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The garden-variety earthworm is a modern interloper in the northern forests. Photo: <a href="">Fir0002/Flagstaffotos</a>, Creative Commons, some rights reserved
The garden-variety earthworm is a modern interloper in the northern forests. Photo: Fir0002/Flagstaffotos, Creative Commons, some rights reserved

Natural Selections: Invasive earthworms

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Earthworms, friend to lawn and garden, are actually an invasive species in northern forests, which developed in the worm-free environment of retreating glaciers 10,000 years ago. Martha Foley and Dr. Curt Stager discuss their return, and the consequences for boreal soil, trees and wildflowers.

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Martha Foley: Tell me about this worm thing. Worms are apparently ruining our northern forests. What is going on?

Dr. Curt Stager: It’s weird, I mean I – I grew up hearing worms are great for the soil, and –

MF: Yeah! What’s wrong with worms? You want them down there. It’s healthy soil if you get worms.

CS: Yeah I mean like, robins eat them, and that’s kinda neat, and kids go fishing with them.

MF: Right!

CS: So they’re just wonderful. But apparently they’re not if you’re a forest ecologist, and if you like wild flowers in the forest. And if you’re concerned about sugar maple growth –

MF: Are they invading the forest? Aren’t they there already?

CS: Well, you know, I sort of look back in my memories of messing around in the soil in forests and I think of finding worms once in a while. So I think they’re supposed to be there. But actually, they’re not, anywhere in the northern half of North America. Anywhere within the formerly glaciated regions from the last Ice Age, there shouldn’t be any worms there. They were all bulldozed away by the ice sheets, more than 10,000 years ago.

MF: So the forests as we know them are sort of based on this wormless –

CS: Worm-free environment.

MF: So they’re grown up and all the little plants and everything have sort of – our forest makeup has evolved without worms.

CS: Right. For the last 10,000 years, northern hardwood forests, especially, have not had worms in them. So that means when the leaves and the twigs and all fall to the forest floor, they do break down because of the fungi and bacteria, but not very quickly. So they would sort of build up, and you’d have sort of a punky, fluffy duff layer, I guess you could say, on the forest floor. Because that was there, wildflowers and other plants that are going to sprout in that hardwood forest have adapted to that sort of a situation, so.

MF: And the worms are upsetting that.

CS: The worms are upsetting that because that’s what they eat, that leaf litter stuff.

MF: Well where are they now? Are they on the march? I mean is there a worm, you know, worm line that scientists are tracing that’s moving north?

CS: Actually, literally yeah. The difference between worm infested and non-worm-infested soil is so striking that you can actually almost see an advancing front from where these things go, where there’s – behind the, I guess you could say behind the enemy lines, they’re a, a small fraction of the native wildflowers are left. So you don’t have the trilliums and the Solomon’s seal and the yellow violets and all. There’s a – you can actually see the difference.

MF: Sure. Yeah, well I’m thinking of the deciduous forests, say in the south. And it is different. They have a different Spring season for sure, for flowers.

CS: The thing is it’s not that there’s like this monolithic wave sweeping across the country. It’s actually from little pockets. These things are brought in in people’s lawns, in the soil, from their potted plants, in farms, in the hooves of animals. And especially from fisher folk, who go fishing with worms, and at the end of the day they think they’re just gonna – “Oh, I’ll free the little wormies.”

MF: Just dump ‘em out.

CS: And you dump ‘em out because you think that’s good for the worms and good for the soil, but it’s actually one of the ways they’re being spread.

MF: Okay, so, so what’s the problem? I mean what’s going to happen?

CS: Well, if these things spread throughout the forest, we’re going to lose a lot of the wildflowers. And economically it can be very important because they also disrupt the regeneration of sugar maples.

MF: Oh.

CS: Which then adds to the whole problem of acid rain, pear thrips, deer grazing and everything. On top of that damage to the sugar maple growth, we now have this problem with earthworms.

MF: So is there anything we could do about keeping the worms at bay?

CS: The better thing to do is try not to encourage their spread. And one of the best things you can do, is if you are a fisher person out in the woods, don’t throw your worms out in the woods.

MF: Save ‘em for your lawn. 

CS: Yeah.

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