Categorized | private lands, Public Lands

Earthworm Invasions: Here’s Something to Make us Squirm

An astounding sixty of the 182 total species of earthworms that occur in the United States and Canada come from other lands. Not all of these foreign earthworms are destructive, but about 16 European and Asian varieties do real damage to our native ecosystems. During the Wisconsonian glaciation, native earthworms in North America were severely impacted by the ice sheets then covering much of the continent. In areas north of this glacial boundary, the absence or lowered populations of native earthworms later facilitated the spread of the exotic invaders.

Invasive, exotic species are recognized as a leading cause in species’ declines and extinctions. Exotic species tend conjure up the most conspicuous instances, for example, pythons invading the Everglades or lionfish in the Florida Keys and Caribbean. On a more subtle level, exotic plants and animals often compete with native species and change entire ecosystems, such as when multiflora rose take over wetlands inhabited by bog turtles. Introduced earthworms probably do not even come to mind as an exotic nuisance. Nevertheless, earthworm invasions and their ability to damage ecosystems have become a global problem. Invasive worms have now spread through almost every habitat, including desert oases. Except for Antarctica, they occur on every continent and many oceanic islands.

Usually out of sight, all worms tend to look more or less the same. And earthworms would seem to be a good thing. Gardeners, composters, fishermen, students needing specimens for dissection, and robins feeding young certainly think of worms in positive terms. The role that earthworms play in building and aerating soils is well known. In the early 1880s Darwin demonstrated that worms on just one acre of land could convert living and dead vegetation into 18 tons of productive soil in just twelve months.

Earthworm invasions have depleted herbaceous plants and seedlings, the litter layer, and led to patches of bare soil in this Wisconsin forest. Photo by Scott Loss

Earthworm invasions have depleted herbaceous plants and seedlings, the litter layer, and led to patches of bare soil in this Wisconsin forest. Photo by Scott Loss, Smithsonian Magazine

Under normal conditions it takes microbes and fungi three to five years to decompose a deciduous leaf to the point that it becomes incorporated into the soil. In a forest infested with introduced night crawlers, however, this process takes as little as four weeks. Although organic duff covering a forest floor may take decades to accumulate, duff can be consumed by introduced earthworms in short order. Accumulation of undecayed and decaying litter is an important feature of temperate forest ecosystems. Exotic earthworms decompose this leaf layer more rapidly than their native counterparts, compromising this micro-habitat on the forest floor. Conditions become unsuitable for seed germination and the various creatures dependent on leaf layer for foraging, humidity, and concealment. In addition, the levels of moisture, temperature, pH, and nutrient levels change substantially. Eventually, this redistribution of organic matter and nutrient loss results in declines in native understory plant cover and an increase in nonnative plants. Soils are often exposed as every leaf, small seed, and tiny twig is devoured by the highly-efficient introduced worms. Soil exposure can then lead to erosion.

Many of the harmful invasive earthworms in the United States arrived in the 18th century. They were accidentally introduced in soil surrounding bulbs and rootstocks of plants brought to the New World by Europeans wanting familiar species for gardens and landscaping. In more recent times, additional species of worms were introduced from Europe and Asia, and cultured on worm farms for use as fish bait. The annual global export of earthworms is a multimillion-dollar business. Unlike many of our native species, these exotic worms are tolerant of disturbed habitats and have high reproductive rates.

Invasions of non-indigenous worms harm woodland wildlife. Worms alter the habitat used by ground roosting bats, ground foraging birds, terrestrial salamanders, and invertebrates that inhabit leaf litter. In addition, changes in leaf litter result in loss of mycorrhizal fungi, thereby paving the way for exotic plants. Because most of our native understory requires a deep, rich, and fertile layer of leaf litter for germination, these invasive earthworms threaten woodland ferns and spring wildflowers like bellworts, trilliums, yellow violets and wild ginger. Invasive earthworms have already denuded forests in the Great Lakes. It’s enough to make you squirm…

David S. Lee

Guest blogger

The Tortoise Reserve, White Lake, NC


J. Christopher Haney, Ph.D.

Chief scientist

Defenders of Wildlife

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- who has written 4 posts on dotWild.

Chris Haney oversees Defenders’ Conservation Science program as its chief scientist. He provides research and analysis to guide and support Defenders’ science-based policy and advocacy needs for endangered species, marine science, wildlife biology, forest ecology and management, and conservation policy. He has published extensively, with his work featured in Conservation Biology, Natural History, Ecological Economics, Limnology and Oceanography, Marine Policy, Natural Areas Journal, and others.

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dotWild is the blog of scientists and policy experts at Defenders of Wildlife, a national, nonprofit membership organization dedicated to the protection of all native animals and plants in their natural communities.