The common earthworms found in gardens and sold as fishing bait are not native to North America. Species like the European nightcrawler and angleworm were introduced by European settlers over the last few centuries, primarily arriving in imported plants, soil, and ship ballast. While earthworms are often seen as beneficial for soil health globally, their presence in large portions of North America represents an ecological invasion. Ecosystems, particularly in previously glaciated areas, developed without these burrowing invertebrates. This historical absence created unique soil conditions that are negatively affected by non-native worms.
The Glacial History of Earthworm Absence
The widespread absence of native earthworms across the northern two-thirds of North America is a direct consequence of the last major ice advance, the Wisconsin glaciation. This massive geological event, which peaked around 18,000 years ago, saw ice sheets covering vast tracts of the continent, extending as far south as central Iowa and New Jersey. The weight and movement of this glacial ice completely scoured the land, stripping away topsoil and bedrock.
The deep freeze and sterilization of the soil eradicated all existing earthworm populations within the glaciated zone. As the ice sheets slowly retreated, the landscape that emerged was “worm-free.” Over the next 11,000 years, northern hardwood forests developed in a unique environment where organic matter decomposition was handled by microbes, fungi, and small soil invertebrates.
Earthworms have limited natural dispersal rates, moving less than 30 feet per year, and were unable to naturally recolonize the vast cleared territory. When European settlers arrived, they introduced species like Lumbricus terrestris (the common nightcrawler) from Europe. These non-native species rapidly established themselves and are now expanding their range into previously undisturbed forests, continuing an invasion that began centuries ago.
Native Species and Their Habitats
While the common worms seen in the northern states are invasive, North America possesses a diverse, though less familiar, native earthworm fauna. These indigenous species survived the glaciation event by inhabiting unglaciated areas, primarily in the South, the Pacific Northwest, and certain coastal refugia. Over 100 native earthworm species can be found in these regions.
In the Southeast, specialized native genera such as Diplocardia contribute to soil processes that co-evolved with local flora. Other native species, like those in the genus Sparganophilus, are adapted to muddy, aquatic habitats near rivers and streams. These native worms did not naturally migrate into the recently deglaciated northern forests due to their slow dispersal and harsh climate conditions.
The native species that survived are taxonomically distinct from the invasive European Lumbricidae and Asian Amynthas species. Their long-term ecological role is different from the European worms; they did not function as primary soil-mixing agents. This distinction explains why northern ecosystems developed a unique soil structure in their absence.
Ecological Consequences of Invasive Earthworms
The invasion of non-native earthworms has significantly altered the balance of North American forest ecosystems that evolved without them. These forests naturally accumulate a thick, spongy layer of organic material on the ground, known as the duff or O-horizon. This layer, composed of slowly decomposing leaf litter and woody debris, is retained by the absence of efficient soil-mixing invertebrates.
The duff layer is functionally important, acting as a natural mulch that retains moisture, insulates the soil, and slowly releases nutrients. It also provides the specific habitat and substrate required for native woodland plants to germinate and grow. Invasive worms, particularly European species, rapidly consume this organic layer, often eliminating it completely within a few years.
The removal of the duff layer exposes the bare mineral soil beneath, fundamentally changing the forest floor environment. This speeds up nutrient cycling, causing essential nutrients like carbon and nitrogen to leach quickly out of the soil profile, making them unavailable to shallow-rooted native plants. The loss of the insulating layer also leads to drier, warmer soil conditions.
Many native woodland plants, such as trillium, ferns, and sugar maple seedlings, rely on the duff layer for protection and moisture necessary for germination. Without this substrate, their seeds fail to sprout, and established plants suffer habitat loss, leading to a decline in biodiversity. The disturbed, bare soil conditions also favor the establishment of non-native, invasive plant species adapted to the altered environment.
Identifying and Preventing the Spread of Invasive Worms
The latest and most aggressive invaders are the Asian Amynthas species, commonly known as Jumping Worms, Snake Worms, or Crazy Worms. They have spread rapidly across the continent and are identifiable by their distinct physical characteristics and erratic behavior. They are often dark brown or gray, possess a sleek, firm body, and exhibit a unique thrashing, snake-like movement when disturbed.
A key identifying feature on an adult Jumping Worm is the clitellum (the reproductive band), which is pale, smooth, and completely encircles the body. This differs from the raised, pinkish clitellum of European nightcrawlers, which only partially wraps around the worm. Their feeding activity is also unique, leaving behind castings that give the soil a distinct, grainy texture resembling coffee grounds.
The primary vectors for the spread of all invasive earthworms are human activities, which inadvertently transport both adult worms and their tiny cocoons. These vectors include:
- The disposal of unused fishing bait into forests or waterways.
- The movement of horticultural materials, including potted plants, mulch, topsoil, and compost.
Preventing their spread requires conscientious action from the public. Gardeners should inspect new potted plants or soil amendments for signs of the worms or their castings before use. Anglers should refrain from dumping unused bait, disposing of it in the trash or a designated location away from natural habitats. Heat-treating compost to temperatures above 104 degrees Fahrenheit can eliminate the cocoons before the material is moved or applied.