Most people associate earthworms with nutrient-rich soil, healthy gardens, and compost piles. It turns out these creatures, which can be so helpful to backyard gardeners, may be harming North American forests.
Researchers suspect that the last ice age wiped out the vast majority of earthworm species in North America; almost no native earthworms can be found living in parts of the continent once covered by glaciers. European settlers reintroduced earthworms to regions that had developed worm-free ecosystems for thousands of years. Many of the worms likely hitched a ride in the soil used for ship ballast and in potted plants Europeans brought to farm in the New World. Other settlers intentionally brought worms to help vegetable gardens grow. Burrowing by worms lets air and water penetrate dense soil, and the worms break down nutrients so that some plants can absorb them more easily.
But hardwood forests are very different from gardens and farms. Before earthworms showed up, fallen leaves decayed slowly on the forest floor, creating a thick layer of organic matter (called the “duff” layer) that allowed many species of ferns, flowers, and saplings to grow. After worms have invaded a forest, they mix the duff layer into the soil and break down organic material much more quickly. Native plants and flowers can have a hard time adapting to this change and usually just die off, often supplanted by other invasive species, such as buckthorn. It’s easy to tell the difference between a worm-free forest and a wormy forest: the former has a thick layer of small ferns and shrubs, and the latter has nearly bare, hard packed soil.
Earthworms also change the chemical consistency of soil. A faster decomposition cycle in a forest’s soil allows active bacteria to release more CO2 into the atmosphere, and worms increase the overall pH of the soil making it harder for native species of plants and insects to survive. They also add potassium and phosphorous, which benefit some plants (especially crops) but not trees.
Northern states have begun asking people to be mindful of what they dump in forests to avoid the spread of earthworms. Fishermen in particular are cautioned not to release unused worms into the soil. Next time you dig up a plant in your vegetable garden and grasp a handful of black, wormy soil, remember that those tiny burrowers are also foreign invaders.
For more about early-American soil chemistry, check out this magazine article.
By Jacob Roberts
Image: A 1998 chemistry set for testing soil quality in CHF’s collections. Photograph by Gregory Tobias.