Earthworms are often seen as beneficial garden helpers, turning soil and aiding plant growth. Many people consider them a sign of healthy soil, assuming they are a natural part of North American ecosystems. However, the reality of earthworms in much of North America is far more complex and, for some habitats, concerning. This article explores the surprising history of earthworms on the continent and their ecological role.
Earthworms Before European Settlement
Much of North America, particularly the northern and northeastern regions, was covered by massive glaciers during the last ice age, ending around 10,000 to 12,000 years ago. These vast ice sheets scoured the landscape, eliminating native earthworm populations in glaciated territories. The soil in these areas developed over millennia without earthworms.
As a result, forests in these formerly glaciated regions evolved unique ecosystems adapted to the absence of these burrowing invertebrates. While some native earthworm species persisted in unglaciated southern and western North America, most earthworms found across the continent today are not indigenous to these northern landscapes. Ecological processes in these northern forests were shaped by different decomposers.
Pathways of Introduction
The widespread presence of non-native earthworms across North America began with European settlement centuries ago. Early European settlers inadvertently introduced them through soil in ship ballasts and root balls of imported plants and agricultural goods. These initial introductions established populations in disturbed areas like farms and gardens.
Over time, their spread accelerated through human activities. Anglers discarding unused fishing bait, often containing European species, became a significant vector for their movement into undisturbed forest environments. The horticultural and landscaping industries also contribute to their dissemination, as earthworms or their cocoons can be transported in potted plants, nursery stock, and compost. Different species, like nightcrawlers and “jumping worms,” arrived at different times and spread through these varied pathways.
Their Impact on Ecosystems
The introduction of non-native earthworms, particularly into northern hardwood forests that evolved without them, leads to ecological alterations. These introduced species rapidly consume the detritus and leaf litter layer, known as duff, which covers the forest floor. This duff layer is an important habitat for diverse native insects, fungi, and microbes.
The accelerated consumption of this organic layer exposes the underlying mineral soil to erosion and compaction. This change alters nutrient cycling processes; nutrients normally slowly released are instead rapidly leached from the soil, becoming unavailable to native plants. Many native understory plants, including ferns, wildflowers, and tree seedlings, depend on the stable duff layer for germination, root development, and moisture retention. Its removal can lead to a decline in their abundance and overall biodiversity in the forest understory.
These physical changes to the soil structure and nutrient availability also affect other forest inhabitants. Amphibians, ground-nesting birds, and small mammals that rely on the leaf litter for shelter and food sources find their habitats degraded. Aggressive species, such as “jumping worms,” exacerbate these impacts by rapidly destroying organic matter and leaving behind granular, nutrient-poor soil that hinders plant growth.
Protecting Native Habitats
Preventing the spread of non-native earthworms is important for protecting North America’s native habitats, especially in previously uninfested areas. One effective measure involves anglers responsibly disposing of unused fishing bait in the trash, rather than releasing it into natural waterways or on land. This simple action can prevent new introductions into sensitive ecosystems.
Individuals should also exercise caution when moving soil, plants, or gardening equipment between locations. Inspect and clean hiking boots, vehicle tires, and gardening tools to remove any attached soil or earthworm cocoons before entering or leaving natural areas. When purchasing plants or compost, be aware of the potential for earthworm presence and choose reputable sources. Spreading awareness about the ecological risks posed by non-native earthworms can empower more people to adopt preventative behaviors and help safeguard local ecosystems.