The silent migration beneath our feet

Understanding the spread of non-native earthworms in northern Canada

  • Mar 08, 2024
  • 1,580 words
  • 7 minutes
Dendrobaena octaedra is a the most common non-native earthworm species we are finding in Canada’s boreal forest. (Photo: Stephen Paterson)
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Earthworms have long been recognized as some of the most ecologically important animals on the planet. Charles Darwin famously wrote of them, “it may be doubted whether there are many other animals which have played so important a part in the history of the world.” However, despite their fame, there remains much to be learned about these humble subterranean creatures. For example, there are about 7,000 described species of earthworms, but it’s expected that another 20,000 have yet to be discovered; the biology and life history of many species is poorly understood; and their ecological roles are diverse and complex.

Stephen Paterson (front) inspects soil for non-native earthworms while Ayla McDonald (field assistant; back) records vegetation and habitat data at a study site in the Yukon. There are faster ways to sample for earthworms, but the most reliable method is to simply pick through a known volume of soil and remove earthworms by hand. (Photo: Robert Cohen)
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My fascination with earthworms began several years ago. I was doing research on insect biodiversity in remote parts of northern Saskatchewan for an organization called Troutreach. When my field partner and I happened upon an earthworm, this seemed an unremarkable discovery to me because, like most Canadians, I was used to seeing them in my yard. My better-informed colleague, however, knew this was an unusual place for an earthworm to be. That’s because earthworms were wiped out in most of what is now Canada and the northern United States during the last ice age. Earthworms migrate very slowly and never recolonized these regions after the icesheets retreated, leaving most Canadian forests naturally earthworm-free. The earthworms most Canadians are now used to finding in their gardens or scattered across the sidewalks after a summer rain are non-native European species that have been introduced over the last few hundred years since the arrival of European settlers.

While working in northern Saskatchewan, I started casually looking for earthworms in my spare time; turning over logs, noting where they were, and wondering how they got there. Beyond my own curiosity, this turned out to be an important question, and I wasn’t the only one trying to answer it. As I learned more about earthworms, I kept reading about one of Canada’s foremost earthworm researchers, Erin Cameron, who is a professor at St. Mary’s University in Halifax. Ultimately, I was fortunate to end up pursuing my PhD in Cameron’s lab.

Samantha Bennett (field assistant) holds a vial of non-native earthworms collected at a study site in northern Saskatchewan. Earthworms are held in a vial like this only briefly before being carefully euthanatized, preserved, and catalogued. In the laboratory each earthworm specimen is measured, identified to species under a microscope, and in some cases, used for genetic analyses. (Photo: Stephen Paterson)
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It’s important to understand the distribution of earthworms to determine their large-scale ecological impacts. Earthworms are known as ‘ecosystem engineers’ because they change the structure of soil and shape the ecosystem around them. Soils harbour a complex and diverse community of microorganisms, fungi, and invertebrates. This community influences the growth of plants and therefore affects the animals that depend on them. Soils also store nutrients that help plants grow, and carbon, which helps to mitigate global warming.

In Canada, earthworms are an unusual case of invasive species. They are culturally and economically important for agriculture, gardening, vermicomposting, and fishing bait; however, in forests, they have predominantly negative environmental impacts. When earthworms are introduced, they can accelerate decomposition, consume organic matter, and mix the soil. These activities can release greenhouse gases and cause losses of soil biodiversity that in turn can affect larger plants and animals. The specific impacts of earthworms depend on the particular species, the type of forest they are living in, and how long they’ve been there. This leads to a critical question: how can we mitigate the negative impacts of earthworms while detracting minimally from their benefits?

Vials containing non-native earthworms collected at a study site in northern Saskatchewan. Earthworms are held in a vial like this only briefly before carefully being carefully euthanatized, preserved, and catalogued. In the laboratory each preserved earthworm specimen is measured, identified to species under a microscope, and in some cases, used for genetic analyses. (Photo: Stephen Paterson)
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Downloading data from a soil temperature and moisture logger. Last year we installed these loggers at 82 sites in the Yukon to collect soil temperature and moisture readings every 15 minutes for one year. These data are important for understanding how temperature and moisture affect the distributions of non-native earthworms at high latitudes. We also submit these data to national and global datasets that are used to build microclimate models that are valuable resources for ecologists. (Photo: Stephen Paterson)
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To make informed decisions on how to manage earthworms, we need to quantify their impacts and predict their migration patterns. To do that, we need to know how widespread they currently are. There are two main pieces to the puzzle when it comes to predicting earthworm distributions. First, to colonize a new patch of land, the worms need to get there. Second, the earthworms need to be able to survive the climate and soil conditions of their new habitat. In our research, we have been looking at both aspects.

