Earthworms Are Bad News for North American Forests

Introduced earthworms are seriously harming North American forests. Ill.: static1.squarespace.com & pixers.uk

When I was a kid, my family used to spend part of every summer at Uncle Don’s cottage on Go Home Lake in Muskoka, Ontario. It was the highlight of our summer, a chance to go swimming, hiking, canoeing and generally just horse around with my cousins. We used to do a lot of fishing, but I can recall my uncle being perplexed by the lack of earthworms. We soon fixed that: we poured our leftover fishing worms onto the soil around the cottage. Within a few years, enough earthworms had settled in so we could harvest them for fishing, and we thought we had lent the environment a helping hand.

But we were wrong.

The Effect of European Earthworms on a North American Forest

North American gardeners are so enthusiastic about earthworms it seems almost cruel to disillusion them, but it should be noted that earthworms also have a dark side.

Yes, as we were all taught when we were kids, earthworms are beneficial to gardens. That’s because they aerate the soil by digging tunnels, devour organic matter and transform it into excreta rich in minerals, promote root development and stimulate the germination of the seeds of many sun-loving plants. But a garden, with its neatly planted rows of plants and constantly tilled soil to which we regularly add amendments, is a hardly a natural environment. In fact, it would be hard to imagine a more artificial one. And outside of our very Eurocentric vegetable beds, flower gardens, lawns and farm fields, the role of earthworms can be very different.

The sad fact is that none of the earthworms typically seen in North American gardens are actually native to North America. The best-known garden worm, Lombricus terrestris, known variously as the common earthworm, nightcrawler and dew worm, is a European species long ago introduced into North American fields for agricultural purposes and is now found just about everywhere. 

And the invading species are doing serious damage to the continent’s natural environment. Indeed, before the Europeans arrived, there were almost no earthworms north of the 45th parallel in North America, the species that once lived there having been eliminated by the glaciations during the last Ice Age. 

Even those species that survived the onslaught of cold by moving to the South and that have slowly been working their way back north since the ice sheets left were never as efficient detritivores (organisms that feed on and break down dead plant matter) as the European species that have been introduced everywhere. Today, it is estimated that of the 182 species of earthworms present in Canada and the United States, 60% come from Europe and Asia. In most northern North American gardens, all the worms you’re likely to run into are exotic invasive species.

Before and After

North American temperate forests evolved largely without the presence of major detritivores like earthworms and one of their major characteristics is an especially thick layer of leaf litter, also called duff (the layer of decomposing leaves that covers the ground). The duff layer can be up to 10 cm (4 inches) thick in sites dominated by sugar maples: that’s pretty much a world record. As a result, the species that grow in these forests are specially adapted to deep leaf litter and many even require it for their survival.

When European or Asian earthworms are introduced to a North American forest, though, they quickly reduce the thickness of the forest litter. It is estimated that in a forest without earthworms, it takes about 5 years for a maple leaf to disappear completely; in a forest with earthworms, it takes less than 2 years. By redistributing nutrients, mixing soil layers and aerating the soil with their tunnels, earthworms completely change the characteristics of the forest’s soil. And they move a lot of the minerals resulting from decomposition deeper underground than normal, sometimes to depths over 6 feet (2 m).

Also, the changes caused have an impact not only on the plants, but on the microbial, fungal and mycorrhizal fauna necessary for the proper development of many plants. Also, the soil becomes more porous and drier, sensitive to erosion.

The result is that many smaller native forest plants and young saplings, with their limited root systems, struggle to survive. Among the species that no longer seem able to regenerate as they once did are trilliums, ferns, Canada yew (Taxus canadensis) and even the jewel of eastern North American forest, the sugar maple (Acer saccharum). Instead, a maple introduced from Europe and which therefore evolved in the company of earthworms, the Norway maple (A. platanoides), appears poised to supplant the native sugar maple as the dominant tree in the “new North American forest”. 

Certainly, this has been the case in most urban and suburban forests in eastern North America, where Norway maple seedlings completely dominate the undergrowth while almost no young sugar maples are seen. 

The effect on the animals of the forest, especially microbes, arthropods, salamanders, and small mammals, has also been disastrous. For example, there has been a decline in the abundance of some insects and mammals living in the undergrowth as well as ground-nesting forest birds. The populations of several species of salamanders are in free fall following the invasion of exotic earthworms, as the insects and other small invertebrates they live on need thick litter in order to thrive.

In addition, earthworms consume tree rootlets—up to a quarter of a tree’s supply per year!—and also break up the mycorrhizal relationship that roots have with beneficial fungi in the soil, increasing the stress to native trees. And the disappearance of leaf litter leaves tree roots exposed to the air and thus drought and sun damage, also weakening the tree’s structure.

No, earthworms don’t kill mature trees … but they weaken them.

And the damages aren’t just to forests: natural prairies also find their soil stripped of precious resources and their soil layers completely disrupted by invasive earthworms.

The Horse is Out of the Barn, But…

In many areas, the damage is already done. And there is no known way to eliminate invasive earthworms from a forest that has been infested. The best we can hope for is to slow their spread … but that, at least, shouldn’t be too difficult.

Left on their own, even invasive species of earthworms don’t seem to penetrate too deeply into forested areas, tending to concentrate on the outer fringes, near the disturbed soils of the artificial meadows created by agriculture and urban development they much prefer. And if they do begin to move deeper into the forest, they do so only very slowly, over decades.

As gardeners, we can simply avoid purposely moving earthworms into forested areas. And fishermen, please don’t release your surplus earthworm in natural forests! Believe it or not, earthworms dumped by fishermen are still the major cause of the continuing expansion of invasive earthworms into forests in North America.

If you have a bit of natural forest in your backyard, you can still help thicken the forest duff even if the sector has been invaded by earthworms. Make a habit of picking up the fall leaves from your lawn, driveway, etc. and, instead of depositing them on the side of the road for the municipality to pick up, spread them in the forest. It may seem like a ridiculously simple gesture, but it can make a good difference in a severely impacted forest.

I still feel guilty about introducing earthworms to the cottage at Go Home Lake, but we didn’t know what we were doing. If you’ve read this far, you now do, so…