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Invasive Earthworms are Becoming a Danger to Forests

canada earthworms

canadian earthwormsCanadian earthworms. Public enemy number one?

A new study details how the common earthworm is disrupting the ecology of Canadian forests and presently wreaking havoc on maple tree stands in Quebec’s Eastern Townships.

For some time now scientists have been investigating the role played by the lowly earthworm in changing the soil composition of Canadian forests. Almost all earthworm species in Canada are non-native, having come across the Atlantic with European settlers. And although the average gardener loves to see them digging in the dirt and helping to decompose plant matter and aerate the soil, earthworms have become a real threat to some plant species in our forests.

The study’s researchers found that the presence of earthworms in the sugar maple forests of southern Quebec corresponded with a thinning out of the forest floor, with fewer new shoots of red and striped maple, American beech and also two fern species.

“The most likely explanation is that the earthworms consume organic matter in forest litter,” says the study’s lead author Line Lapointe, professor at the Université Laval’s Faculty of Science and Engineering. “This results in soils that can’t hold as much moisture and that in turn interferes with seed germination and the ability of some species’ plantlets to survive.”

Are earthworms native to North America?

No, earthworms are not native to North America. Most species of earthworms found in North America were introduced from Europe by early settlers in the 1600s and 1700s. The native earthworm fauna in North America was wiped out during the last glacial period, which ended about 12,000 years ago.

The introduction of non-native earthworms has had both positive and negative impacts on North American ecosystems. Earthworms can improve soil structure and nutrient cycling, which can benefit some plant species. However, non-native earthworms can also alter soil chemistry, reduce plant diversity, and change the composition of forest litter, which can have negative effects on soil microbes, fungi, and other wildlife species that depend on leaf litter for food and habitat.

Some native earthworm species have since been reintroduced to certain parts of North America, but they are still relatively uncommon and not as widespread as non-native species.

How earthworms spread

The dew worm or Canadian night crawler (Lumbricus terrestris) is the species currently causing damage to Canadian forests. (The name is deceiving – it too came from Europe.)

Stephen Westcott-Gratton of Canadian Gardening explains that, unlike in the backyard garden where fast turnover of soil is a plus, forests need to work on slower timelines, hence the trouble with the earthworm. “Once present, the earthworms gorge on the forest floor’s rich leaf litter, leaving the underlying mineral subsoil exposed and effectively wiping out tree seedlings, flowering plants and ferns who suddenly find themselves without a root zone.”

The issue is not one of earthworms charging across the country on their own – they only travel about 5 to 10 metres per year or one kilometer every 100 years – but of human-caused distribution. In Alberta, scientists only began noticing earthworms starting in the mid-1980s. Now, attempts are now underway to limit their spread. The Alberta Worm Invasion Project (yes, such a thing exists) studies the earthworm’s effects on local forests and advocates for more vigilance to prevent their spread.

Researchers with the Project list a number of ways that citizens can help. At the top of the list, anglers need to make sure that they don’t dump unused earthworms after fishing, and off-road enthusiasts should clean the tires of their ATVs to remove earthworms and cocoons. As well, it’s important to avoid transporting leaves, mulch and soil between sites unless you are sure they don’t contain earthworms or cocoons.

But help may be on its way from an international source. Recently, the European Research Council awarded a 1.5 million Euro ($2.27 million dollars Cdn) grant to the German Centre for Integrative Biodiversity Research in Leipzig, to study the effects of European earthworms in North America. According to the Centre, the ECOWORM project grant “will fund the first systematic study on the impact of earthworms on plant communities and soil food webs.”

Aren’t earthworms good for the soil?

But wait, you say. I thought earthworms were good for the soil and for the environment? That can be attributed to none other than Charles Darwin, who studied earthworms for decades and even wrote a book about them called The Formation of Vegetable Mould through the Action of Worms with Observations on their Habits, in which he offered nothing less than this rave about the slimy brown creatures.  “It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures”.

That page turner did unveil some previously unknow facts about earthworms, including that they can move as much as ten tons of soil each year and have some rudimentary problem solving ability.

The issue is that there is a bit of a misunderstanding about health soil.

