Until about 10,000 years ago, a vast ice sheet covered the northern third of the North American continent. Its belly rose over what is now Hudson Bay, and its toes dangled down into Iowa and Ohio. Scientists think it killed off the earthworms that may have inhabited the area before the last glaciation. And worms—with their limited powers of dispersal—weren’t able to recolonize on their own.
For someone like me, who grew up in the Midwest seeing earthworms stranded on the sidewalk after every rain, this was a shocking revelation. With the exception of a few native species that live in rotting logs and around wetlands, there are not supposed to be any earthworms east of the Great Plains and north of the Mason-Dixon Line.
But there are, thanks to humans. We’ve been moving worms for centuries, in dirt used for ship ballast, in horticultural plants, in mulch. Worms from South America now tunnel through the global tropics. And European earthworms live on every continent except Antarctica. Dobson, a forest ecologist at Yale University, calls it “global worming.”
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Gardeners now rejoice to find earthworms in their soil, and you can purchase a 1,000-pack of “Nature’s Wonder Workers” on Amazon for $45. [...] But Peter Groffman, a soil ecologist at the City University of New York, says that while worms may do some good in your compost bin, they don’t deserve all the credit for your bumper crops and lush ornamentals. “The earthworms are in the soil because the soil is healthy,” he says. “They are not necessarily doing anything for it.”
And though they can be helpful for breaking up compacted soils and breaking down organic matter, worms can also cause trouble in agricultural fields. Their burrows create channels that allow nutrients and pesticides to leak from fields into nearby waterways, and carbon dioxide and nitrous oxide to escape into the atmosphere. In fact, a 2013 review of recent research found that worms likely increase greenhouse-gas emissions.
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In the absence of worms, North American hardwood forests develop a thick blanket of duff—a mille-feuille of slowly decomposing leaves deposited over the course of years, if not decades. That layer creates a home for insects, amphibians, birds, and native flowers. But when worms show up, they devour the litter within the space of a few years. All the nutrients that have been stored up over time are released in one giant burst, too quickly for most plants to capture. And without cover, the invertebrate population in the soil collapses.
Where millipedes and mites once proliferated, now there are only worms. “If you were to think about the soil food web as the African savanna, it’s like taking out all the animals and just putting in elephants—a ton of elephants,” Dobson says.
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As of late 2017, there had only been one sighting of jumping worms in Canada, but the country’s vast tracts of carbon-rich, worm-free boreal forest are already under siege by their European cousins. And scientists there know it’s only a matter of time before the jumping worms follow.
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