Earthworm Invaders

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Capital News Service
LANSING – Compost box heroes, or the root of all ecological evil? Earthworms in Great Lakes forests are not what they seem.
Trilliums are smaller, algal blooms are more common and hummingbird populations are decreasing.
All of these are worsened by non-native earthworms in Great Lakes soil, according to a new study that identified four key minerals that earthworms remove from soil and that native plants need to grow.

The loss of calcium, magnesium, potassium and phosphate reduces the richness of the soil and limits what plants the soil can support, said the study by scientists at the U.S. Forest Service, University of Minnesota, Stroud Research Center and U.S. Geological Survey and published in the journal “Ecosystems.”
The impacts of the earthworms are visible in a variety of places, said Tara Bal, a research assistant professor at Michigan Technological University’s School of Forest Resources and Environmental Science. Soil is degraded when earthworms mix different levels, and tree roots become exposed when worms consume fallen leaves and other ground material.
This root exposure can hurt the tree and cause the canopy to thin and die back, said Bal. It’s possible to look at a cross section of a tree and identify roughly when worms entered the soil in that area.
There are better indicators of which soils and forests have worms though, and some of them are odd enough to almost be funny, she said. One of the most common methods to sample for worms is dousing a test area with mustard because mustard is a highly effective skin irritant, and spraying it on the earth causes the worms to exit the soil quickly.
Jan Schultz, a botanist for the Eastern Region of the U.S. Forest Service, said earthworm composting has long had a reputation of saving humans from total burial under organic material\
But the Great Lakes region hasn’t had worms in its soil since the glaciers wiped them out and forests adapted to not needing them in the thousands of years since — speeding the pace of decomposition, Schultz said.
But, introduced as a byproduct of gardening and fishing, earthworms have returned and now are causing ecological ruckus, while the pace at which they consume organic material is much too fast for Great Lakes forests to keep up. This leads to nutrient-deprived soil, which stresses “the entire forest community,” Schultz said.
But like the proverbial snowflake that started an avalanche, nutrient-depleted soil directly leads to a whole host of other problems. The minerals and nutrients that earthworms consume contribute to an ecological cascade that impacts almost every aspect of Great Lakes plant and animal life.
Leaping up to take the place of waning undergrowth are invasive species like garlic mustard and buckthorn, which can grow rapidly and don’t require the nutrient concentrations that native orchids and tree saplings do, said Schultz.
Much of what looks like healthy forest undergrowth is actually invasive plants, Schultz said, which can be deceiving to passerby. “People see green and think green is good. The entire forest community is at stake.”
Other species of invasive earthworms operate at different levels in the soil, causing mayhem in other ways.
For example, Lee Frelich, director of the University of Minnesota Center for Forest Ecology, said the thinning organic layer provides less cover for budding trillium and exposes tree roots to damaging elements of nature.
Orchids suffer, and as a result are smaller and less common, said Frelich. That creates a food shortage for hummingbirds. Populations of both decline, and more invasive species take their place.
Soybean aphids and Japanese ladybeetles, which Frelich describes as “those annoying bugs you pick off the side of your house,” increase as well. Garlic mustard and buckthorn attract invasive and pest insects so the populations of both wedge themselves into the existing ecosystem.
Algal blooms can increase due to the nutrients broken down by the worms that make their way into bodies of water, Frelich said. The increase in dissolved nutrients overloads the water with minerals that help plants to grow.
So what can be done to combat the invading forces of nightcrawlers and earthworms causing such damage?
Michigan Tech’s Bal said there’s a need for education about how to avoid spreading worms any further, and she recommends small measures such as washing down hiking shoes and car tires whenever traveling between areas. There are also websites, such as Great Lakes Worm Watch, that use general public submissions and reports, or ‘citizen science,’ to record, observe and collect data.
Chris Symons writes for Great Lakes Echo.
Great Lakes Worm Watch:

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