.05) in plots with Amynthas versus those without, but the abundances of Amynthas and L. terrestris were negatively correlated (p < 0.001), suggesting specialized habitat preference by earthworms or antagonistic relationships between these invasive species. The lack of apparent negative effects on vegetation and leaf litter from Amynthas may be due to the recentness of its invasion in this location or its preference for soil biota over leaf litter, which may take a longer time period to produce measurable site changes. Although our results do not show substantial impacts of this Asian worm, the understanding of earthworm-based changes in northern forests will inform land use managers and the general public about this new ecological invasion and the potential threat it poses to the Midwest’s hardwood forests.