Earthworms migrate slowly on their own (only about 10 metres per year), so are usually spread over farther distances by humans. Sometimes they are introduced purposefully for use in gardens, compost, or as live fishing bait, but they are often relocated accidentally. Earthworm cocoons (eggs) and juveniles are very small and can be transported even in trace amounts of soil. Recently, I noticed an earthworm in one of my house plants and upon closer inspection found it was an invasive species from Asia that had not yet been recorded in Nova Scotia, where I now reside. The invasive earthworms likely travelled with the plant, which was imported from Florida.

Ayla McDonald (field assistant) records data at a study site in the Yukon. In addition to sampling for earthworms, we collect a wealth of data on everything that may affect them, such as soil properties, ground cover, plant communities, climate, and human disturbances. (Photo: Stephen Paterson)
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In 2021, we designed a study to determine if earthworms are also spreading through lakes and rivers. This is difficult to study in nature, but we found a great place to answer this question. Lac La Ronge is a large lake in northern Saskatchewan with over 1,300 islands. While working in the area, we came to realize that many of these islands have non-native earthworms on them. By understanding the distributions of earthworm populations on these islands in relation to human activity and island geography we can determine how they are spreading. This project is ongoing, but it seems that at least one species of non-native earthworm regularly disperses though water in northern Canada.

To better understand the factors affecting the spread of earthworms in Canada, we are also determining their occurrence at the northern margins of their distribution. Distributional data on earthworms is very limited in Canada and records of earthworms in the Canadian territories are especially few and far between. It is unclear if this is because earthworms are in fact rare at these high latitudes, or because almost no one is looking for them. So, we set out to do the first structured and quantitative survey of earthworms in the Yukon.

The concept is simple, but collecting the data is tough. Each of the last three summers, I’ve spent about three months conducting fieldwork in remote forests, working long days rain or shine, and often living in a tent. I work with a team of research assistants to systematically sample northern forests to see if earthworms are there, and if so, how many, and what species. I then relate these data to factors that might explain why the earthworms are (or are not) there, and will use statistical models to make predictions about their distribution across the landscape.

An island archipelago on Lac La Ronge, northern Saskatchewan. Islands have long been recognized by scientists as natural laboratories that can reveal mechanisms that drive large-scale biogeographic and evolutionary patterns. Understanding how non-native earthworms colonize these remote islands will help us to understand how they will spread more broadly across northern Canada. (Photo: Stephen Paterson)
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In 2022, after three long months of sampling in the Yukon, we determined that non-native earthworms are common around some cities, but for the most part, have not spread beyond the urban environment as they have in more southern parts of the country. It was unclear if this pattern marks the early stages of a biological invasion in the Yukon, or if earthworms are limited to urban environments at these high latitudes. Cities are likely places for earthworms to be introduced through gardening, compost, or transportation of soil, but cities may also be warmer and provide more hospitable soil conditions. To investigate this further, I returned to the Yukon in 2023 to collect thousands of soil samples and install long-term soil temperature and moisture sensors to determine the environmental tolerances of these introduced earthworms. Collection of these data will be completed in 2024 and will allow us to better predict the future spread of these potentially invasive species.

This research, in combination with a lot of work that has preceded it, will help us to make informed decisions on how to manage our environment. Preventing earthworm introductions is currently the only effective strategy to limit their impacts, so we need to be proactive in understanding how they are spreading, where they are likely to spread to, and what their impacts will be. The good news is that most earthworm species are predominantly introduced through human activity, meaning their spread is largely in our control. In places where it’s worthwhile to prevent earthworm introductions, there is potential to make impactful changes with relatively low-cost options such as education programs and restrictions on transportation of live earthworms. Right now, you can go online and have live earthworms shipped directly to your door almost anywhere in Canada, but there’s a good case to be made that we should be more cautious about transporting and releasing these impactful animals into new environments.

Funding acknowledgement

Stephen Paterson is a PhD Candidate at Saint Mary’s University. His work is supported by the Weston Family Foundation, the Wildlife Conservation Society of Canada’s Weston Family Boreal Research Fellowship, the Natural Sciences and Engineering Research Council of Canada, the Northern Scientific Training Program, the Saskatchewan Fish and Wildlife Development Fund, the Royal Canadian Geographical Society, and in-kind support from the Saskatchewan Wildlife Federation and Troutreach Saskatchewan.

Land acknowledgement

This research was conducted on Treaty 6 and Treaty 10 territory, the traditional territory of the Cree and Métis, and on the traditional territories of the Kwanlin Dun, Ta’an Kwäch’än, Little Salmon Carmacks, Selkirk, Na-Cho Nyäk Dun, Tr’ondëk Hwēch’in, Champagne & Aishihik, White River, and Kluane First Nations.


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