Earthworms are often praised as beneficial soil engineers in agricultural settings, where their activities improve soil structure and nutrient availability. This positive perception contrasts sharply with their role in many North American forests. In regions formerly scoured by the last continental glaciation, the presence of earthworms is a destructive invasion. These non-native species disrupt established forest ecosystems that evolved in their absence, leading to significant ecological damage.
The Invasive Status of Earthworms
The current earthworm populations in glaciated North America are almost entirely non-native, primarily of European or Asian origin. The last Ice Age wiped out native populations in the northern regions, meaning subsequent forest ecosystems developed without these efficient detritivores, creating a unique soil structure. European settlers initially introduced these earthworms starting in the 18th century, often unintentionally in the soil of imported plants or as fishing bait.
The European nightcrawler, Lumbricus terrestris, is a widely recognized invader commonly sold as bait. This species and other European relatives have been prevalent for centuries, slowly spreading throughout the northern forests. More recently, a second, more aggressive wave of invaders known as “jumping worms” (Amynthas species) has emerged from East Asia. These Asian species, which include the “crazy worm” and “snake worm,” are concerning due to their rapid reproduction and voracious consumption of the forest floor.
Jumping worms are distinct from their European counterparts. They are annual species, meaning the adults die with the first frost, but their cocoons survive the winter. They reproduce asexually (parthenogenetically), allowing a single worm to quickly establish a large population without a mate. This allows them to spread quickly and aggressively displace established European earthworms, leading to greater ecological disruption.
Ecological Impact on Forest Floors
The primary ecological harm caused by invasive earthworms is the rapid destruction of the organic layer of the forest floor, known as the O horizon or duff layer. This layer, composed of slowly decomposing leaf litter, is a defining feature of forests that developed without earthworms. The duff layer provides a stable, insulating blanket that retains moisture, regulates soil temperature, and slowly releases nutrients.
Invasive earthworms, particularly the surface-dwelling European and Asian species, consume this protective layer at an accelerated rate, often reducing its thickness by up to 85%. This process strips the forest floor bare, exposing the mineral soil beneath. The resulting consequences damage the forest ecosystem.
The removal of the duff layer leads to increased soil erosion and nutrient leaching because the protective cover is gone. Many native forest plants, such as wildflowers and tree seedlings, rely on the cool, moist, and nutrient-rich duff layer for seed germination and initial growth. When this habitat is lost, native understory plant diversity decreases. The forest floor also becomes more hospitable to invasive plant species, such as garlic mustard, which thrive in the altered soil conditions.
Furthermore, the excrement of jumping worms, known as castings, often leaves the soil with a granular, pellet-like texture resembling coffee grounds. This change in soil structure inhibits water infiltration, increases surface runoff, and makes the soil inhospitable for native plant roots and the beneficial fungi they rely on.
Preventing the Spread of Invasive Earthworms
Earthworms generally spread slowly on their own, moving only about 10 meters per year. Therefore, human activities are the primary vector for long-distance invasion. Anglers play a role in this spread, as common bait species like the European nightcrawler are frequently discarded near lakes and streams, allowing new populations to establish. To prevent this, all unused fishing bait should be disposed of in the trash, not released onto the ground or into waterways.
Gardeners and landscapers must be cautious, as earthworms and their cocoons are easily transported in soil, mulch, and potted plants. When purchasing new plants, inspect the soil for jumping worms, which are identifiable by their thrashing, snake-like movement when disturbed. Transferring plants bare-rooted or ensuring that all mulch and topsoil comes from a known, uninfested source reduces the risk of introduction.
For those engaging in composting, the cocoons of invasive earthworms can survive and spread if proper procedures are not followed. To eliminate the cocoons, compost, mulch, or topsoil should be heat-treated, reaching an internal temperature of 105 degrees Fahrenheit for at least three days, or frozen for a minimum of one week. These actions help slow the invasion and protect uninfested forests.