Invasive earthworms are spreading into the northern forests of Canada and the US, disturbing the soil and transforming ecosystems, reported researchers at the American Geophysical Union fall meeting in San Francisco last week. This “global worming” could contribute to climate change by potentially unleashing the large stocks of carbon stored in these northern soils.
The boreal forests—large expanses of wild lands in the planet’s northern latitudes—are the Earth’s largest terrestrial carbon sink. Their soils hold an estimated 200 billion metric tons (t) of carbon. And 60 billion t of that is in Canada’s boreal forest soils, says Justine Lejoly, a researcher in the lab of University of Alberta soil biogeochemist Sylvie Quideau.
Until about 20 years ago, scientists didn’t think it was possible for earthworms to survive in the relatively cold, acidic soils of the Canadian boreal forests. But just about everywhere researchers look these days, they find them. People spread the worms when they use the critters as live fishing bait or for help improving soil for farming and gardening. The species present in Canada originated in Europe, Lejoly says. And as the northern latitudes have warmed, earthworms’ habitable range has expanded. That could be bad news for the boreal forests’ carbon stocks.
Earthworms are the farmer’s friend because they can loosen up the soil and spur nutrient cycling between the soil and plants. “The story for the forests is very different,” Lejoly says. She and Quideau are trying to work out the details. Earthworms in the boreal forests of North America have the potential to unleash a “carbon release on the scale of wildfires,” Lejoly says.
Earthworm invasions occur over the course of decades—which makes them difficult to study in the lab. So the University of Alberta team visited three boreal forest field sites in the province, each in different stages of an invasion. They did a deep dive into the soil chemistry to determine how the worms were affecting carbon storage.
There are three stages to an earthworm invasion and each wave brings different species with distinct ways of life. First, relatively small worms sweep in, consuming leaf litter and other organic matter in the top layer of soil—a process researchers believe releases carbon dioxide into the atmosphere. Once these critters clear the ground, the next group of larger worms burrows in deeper, beginning to churn up the soil as they move into its mineral layers. Finally, larger worms that feed both on the top layers and deeper underground take hold, further disturbing the soil as they move and feed on the surface and below it.
At the AGU meeting, Lejoly presented the team’s preliminary results. At some of the sites, “the forest floor almost disappeared,” she says. Before the invasion, the top layer of soil at the Alberta sites stored about 30 t of carbon per hectare. After, they held just 2 t per hectare. The implications for the underlying mineral soil are less clear. The carbon was still there, but it was stored in a less stable form. The clumps in the soil were smaller, which means it will probably release carbon more readily as temperatures increase, Lejoly says.
Quideau and Lejoly are currently studying a group of field sites in Quebec to see if something similar is happening there. Forest service staff in Quebec told the team they wouldn’t find any earthworms, but Lejoly says they have found an invasion well underway.
Kyungsoo Yoo, a soil biogeochemist at the University of Minnesota, says these kinds of worm-triggered changes can cascade through forest ecosystems in unexpected ways. It’s possible that later stages of an invasion will put carbon back into the soil, as the worms burrow in deeper and leave their droppings behind. However, the invading worms can also have wide-ranging and negative effects on the forest ecosystem that in turn lead to release of carbon. Clearing off the forest floor can make sugar maple seedlings more visible and therefore prone to being eaten by deer, for example. And in New England and the Great Lakes region, he says, invading earthworms have shifted the soil microbiome from fungal to bacterial dominance. This shift causes a disadvantage for plants that live in symbiosis with fungi, exchanging sugars for other nutrients through their roots.
Also at the American Geophysical Union meeting, Yoo presented data from a large survey of earthworm populations in Alaska. His maps show worms spreading out from fishing sites and gardens. His group is still studying the impacts on the region’s carbon stock. They’re doing similar studies in Scandinavia, where they’ve found earthworms north of the Arctic circle.
These earthworm invasions and their ecological consequences are “an example of what humans can do without even knowing it,” Yoo says. “No one would have thought the worms they throw out after fishing could cause so much trouble for the forests.” But he says humans can make a positive difference too. He says live bait should be banned in Alaska to slow the worm invasion in that state, and keep more carbon in the soil.