by Anne Pearce, Wisconsin First Detector Network
Many of us have learned that earthworms are some of nature’s busiest engineers, helping our gardens grow by aerating the soil and breaking down organic material into nutrients for our flowers and vegetables. But the same earthworm engineering practices are less helpful, and even damaging, in our woodlands. Now that “jumping worms” are spreading across the upper Great Lakes region, it’s a good time to look at what we know about earthworms in forests.
The Ecology of Worms
Earthworms are generally divided into three broad categories based on their ecological impacts. Epigeic earthworms are small, red-brown worms that live and feed only in the leaf litter, at the soil surface. Endogeic earthworms are small, light colored worms that live and feed in the top layer of mineral soil. They build and use shallow horizontal burrows. Anecic earthworms are large, reddish-brown worms that feed on both leaf litter and soil. These worms are the busiest of the earthworm engineers as they build and use extensive vertical burrows and pull leaf litter from the surface of the soil down into their burrows.
(Credit: Great Lakes Worm Watch)
In northern temperate forests of the United States and Canada, the retreat of the glaciers left behind a landscape with no native earthworms. The plant communities that grew back after the ice was gone evolved in a system where leaf litter from trees built a protective “skin” over the soil and the litter was gradually broken down, mostly by fungal activity. The perennial plants and trees in our forests are adapted to the slow release of nutrients from the fungal breakdown of leaf litter and depend on the protective skin.
The arrival of European settles brought the arrival of European earthworms from the family Lumbricidae. Famous among this group are the nightcrawlers (Lumbricus terrestis), which are deep-burrowing anecic worms. Since these worms have been around for a few hundred years, we have a relatively good understanding of how they impact forests, and you can read a comprehensive summary of these impacts at Great Lakes Worm Watch. The short version is that nightcrawlers deplete the leaf litter layer, releasing nutrients too rapidly for our plants and trees to use and removing the skin of the forest that protects and nourishes everything from seeds to mature trees.
The New Invaders
More recently, “jumping worms” are spreading into forests across the Northeast and Great Lakes regions. Several Amynthas and Metaphire species comprise this group of earthworms that originate from Asia. Jumping worms were first reported in the south in the early 1900s and have been present in arboreta, nurseries, and greenhouses in the southeastern United States and further up the Eastern Seaboard since the 1950s and 60s. Eventually, jumping worms were found in forests, and since 2010, they’ve been newly discovered in forests in Vermont, New Hampshire, Maine, Illinois, Wisconsin, Minnesota, and Ontario.
You may have heard some hubbub around the arrival of jumping worms and wondered, “What’s the big deal? We already have earthworms; this is nothing new!” As it turns out, jumping worms differ quite a bit from our familiar European earthworms. Notably, jumping worms reproduce asexually, meaning it takes only one adult jumping worm to develop cocoons. A single worm, or a single cocoon (which looks like a particle of soil), can start a whole new infestation. While jumping worm adults die with the onset of winter (many European earthworm adults overwinter deep in the soil), their cocoons withstand a wide variety of conditions, including cold northern winters. Cocoons hatch throughout the early spring and summer, and mature jumping worms are typically present and depositing cocoons from July through autumn.
Recent studies show that newly-arrived jumping worms interact with established European earthworm populations. Jumping worms tend to feed more on soil biota, and European nightcrawlers feed more on leaf litter, so they don’t necessarily compete directly for food. However, jumping worms have a more flexible diet that favors them in areas where they exist with European earthworms (1). In forests previously dominated by European earthworms, newly invading jumping worms become dominant within a year or two (2, 3). Jumping worms have been documented to have negative impacts on abundance and diversity of millipede populations in the soil (4) and disrupt habitat use by vulnerable lungless salamanders (5). In addition to these impacts on other species, jumping worms have been shown to significantly alter soil habitats and processes. While the long-term impacts of jumping worms aren’t yet documented, studies suggest that the changes wrought by jumping worms may be as ecologically important and detrimental as those documented for European earthworms (6, 7).
Identifying Jumping Worms
If you’re wondering whether you have jumping worms in your woodland, there are a few things to look for. First, jumping worms leave a characteristic soil signature. Infested areas will have depleted (though not necessarily absent) leaf litter and loose topsoil that looks like coffee grounds. Jumping worms have been classified as epi-endogeic, so you will find them at or near the surface of the soil, often right under the leaf litter. They are large worms, like nightcrawlers, but instead of feeling soft and slimy, they have rigid, snake-like bodies. And the name jumping worm? That comes from the unique writhing motion the worms make when they are handled or disturbed. Adult worms (typically present from July through fall) are dark gray/brown and glossy with a smooth clitellum (the circular band around the body). The clitellum is a light tan color and wraps around the entire body. For more I.D. tips and a comparison with nightcrawlers, check out Wisconsin DNR’s jumping worm page.
Typical soil signature of jumping worms (Credit: Anne Pearce)