They disappeared 10,000 years ago during the Ice Age in North America, but earthworms have returned and they are adding to the carbon overload that affects our planet’s climate change.

Cindy Shaw, a carbon research scientist from the Canadian Forest Service, studies the boreal forest, the world’s most northerly forest, which circles the top of the globe like a ring of hair around a balding head.

While conducting a study in northern Alberta a few years ago to see how the forest floor was recovering after oil and gas activity, Shaw saw something she had never seen there before: earthworms.

“I was amazed,” she said. “At the very first plot, there was a lot of evidence of earthworm activity.”

Now invasive earthworm species from southern Europe — survivors of that frozen epoch and introduced to this continent by European settlers centuries ago — are making their way through northern forests, their spread hastened by roads, timber and petroleum activity, tire threads, boats, anglers and even gardeners.

“Earthworms are yet another factor that can affect the carbon balance,” Werner Kurz, a researcher with the Canadian Forest Service in Victoria, British Columbia, wrote in an email. His fear is that the growing incursion of earthworms, not just in North America, but also in northern Europe and Russia, could convert the boreal forest, now a powerful global carbon sponge, into a carbon spout.

Moreover, the threat is still so new to boreal forests that scientists don’t yet know how to calculate what the earthworms’ carbon effect will be, or when it will appear.

The relationship between carbon and earthworms is complex. Earthworms are beloved by gardeners because they break down organic material in soil, freeing up nutrients. This helps plants and trees grow faster, which locks carbon into living tissue. Some types of invasive earthworms also burrow into mineral soil and seal carbon, there.

But as earthworms speed decomposition, they also release carbon dioxide into the atmosphere. As they occupy more areas of the world, will they ultimately add more carbon to the atmosphere or subtract it?

Invasive earthworms also have spread to parts of Alaska’s boreal forest, including the Kenai National Wildlife Refuge.

And as scientists analyze the effects of the earthworms they know about, they also are keeping an eye on a new invader: Asian earthworms, which have made their way to southern Quebec and Ontario.

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