While earthworms are often seen as beneficial for soil health in gardens and agricultural settings, certain non-native species can be highly destructive. These invasive earthworms disrupt ecosystems that have not evolved with their presence, causing environmental or economic harm.
Understanding Invasive Earthworms
Many regions of North America, especially those north of Pennsylvania, were largely devoid of native earthworms for thousands of years following the last glacial period, which ended about 10,000 to 12,000 years ago. The vast ice sheets of the Wisconsin glaciation scoured the landscape, eliminating most native earthworm populations. Consequently, northern forests developed unique ecosystems without the influence of these invertebrates.
The introduction of non-native earthworms began as early as the 1600s with European settlers. These worms arrived in various ways, including in the root balls of imported plants, in soil used as ship ballast, and through the horticultural trade. More recently, the widespread use of earthworms as fishing bait has contributed significantly to their spread into more remote natural areas. Common invasive species include European nightcrawlers (Lumbricus terrestris) and red wigglers (Lumbricus rubellus), both originating from Europe. A more recent concern is the spread of Asian jumping worms (Amynthas agrestis, Amynthas tokioensis, and Metaphire hilgendorfi), also known as “snake worms” or “crazy worms,” which are characterized by their thrashing movements when disturbed and their ability to rapidly alter soil structure.
Ecological Impacts
The presence of invasive earthworms profoundly alters forest ecosystems. One of their most noticeable effects is the rapid consumption of the leaf litter layer on the forest floor. This layer, composed of decaying leaves and organic matter, is a foundational component of these ecosystems, providing habitat for native plants, fungi, and invertebrates, while also retaining moisture and slowly releasing nutrients.
As earthworms consume this litter, they reduce its depth and quantity, exposing the bare mineral soil. This change accelerates nutrient cycling and can lead to increased nutrient leaching, making essential nutrients less available for native plants that have evolved to thrive in soils with a thick organic layer. The physical structure of the soil also changes, becoming more compacted and granular, which can reduce water infiltration and alter soil pH.
These alterations have cascading effects on the entire forest community. Native plant communities, including tree saplings and wildflowers, struggle to grow and reproduce in the changed soil conditions, leading to a decrease in biodiversity. The loss of leaf litter can eliminate habitat for ground-nesting birds and various insect populations, impacting the food web. The presence of invasive earthworms can also facilitate the spread of other non-native plant species, creating a further imbalance in the ecosystem.
Preventing Their Spread
Preventing the spread of invasive earthworms relies on public awareness and careful practices. Once established, complete eradication is unlikely.
Anglers should never release unused bait worms, such as European nightcrawlers, into lakes, rivers, or surrounding natural areas. Instead, dispose of them in the trash.
Gardeners and landscapers play a significant role in prevention. Thoroughly clean gardening tools, boots, and vehicle tires to remove any soil or worm cocoons before moving between natural areas or garden beds. When purchasing plants, inspect them carefully for worms or their distinctive, granular castings that resemble coffee grounds.
Consider purchasing bare-root plants or repotting plants with sterile, bagged potting soil to avoid introducing contaminated soil. If using mulch or compost, ensure it comes from a reputable source that has heated the material sufficiently (e.g., to 130 degrees Fahrenheit for at least three days) to kill any worm eggs. Educating others about these prevention measures can help protect vulnerable ecosystems from the damaging effects of invasive earthworms.