Submitted by Russel Barsh, director of Kwiaht.
Ask any extension program or gardening blogger, and they will probably assure you that worms are good for soil health. Check online, and there are literally hundreds of offers of garden worms for sale. How important are earthworms, and do you need to buy them?
Earthworms demonstrably help till and aerate soils. As detritivores, they eat decaying vegetation and help recycle nutrients underground. They are not alone at this job, but part of an ecological guild — a group of organisms that eat the same things and share the same functions. The compost guild in soils includes many species of beetles, ants, mites, bacteria and fungi, as well as the most mysterious of six-legged animals, the Collembola or springtails, which do double duty as detrital leaf-shredders and dispersers of bacteria and fungi. Like ants, Collembola tend and harvest tiny gardens beneath our feet. Healthy soils have representatives of this entire guild. But to be fair, earthworms are the largest and most conspicuous members of the compost guild in temperate soils. What exactly, then, is an “earthworm”?
Segmented worms (Phylum Annelida) are one of the largest and most diverse groups of animals on earth, with an estimated 22,000 species worldwide. Within the Annelida are thousands of species of worms that generally live in soils and mud, eating detrital organic matter: the Oligochaeta or earthworms, broadly speaking. The most widespread and successful families of earthworms are the Lumbricidae, with 670 species thus far recognized, most of them native to northern Eurasia and North America. About 30 Eurasian Lumbricid species have proven to be highly invasive, and over the past few centuries of empires, colonization and global trade in plant species, these Old World earthworms have been displacing the native earthworms of the Americas.
According to Canadian earthworm taxonomist John Warren Reynolds, there are 161 earthworm species at present in the United States and Canada, of which 116 (72%) are native to North America. Non-native species literally outweigh the biomass of native species, however, and this imbalance is growing due, at least in part, to the promotion of Eurasian earthworms in the garden trade and organic farming. Lee Frelich and his colleagues at the University of Minnesota recently published data showing that Eurasian Lumbricid earthworms quickly colonize disturbed land such as forests and gardens where soils have been churned and native worms at least temporarily extirpated.
Proponents of propagating Eurasian worms such as Lumbricus terrestris here unanimously argue that the continental glaciers killed all of our native North American worms. This is simply untrue. No glaciation in the last million years covered all of what are now the 48 contiguous United States, and no more than a dozen states were ever completely covered in ice. Only the northern half of Washington state was periodically glaciated. The last time was 12,000 years ago. It is a stretch to assume that native worms were stuck in the prairies of southwestern Washington for all of those millennia, and that there were no worms at all around Puget Sound until Europeans arrived.
To be sure, the San Juan Islands were scraped quite bare by the time the Vashon ice sheet melted. Since that time, the islands have been recolonized by plants and animals, many of which could not swim or fly, so they rode floating rafts of trees torn off riversides on the mainland by spring floods, or the muddy feet of migratory waterfowl. There are native Northwest beetles, springtails, centipedes, snails and other invertebrates in the islands’ soils that almost certainly returned long before European sailing ships. Why not earthworms?
In fact, the glacial extermination of native worms is a myth. In New York state, for example, which was completely covered in Pleistocene glacial ice, 30 species of earthworms have been identified, including five native species. Here in the Salish Sea lowlands, also previously glaciated, there are 21 earthworm species, of which six are native. Most native Salish Sea earthworms belong to the genus Arctiostrotus and have been found only in coastal areas of Oregon, Washington and British Columbia.
Native Northwest earthworms were only fully recognized in the 1990s as a result of the work of Portland State University zoologist Dorothy McKey-Fender, and were first considered for protection as threatened species in 2024 by British Columbia. They are rarely included in ecological surveys, often overlooked or misidentified. They are difficult to distinguish by gross visible morphology, and genetic barcoding of earthworms is not yet reliable for this purpose.
For the time being, native earthworms, like native bees, continue to be displaced by farmers and gardeners who think they must import commercially available Eurasian species to make up for a supposed lack of native ones.
