What does science say? Global warming and the Great Lakes (yes, you read that right)

What does science say? is a monthly column written by now Great Lakes contributor Sharon Ostuk that explores what science can tell us about what’s happening below and above the waves of our beloved Great Lakes and their watershed.


Invasion of earthworms! It sounds like a bad Hollywood movie, but science can be stranger than fiction.

Like most people who grew up in the Great Lakes region, I thought the worms in my garden were native and a sign of soil health. You know the ones – reddish, brown and ranging from about four to eight inches.

It turns out that the story is more complicated and much more interesting.

As I recently discovered, my garden worms come from Europe and their Latin name is Lumbricus terrestris. Most of us know them as dew worms or night crawlers, popular bait worms.

The first wave of night crawlers arrived in North America from Europe about 400 years ago. They hitchhiked with the settlers’ crops and garden plants, and the worms have been quite happy here ever since.

But nocturnal worms were neither the first invader nor the last. There are now more than 22 non-native species of worms from Europe and Asia in the Great Lakes region. The latest arrivals consist of several species of jumping worms from Asia that writhe and jump like a snake when disturbed.

As the cumulative impact of all these worms becomes clearer, scientists are sounding the alarm about their effect on hardwood forests.

Michigan Tech University researchers recently suggested that invading earthworms are responsible for the death of maple trees in the Upper Peninsula, northern Wisconsin and eastern Minnesota.

Gardeners and farmers usually like to see earthworms because they create holes in the soil, making it permeable and allowing rain and air to penetrate. What’s more, worms deposit nutrients that plants need to grow. They do this by eating dead vegetation. As the worm’s gut digests these leaves and twigs, it excretes nutrients such as nitrogen and phosphorus in its feces. When they poop, worms leave these nutrients behind.

So in farmers’ fields and backyard gardens—places full of non-native plants—these worms are actually beneficial, says Michael McTavish, a postdoctoral fellow at the University of Toronto’s Smith Forest Health Lab and an expert on invasive worms.

The problem starts, McTavish says, when people help the worms travel in hardwood forests. We do this when we move plants with soil from one area to another, when we dump buckets of bait, or even through cocooned worm-infested dirt on our trunk protectors and car tires.

Jumping worm cocoons. (Photo: Sharon Oosthoek)

So what’s the big deal?

Once established in a hardwood forest, the numbers and appetites of invading worms are a problem. There, they rapidly break down plant debris on the forest floor, releasing pulses of nutrients.

The problem is that hardwoods, including maples, are used to a slow drip of nutrients. What they cannot absorb is quickly washed away in the next rainfall or used by non-native plants better adapted to the food pulses.

That’s not all By eliminating this waste too quickly, invasive earthworms are removing the material that many native plant seeds like trillium and blue cohosh need to start life. Without leaf litter, the forest floor also heats up and loses moisture, and the soil is more vulnerable to erosion and freeze-thaw cycles.

Any native seeds that manage to grow into seedlings are more visible on the bare forest floor, making them easy to pick for viewing deer.

The evolution of forests happens slowly. Unlike Europe and Asia, our forested areas simply haven’t had enough time to develop to cope with these changes.

The forests most affected are those with trees that emit leaves with low lignin and tannin content. This means maple, acacia and willow.

If their favorites are not available, the worms will settle for leaves that have higher levels of lignins and tannins – birch and ash, then oak and beech. They don’t particularly like conifer needles because they are waxy and stringy and have less nutrients.

“If you give them a buffet, they will choose,” says McTavish. “But eventually they’ll eat anything, even conifer needles.”

What about native earthworms?

Canada and the northern US had native earthworms. But during the last glaciation 20,000 years ago, they were scraped away – along with most of the fertile soil.

Small pockets survived here and there, but when the glaciers retreated north about 11,000 years ago, the native worms that survived mostly remained. Without human intervention, they move slowly, advancing a little more than half a mile every 100 years.

In Ontario, scientists believe there are only two native species, but they are extremely rare. There are only a handful of native worms in the entire Great Lakes region. Most are invaders.

In fact, exotic earthworms are found on every continent except Antarctica and in almost every type of ecosystem, McTavish says. It’s such a widespread infestation that some researchers have started calling it the “global worm.”

Some scientists proposed chemical poisons or biocontrol using the predatory New Zealand flatworms or parasitic flies, but these could have unintended consequences and therefore did not gain much support.

“There’s really no going back,” McTavish says. “We can’t completely eliminate them, and there’s no real safe or targeted way to get rid of them. The reality is that our forests will probably look different.

Yet the situation may not be as dire as it sounds.

We still don’t know enough about how our forests are changing as a result of these earthworms from elsewhere. For example, worms are a high-quality, protein-rich food for robins, salamanders, and bears. And some plant seeds germinate better after passing through the worm’s gut.

So maybe we’d better stop thinking of them as scary invaders, McTavish says.

“There’s a new soil ecosystem engineer here, even if it ‘shouldn’t be,'” he says. “At what point do we consider them a normal part of soil biodiversity?”


See more news at Great Lakes Now:

What does science say? The weight of the world rests on a small Canadian lake

What does science say? How airlifting wolves saved the Isle Royale ecosystem and sparked a conservation debate


Featured Image: Jumping Worm. (Photo: Sharon Oosthoek)

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