Otherwise beneficial earthworms invading the boreal forest in Canada

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Researchers have revealed through an ongoing research project that by 2056 earthworms will expand their territory from 3 per cent of the Alberta boreal forest to a whopping 39 per cent.

The research project which aims to bring a new weapon to bear against invasive earthworms is a collaborative effort between researchers at The Ohio State University, the University of Alberta and Simon Fraser University. Researchers are using statistical analysis to forecast one worm species’ spread, in hopes of finding ways to curtail it.

Focusing on the boreal forest of northern Alberta, which doesn’t have any native worms and has been a region free of worms since the last ice age 11,000 years ago, researchers found that there has been an invasion of European species of earthworms in these areas, which began working their way across the United States and Canada.

Published in the journal Computational Statistics and Data Analysis, the research projects that in the next 41 years, one of the invasive species of earthworms — Dendrobaena octaedra – sometimes also known as the octagonal-tail worm will expand its territory from 3 per cent of the Alberta boreal forest to 39 per cent.

According to latest statistics, there are isolated pockets of regions where D. octaedra is present; however, they are spreading deeper into the forest by around 52 feet each year.

The Real Picture

Oksana Chkrebtii, assistant professor of statistics at Ohio State University says that one of the goals of their project is to get a more realistic picture of earthworm spread, which involves a lot of uncertainty. Chkrebtii adds that they are not able to observe the worms directly as they are underground and the only way to infer population dynamics based on the information they get is by using statistical analysis.

The worm is less than an inch long, and its eggs are about the size of sesame seeds. One of the team’s findings is that the worms are entering the forest via roadways, which suggests that the tiny eggs are becoming lodged in tire treads and then dropping onto the ground as people drive through the forest.

Before she joined the study, Chkrebtii thought—as many North Americans probably do—that worms are always good for the soil.

“I thought they were beneficial, especially for agriculture, but maybe the boreal forest is not the best place for them,” she said.

D. octaedra eats leaves that fall to the forest floor. The worms burrow beneath the surface where they mix different layers of soil and change the soil pH. Ultimately, these changes alter how organic and inorganic matter decomposes and result in fewer small invertebrates in the soil. Other types of worms have even been found to cause native plants living on the forest floor to die and birds that nest there to lose their habitat.

Since the forest hosts no native worms, D. octaedra and other invasive earthworms are able to spread unchecked.

Study co-author Erin Cameron and her team at the University of Alberta conducted the first field studies of D. octaedra in Alberta’s boreal forest.

Based on worm counts they made at 78 sites within the forest in 2006, Chkrebtii and co-authors estimated the geographic spread of the worm population. They also modeled the likely number of introductions of worms along roads every year.

The Statistical Spread

By using a technique called approximate Bayesian computation in a new way, the team was able to estimate population spread and new introductions simultaneously, and then use them to get an overall 50-year worm forecast for northern Alberta.

The finding: D. octaedra populations currently expand about 16 meters (around 52 feet) per year. At that rate, a single worm and its descendants—they reproduce asexually, so one worm reproduces on its own—could expand to cover the length of an American football field in six to seven years.

As to introduction rates, the model determined that one new worm is likely deposited in the forest every year for every 100 kilometers (62 miles) of road that runs through it. The section of the forest used in the study contains around 22,000 kilometers (a little less than 14,000 miles) of road, so the model suggests that more than 200 worms are likely to be dropped into the forest per year.

In total, the population for this worm species is expected to expand to cover 39 per cent of the boreal forest floor in northeastern Alberta by 2056.

Earthworms Hitch a Ride

Americans and Canadians alike travel into the forest for hunting and fishing, so in some cases the worms are able hitch a ride for many miles before being deposited there.

The changes wrought by the worms are slow but profound—and the spread is difficult to model, Chkrebtii said.

“When you think of invasive species, you might think that one worm gets in and suddenly you have a forest full of worms, but it’s much more complicated than that. And here the worms are getting re-introduced all the time,” she explained.

Cameron added that the information gained in this study could be used to inform both tourists and locals.

“There is no way to stop the spread of earthworms once they are introduced, but having a more realistic estimate of future spread—for example, if we follow business as usual in terms of road construction—may increase awareness of the magnitude of the issue, and ideally encourage people to avoid introducing worms,” she said.

“This model will allow us to make more accurate estimates of earthworm effects at large spatial scales,” Cameron continued. “For example, we are working on modeling the effects of earthworms on carbon storage in the soil, so we’ll be using the maps of earthworm spread from the more accurate statistical model to estimate overall changes in carbon storage due to earthworms for this region in the future.”

Next Steps

Chkrebtii wants to expand the statistical model to examine how worms that are already in the forest can be transported within it—for example, between campsites—and how they spread to other areas outside the forest.

Meanwhile, Cameron has created a website that suggests ways for people to avoid unwittingly introducing non-native earthworms, and a smartphone app called “Worm Tracker” that lets elementary and middle school students track worm populations in their local area.

Though the app is geared toward students in Alberta, Cameron said that the information it is gathering, as well as the rest of the study, will be useful to researchers elsewhere.

“People in the US should care about this especially, because earthworm invasions have been ongoing there for a longer period of time, and larger impacts have been observed than in western Canada,” she added.

She pointed to the Great Lakes Worm Watch, a project out of the University of Minnesota that partners with students in the U.S. to track invasive worms.