Thursday, July 25, 2013

Are Earthworms An Invasive Species?

earthwormsAldo Leopold, the famous conservationist, once wrote: “One of the penalties of an ecological education is that one lives alone in a world of wounds.” This whole idea puts nature writers in an odd position. On the one hand, it’s our job to raise the alarm when we see something amiss, but on the other, we run the risk of spending so much time dwelling on nature’s wounds that we end up giving people the impression that everything has gone to hell, which of course it has not.

So what to make of earthworms? We’ve been told for years that worms are good. Darwin was a great admirer. They make our gardens grow. But as invasive plant and animal awareness grows, we’re now being told they’re invasive animals that have the potential to destroy whole forest ecosystems.

It is true that earthworms are not native in the Northeast, though neither are apples or honeybees. Conventional ecological thinking holds that our native earthworms got smooshed by the last glacier, and that the earthworms here today are European immigrants that arrived with the first settlers and have continued to migrate ever since. When you’re done fishing and you dump your surviving nightcrawlers (Lumbricus terrestris) out on the bank of a creek, you’re contributing to (or establishing a new) non-native population. And, of course, introducing a new variable into an equation causes a disruption.

So all this we know. What ecologists are still trying to get a handle on is what, exactly, worms are doing to the forest. And there are conflicting reports. Tim Fahey, a forest ecologist at Cornell University, has been studying New York’s 20 species of exotic earthworms for years (Vermont and New Hampshire have between 15 and 20 species), and some of what he’s found is encouraging. Earthworms don’t move quickly – maybe five or ten meters a year. And in the area where he’s studied, Fahey’s observation is that the worms aren’t really invading anymore. They exist in rich soil sites in the forest, adjacent to farm fields and roads and fishing ponds; they aren’t found in areas with acidic soils. Barring major ecosystem change, he guesses that they’re pretty much where they’re going to be.

Where earthworms exist in large numbers in a forest (large would be about 150 per square meter), they destroy the layer of leaves that blankets the forest floor. This can be bad. There’s evidence in some places that native plants, like wild sarsaparilla and bellwort, are negatively affected, and the exposed dirt may make it that much easier for an invasive plant like garlic mustard to get a foothold. But in other places the woods seem fine. I went for a worm walk with Fahey and a conservation group in eastern New York, and we found earthworms in an ecosystem that supported a good diversity of trilliums, wild ginger, jack-in-the-pulpit, and other native forest herbs. If we hadn’t doused the ground with mustard-laced water, an act that prompted the worms to race to the surface, we might not have even known they were there.

Now the fine print.

Earthworms do seem to affect the amount of carbon a forest can sequester, something to consider as atmospheric carbon dioxide levels continue to rise. Fahey says that in one recent study, wormy forest soil contained about a third less carbon than non-wormy forest soil.

Also, earthworms eat tree roots – as much as one fourth of a tree’s roots per year – and break up the beneficial mycorrhizal relationship the roots have with fungi in the soil. Fahey hasn’t seen this kill a tree, but it’s one more stress they have to endure.

And there’s a new crop of Asian worms in the genus Amynthas that have ecologists concerned – one’s been dubbed the “crazy snakeworm.” These new worms are reportedly much more aggressive than our lumbering lumbricus species, though they’ve yet to become widely established.

The take home seems to be that earthworms are a non-native species that affect forest ecosystems. There’s nothing we can do about it, but next time you go fishing you may as well dispatch your bait instead of dumping it and contributing to the problem. And Asian snakeworms may be on the way. Or maybe not. If the ambiguity here is terribly unsatisfying, and why wouldn’t it be, I’ll leave you with Fahey’s worm conjuring recipe. Mix 20 grams of Coleman’s hot powdered mustard with a gallon of water. Dump on ground near nightcrawler holes. Wait for worms to come to surface (about 30 seconds). Collect. Go fishing.

Dave Mance III is the editor of Northern Woodlands magazine. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation:

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6 Responses

  1. Pete Klein says:

    So throw the worms in the lake for the fish to eat.

  2. It wouldn’t shock me if some here considered mankind to be an invasive species…..

  3. Wally says:

    Interesting article. Thanks. I have read that the problems caused by worms are more severe in the upper Midwest. As for humans, not invasive, but certainly beyond ecological control currently.

  4. Ellen says:

    Throwing worms overboard is part of the problem. Earthworms can get oxygen through their skins and contrary to popular belief, do not drown; they are able to make their way back to land to cause damage. Humans have been the main way that worms have traveled distances in northern Minnesota since the ice ages left us with no native earthworms.