“The earthworms are in the soil because the soil is healthy. They are not necessarily doing anything for it,” says Peter Groffman, a soil ecologist at the City University of New York.

Groffman, in a recent talk with NPR’s Ira Flatow, expanded on this explanation.

“The soil’s probably pretty well aerated anyway. Certainly if we have soils that are compacted, say, that have seen a lot of traffic, people walking on them or driving on them. Compacted soils, earthworms can have very beneficial effect by aerating the soil. Most of our research is focused on forests and the effects that earthworms have in forests are – forests are naturally very well aerated. And so the benefit of the aeration in forests is probably very limited.”

Part of the problem is that the small tunnels worms create allow nutrients and pesticides to leak into nearby waterways.

“During the Pleistocene Epoch that began about 2.6 million years ago and lasted until about 11,700 years ago, ice sheets covered Ontario,” explains a paper by Cathy Kavassalis for the The Halton Region Master Gardeners website. “When they retreated, no earthworms were present in Ontario. Over the next 10,000 years, Ontario ecosystems evolved without earthworms. Their introduction over the past few centuries is dramatically impacting Ontario ecosystems. Gardeners need to reduce their spread to protect Ontario forests and grasslands.Earthworms move through the soil, ingesting organic and mineral matter as well as microorganisms. As they do, they mix soil layers and create new soil profiles. They change the physical, chemical and biological activity of soils. They impact soil microbia and reduce mycorrhizal diversity. Plant species supported by new soil systems change.

New invasive “jumping worms”

So that’s the worst of it, right? Some worms came over here centuries ago and they are messing with our soil? Not so fast. There is a new threat in the world off worms and they come from Japan.  If “Jumping worms” sound mildly terrifying, it’s because they are. Don’t try to put these worms on a hook, they will “thrash and snap” their bodies in self defense. They also have what has been described as an “insatiable” appetite and they leave behind much depleted soil. Oh, and did we mention they can reproduce with mating?

“The jumping worm looks like a regular earthworm, but it’s not,” explained Shari Kulha for the National Post. “It varies in size but often runs large, plump and up to 20 centimetres in length, and most unusually, it jumps and shakes when disturbed. When it’s going about its business — which is devouring the nutrients in the earth — it slithers along the surface like a snake. Like other worms, it reproduces on its own and can regenerate a tail if it pops off in a battle. Sometimes called a “snake worm,” it also congregates in masses — as many as 100 in a single metre of earth.

What can be done about invasive earthworms?

Okay, before you run out to your garden and start ripping apart the soil and tossing earthworms into your fire pit, understand that there are things that can be done about invasive earthworm species. Ontario’s Invasding Species Awareness Program has four tips in particular.

  • When moving soil or plants be careful to avoid moving earthworms into new areas.
  • Clean your boots! Soil on boots and equipment can easily transport worm eggs and other seeds into new areas.
  • Dispose of bait in an area of known worm infestation such as the garden at home, rather than dumping bait on land in natural areas.
  • Don’t move earthworms into new natural areas such as forests.


Are earthworms dangerous?

Earthworms are generally not dangerous to humans or pets. In fact, they are beneficial to soil health and play an important role in the ecosystem.

Earthworms can help improve soil quality by aerating the soil, breaking down organic matter, and increasing nutrient availability for plants. They also provide food for many bird and mammal species.

While earthworms are generally harmless, it’s important to note that some species can cause harm if they are ingested. The giant redworm, for example, can cause vomiting, diarrhea, and other symptoms if ingested by pets or humans.

Additionally, some species of earthworms can be invasive and cause harm to ecosystems if they are introduced to areas where they do not naturally occur. For example, the Asian jumping worm is an invasive species that can alter soil structure, reduce plant diversity, and change the composition of forest litter.

Overall, while there are some potential risks associated with earthworms, they are generally considered to be beneficial to the environment and not dangerous to humans or pets.

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About The Author /

Jayson is a writer, researcher and educator with a PhD in political philosophy from the University of Ottawa. His interests range from bioethics and innovations in the health sciences to governance, social justice and the history of ideas.
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