You might think scientists know everything there is to know about earthworms. After all, how hard could it be to study a creature that you can dig up in almost any backyard?
But there’s a lot you probably don’t know about earthworms and a lot even the scientists don’t know – and you might be able to help them find out more.
Let’s start with this: Did you know that there’s more than one species of earthworm? The species fall into three groups that live in different parts of the soil and have different feeding habits. Here’s another fact that might surprise you more: Chances are, that earthworm you dig up in your garden is actually an invasive species.
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When we think of invasive species we often think of competition among similar creatures – one fish or bird taking over the habitat of another fish or bird. But as with every other invasive species, introducing worms to an ecosystem that developed without them has an effect on everything that lives there.
Non-native earthworms have been in North America for a couple hundred years, brought over from Europe with soils and plants. More recently, Asian species, sold as fishing bait and for composting, have started to spread. “They have such voracious appetites and can live at such high densities that they’re the ones people are trying to focus attention on now,” says Ryan Hueffmeier of Great Lakes Worm Watch.
This concern is very clear in the Great Lakes region, where there are no native earthworms, Hueffmeier says. “Where the glaciers came down and scraped everything down to bedrock, the ecosystems developed earthworm-free.”
We think of soil as just dirt, but there’s actually quite a complicated ecosystem under our feet, so any new species can upset the balance. In these earthworm-free forests, when leaves fall onto the forest floor and plants and animals die, they build up into what’s called the duff layer. “In this area, it was just fungus and bacteria that broke down the organic material for plants to consume and take up,” Hueffmeier says. “Earthworms come in, they do their job and do it extremely well – they take that forest floor and convert it into the rich black soil you find in your garden.”
That sounds great – but the local plants and animals in the Great Lakes region evolved to live with the duff layer, not in the rich soil associated with earthworms. In addition, the excess nutrients left by earthworms can wash out in a heavy rain, causing problems similar to fertilizer runoff. The differently textured soil is an issue as well. “There’s a mat of fine roots in that duff layer – that’s where a lot of our native plants put their root system,” Hueffmeier says. “When that duff is gone, it’s a lot harder for them to put their roots into the mineral soil.”
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Working Against the Natural Order
One important plant, the sugar maple, is a good example of the changes wrought by earthworms. “When you go to an earthworm-free sugar maple stand, there’ll be carpets of seedlings and saplings,” Hueffmeier says. “Go into a heavily invaded forest and there’s a sapling here, a seedling there, and they don’t look healthy. It’s not that they can’t grow – they do, but their density and success rate goes down.”
Hueffmeier compares the effect of earthworms to that of some much bigger animals. “They’re a keystone species, just like a beaver – or just like us,” he says. “When [these keystone species] come into an environment, they change it to their liking. And that’s exactly what happens under our feet with earthworms.”
Earthworm-free forests aren’t the only places where non-native earthworms are a concern.
In Kansas, Bruce Snyder of Kansas State University says that he’s mostly finding them where natives have already been driven out by soil disturbance via development, or where the frequency of prairie fires has been altered by ranchers using controlled burns to encourage grass. But just because they’re not competing with native earthworms doesn’t mean they’re not competing with other native creatures and potentially changing the balance, as Snyder knows from his work elsewhere.
“I did my Ph.D. dissertation on an invasive earthworm moving into the Great Smoky Mountains National Park,” Snyder says. “The other things I worked on were millipedes. These earthworms weren’t affecting native earthworms much, but they were having a big impact on the number of individual millipedes and the number of millipede species.”
He says that earthworms can also change the way an ecosystem functions by altering the way carbon and nitrogen cycle through the system. “It’s hard to see, but it can be a big problem. Changes like that can cascade to things like the plant community.”
Simple Ways to Save the Soil
So how can you help stop the spread of invasive earthworms? If you fish, don’t dump your leftover bait in the water – it won’t drown, and it’s most likely a non-native species. If you buy worms for composting, which are increasingly the Asian type, keep them contained, and don’t use the soil itself – use compost tea as a fertilizer. Even sifting the soil carefully isn’t enough. “There’s going to be eggs, cocoons, baby worms – they are super tough to find,” Snyder says. “You’re going to miss some.”
Snyder adds that there is still a lot we don’t know about these creatures living right beneath our feet – so much so that even undergraduate research projects can turn into scientific publications. But you don’t have to be a scientist to get involved: Great Lakes Worm Watch gives average people a way to contribute by teaching them to identify different species of earthworms. Contact them to find out how you can help collect information on where these worms occur. Although the project concentrates on the Great Lakes area, Hueffmeier says he’ll take data from anywhere in the country.
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