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As a gardener, I know humans have introduced many invasive species like Dandelions, Japanese Knotweed, and Phragmites, into many of our native North American habitats. I have witnessed it, and to some extent participated (in ignorance) of the problem.

As little as two hundred years ago the forests surrounding the Great Lakes lacked earthworms. The Ice Age and glaciers eliminated the native species, and during the 10,000 years since the last glacier, our trees and plants evolved to live in the wormless local soils. This became a thick layer of duff or the debris of trees and shrubs that lined the forest floor. Duff remains for years, slowly decayed by fungi, but providing a habitat for many Michigan ferns, delicate wildflowers, and small animals like salamanders.

Since settlers first moved into this area, they introduced European earthworms like the red wriggler to encourage composting. Gardeners still think of worms as a desirable asset to their garden’s soil. These worms eat decaying matter, aerate the soil, and leave humus-rich worm casting behind as they burrow. In general, they do good things for garden soil.

In the forests, however, worms eat the duff in as little as two years, lowering the soil’s acidity and making nutrients more available. The plants in our forests survived in the poor fertility and the high acidity of our original native soils. While the introduction of worms won’t necessarily harm the mature trees growing there, the duff’s absence affects the number of seedlings growing in an area. It also makes an advantageous situation for unwanted invasive species that couldn’t have grown in the more acidic original soil.

Bloodroot

As this debris layer disappears, so does the habitat for plants like Trillium, Solomon’s Seal, Blue Cohosh, Sweet Cicely, Mayflowers, Wild Ginger, Lady Fern, Bloodroot, Bellwort, and the tiny Goblin Fern which might already be extinct in this area. They adapted to germination and growth in the thick layer of duff that provided them nutrients, moisture, protection from extremes in temperature and predators, and microorganisms like fungi necessary for their survival.

It is also believed the loss of the duff effects the salamander population that fed on insects living in the debris. The salamanders, in turn, provided food for snakes, shrews, thrushes, and screech owls, so the earthworm invasion affects a whole chain of living organisms.

Worms were introduced from dumped fishing bait, on the root-soil clumps of purchased plants, and on the tires of vehicles entering the local forest. The most severe invasions are usually near roads. This is one reason the U.S. Forest Service is restricting logging and road-building in certain forest areas in states around the Great Lakes.

Trillium

Once the earthworms infest an area, little can be done to remove them without doing more harm to the forest. We know many of the above plants grow in worm-infested soil because we grow them in our gardens, but a study in the Chippewa National Forest in northern Minnesota showed these species do not return to worm-infested forests.

Some ecologists hope the ecosystem of the forest may eventually come to a new balance. In the meantime, don’t spread earthworms into wormless areas, don’t dump worm bait in forested lands, and keep your vehicles on roads and trails. Gardeners in wooded areas need to keep compost piles and worm contaminated soil from contaminating the woodlands. These may seem like small measures, but it was a little thing that started this change.

(Updated. Originally published in the Cadillac News in 2002.

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