Earthworms are absent in the fossil record but are estimated to have been around for ~200 million years. There are now ~6,000 distinct earthworms, living in and on soils around the world. Of the more than 180 earthworm species in North America, 60 were brought over from the Old World – and they’re causing trouble.
The glaciers that crawled across Canada and the northern United States during the last ice age wiped out the earthworms in those regions. 10,000 years later, earthworms found this fertile land suitable for residence, having arrived with 17th-century settlers who thought that earthworms would improve the soil, or accidentally brought in shipments of plants, or in dirt used as ballast in ships.
The northern forest evolved after the glaciers retreated, yielding an ecosystem that does not benefit from earthworms. These forests need duff: a deep layer of slowly decomposing leaves (leaf litter) and other organic matter that overlays the soil. Invasive earthworms eat duff. This leaves fewer nutrients for young, growing plants; and the soil, instead of being loose and aerating, compacts.
The combined effects damage the prospects for trees such as sugar maples and to many forest herbs and understory plants, such as trillium, rare goblin ferns, trout lilies, and other forest-floor dwellers. In some areas, oak forests have been overrun by buckthorn; in others, earthworms have facilitated the invasion of Japanese barberry.
As duff disappears, so do the insects and other small creatures that depend on duff to survive. Animals, such as salamanders, that feed on duff eaters lose a key food source. Their populations decline. Earthworm burrows also accelerate the passage of water through forest soil – another change that is a negative in the northern forest.
“Earthworms are yet another factor that can affect the carbon balance,” worries Canadian forester Werner Kurz. Earthworms speed decomposition and release CO2 into the atmosphere.
“The global boreal forest is a muscular part of Earth’s carbon cycle. At least 1/5th of the carbon that cycles through air, soil and oceans passes through the boreal,” says Canadian soil biogeochemist Sylvie Quideau. Currently, the boreal absorbs more carbon from the atmosphere than it adds. But that is changing, and earthworms aren’t helping. “Although earthworms are largely beneficial to soil fertility, they increase soil greenhouse-gas emissions,” concludes Dutch soil ecologist Ingrid Lubbers. “Even though worms themselves are tiny and don’t individually seem to constitute a threat, when you think of how many of them there are, they’re very important organisms,” exclaims American earthworm expert Adrian Wackett.
There is no way to eradicate earthworms from the boreal. Their impact is lasting.
The only good news is that earthworms don’t bust a move: they only migrate 9 meters at most a year.
Erin K. Cameron et al, Modelling interacting effects of invasive earthworms and wildfire on forest floor carbon storage in the boreal forest,” Soil Biology and Biochemistry 88: 189-196 (September 2015).
Erin K. Cameron & Erin M. Bayne, “Road age and its importance in earthworm invasion of northern boreal forests,” Journal of Applied Ecology 46: 28-36 (2009).
Alanna Mitchell, “‘Earthworm dilemma’ has climate scientists racing to keep up,” The New York Times (20 May 2019).
Ingrid M. Lubbers et al, “Greenhouse-gas emissions from soils increased by earthworms,” Nature Climate Change (3 February 2013).
Frank E. Anderson et al, “Phylogenomic analyses of Crassiclitellata support major Northern and Southern Hemisphere clades and a Pangaean origin for earthworms,” BMC Evolutionary Biology (30 May 2017).
Frank Anderson & Samuel James, “The evolution of earthworms,” BMC Series blog (1 June 2017).