See the paper: Impacts of European Earthworms on Maple-Basswood Forests

Background

Did you know that earthworms are not native to Minnesota? While there may be some evidence that North American earthworms existed, the glaciers eradicated any historic species of earthworms, and Minnesota’s ecosystems developed without their presence. All species of earthworms in Minnesota now have been introduced from Europe and Asia, and we are left questioning how these non-native species are impacting our ecosystem on a broad scale and locally. 

Healthy hardwood forests have a layer of partially decomposed leaves and other plant material on the ground. This layer is important for new plants and seeds to be able to place roots and get the right amount of nutrients. Introduced earthworms eat much of this layer and make it difficult for new plants to grow. But other species, like people and deer, affect this layer of vegetation, and not all forests are impacted by earthworms in the same way. How might earthworms be impacting our forests in southeastern Minnesota? A student at St. Olaf sought to investigate this very question in 2001. 

The Study

Amanda Ista (Class of 2001, B.A. Environmental Studies) chose three sites to study in separate hardwood forests in Rice County. One in Nerstrand Big Woods State Park, one in Cannon River Wilderness Area, and another in the St. Olaf Natural Lands. Ista looked at the types of plants growing in each site, took soil samples and analyzed their chemical composition, measured the amount of organic matter on the forest floor, and counted the number of earthworms present. All of these data are important to help establish a baseline condition for an area. 

The Findings

Ista found that compared to the other sites, the St. Olaf Natural Lands (areas in Norway Valley and Heath Creek) had the highest number of earthworms: approximately 150 per square meter. St. Olaf also had the lowest vegetation density, or the lowest amount of leafy plant species growing on the forest floor. This suggests that in the Natural Lands, earthworms are decreasing the ability of understory plants to grow. 

Ista’s study also found that the soil pH (the acidity of the soil) in Heath Creek and Norway Valley was within optimal levels for earthworms to live in. 

Relevance

Over 20 years later, earthworms still populate Minnesota and are found to have a massive impact on our forests. Learn more about Minnesota’s earthworm populations here and how you might be able to help stop their spread. 

Citation

Ista, A. 2001. Impacts of European Earthworms on Maple-Basswood Forests. St. Olaf College Local Ecology Papers.