The Trouble with Earthworms

An invasion of worms is threatening forests in the Northeast and Midwest. Peter M. Groffman, senior scientist at the Institute of Ecosystem Studies in Millbrook, N.Y., explains why worms might not be your garden's best friend.

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IRA FLATOW, host:

For the rest of the hour, something you probably didn't expect to talk about today when you turned on the radio. And that is earthworms. Earthworms.

There is a trouble with earthworms. If you're a gardener, you have no doubt heard the conventional wisdom that to get good soil you need lots of earthworms.

But while that may be true for vegetable gardens - and I emphasize may be true because my next guest may even question that knowledge that we think we know about - earthworms are not so good for other types of ecosystems.

Scientists in the Northeast are reporting a real problem with earthworms. The invasive worms are destroying the rich carpet of organic matter on the forest floor and may be causing a major shift in the delicate balance between the soil and the plants and the animals that need it to live.

Joining me now to talk more about it is Peter Groffman, senior scientist at the Institute of Ecosystem Studies in Millbrook, New York. He joins us by phone. Welcome to SCIENCE FRIDAY.

Mr. PETER GROFFMAN (Senior Scientist, Institute of Ecosystem Studies): Very glad to be here.

FLATOW: Hey, now we've all heard that earthworms are great for our garden.

Mr. GROFFMAN: Yeah. You know, it's interesting what you just said. You said that in order to have good soil you have to have earthworms. We don't really know if that's true.

We certainly know the other thing is true, that if you want earthworms - earthworms like good soil. And so if you have good soil, you definitely have earthworms. But you can probably have good soil without earthworms.

FLATOW: We heard they aerate the soil, things like that.

Mr. GROFFMAN: Yes, they do. But the soil's probably pretty well aerated anyway. Certainly if we have soils that are compacted, say, that have seen a lot of traffic, people walking on them or driving on them. Compacted soils, earthworms can have very beneficial effect by aerating the soil.

Most of our research is focused on forests and the effects that earthworms have in forests are - forests are naturally very well aerated. And so the benefit of the aeration in forests is probably very limited.

FLATOW: So no one has really studied whether earthworms actually help your garden soil or not?

Mr. GROFFMAN: People have studied it. But most of the work's been anecdotal. So some of my colleagues have been involved in work on earthworms in agricultural systems, and manipulated levels of earthworms to see was there an improvement in the environmental performance of those systems. And they were unable to find any.

FLATOW: Hmm.

Mr. GROFFMAN: And in fact earthworms, you know, by making holes in the soil, accelerate the movement of water through the soil. And so in a certain way that's good, and it stimulates infiltration of water into the soil, but it also stimulates leeching of water through the soil profile and into the groundwater.

So sometimes we might find actually more water-quality problems in gardens or in agricultural systems because earthworms are accelerating the way water and nutrients move through the system.

FLATOW: And now you've got even worse news about earthworms in forests.

Mr. GROFFMAN: Yeah, so the interesting thing - and one of the things that's nice about this is it's an interesting mixture of basic and applied science, where we've learned a lot of - by studying earthworms, we've learned a lot about the soil ecosystem and the ecosystem services that go on in the soil, which is something - we soil scientists sometimes feel that society doesn't think enough about the soil and the many good things that it does.

But so many forests - forests pretty much north of the glacial boundary, for the most part have no earthworms. So if you think about the glacial boundary runs from, say, central New Jersey across the middle of Pennsylvania and through Ohio and then to the West. Fifteen thousand years ago, all that area was covered by glaciers. And the glaciers - the hypothesis is that the glaciers killed the earthworms and eliminated all the native species of earthworms.

And so when we go out now, we see large areas of the northern forest have no earthworms. And instead they have these very thick organic layers, organic forest floors that you mentioned before, and we have a lot of ideas about the importance of that organic layer in forest soils. It protects the soil from erosion. It's where the roots of trees grow. We have lots of valuable plants, say spring beauty and trillium and trout lily. They're all rooted in the forest floor. And there are lots of animals that live in the forest floor, in particular salamanders and a series of beetles and insects.

And so for many years we studied these northern forest soils. We studied these organic horizons and the biodiversity associated with those, but then people started to notice that patches of the northern forest were colonized by earthworm species from Europe and from Asia, and they noticed that the forest floor would disappear.

In a period of maybe three to five years, it would go from this thick organic carpet, and then all of a sudden that would be gone and you would have just bare mineral soil at the surface. And so people became very concerned about the loss of these ecosystem services that come from these forest soils.

FLATOW: You mean all the leaves and the organic matter we walk on and trudge through happily through the forest would be sort of, like, missing?

Mr. GROFFMAN: Yes! And again, people, you know, people probably don't pay enough attention to the soil, but enough people do that they noticed that this was a change. And then instead of seeing this carpet that you could pick up, you'd have a couple of leaves and then a lot of earthworm casts and then pretty much some bare soil.

So people became concerned about are our forest soils going to be eroding, which is a huge problem in forests. And they also became concerned about the loss of some unique biodiversity in terms of these spring ephemeral plants. This time of year, people go out in the woods and they're all excited to see the trillium or the trout lily or the spring beauty bloom, and earthworms can be very tough on those.

And then there are a variety of other ecosystem services that the soil provides related to - like, we hope that our forests are pulling carbon dioxide out of the air and then storing that carbon dioxide in the soil. And if the earthworms are churning up the soil, they're taking a lot of that carbon and putting it back in the atmosphere, and so there's an ecosystem service that is decreased by earthworms.

FLATOW: Wow.

Mr. GROFFMAN: And we also want our forest to pull nitrogen out of the air and keep that nitrogen out of the groundwater or out of estuaries, and we have active research now to determine, if we have this earthworm invasion, does that also reduce that nitrogen ecosystem service.

FLATOW: When you say an earthworm invasion, you're basically saying, if I hear you correctly, they're recovering from the last ice age.

Mr. GROFFMAN: Well that's a very good question because there are native species of earthworms in North America, but we don't see them. Rather, what we see are these European and Asian species. Even if you go south of the glacial boundary into areas where there are native North American species, for the most part those have been replaced by bigger, more aggressive European and Asian species.

FLATOW: So wait, let me just stop you. So here in Central Park in New York, when I walk through the park and it's certainly a glacial - it was covered, as you all know, in the glacier, these are all non-native earthworms?

Mr. GROFFMAN: They are, and some of them have been there for a long time. And actually some of the earliest work on earthworm invasions was done not in Central Park but in the Bronx in Van Cortlandt Park.

FLATOW: Sure.

Mr. GROFFMAN: There was a student at Rutgers who now works for the Forest Service, Rich Pouyat, who was doing his dissertation research on forests along an urban to rural gradient. And these were forests that were very similar, in similar trees, similar soils, but the forests in the city had extraordinary densities of earthworms in them. And so then he began to raise these questions about, well this is really changing the way these forests function.

And some of these earthworms have been there for a long time. So the common nightcrawler, Lumbricus terrestris, which we probably all grew up with, that is a European species never found as a native species in North America.

FLATOW: The one we're all fishing with.

Mr. GROFFMAN: Well, we fish with a lot of different ones, so there's actually quite a bit of commerce in earthworms because of the many useful things that earthworms do. So they're excellent for bait. They're excellent in compost piles. They're excellent in compacted soils.

FLATOW: But not in the forest.

Mr. GROFFMAN: It would be a good idea to keep them out of the forest.

FLATOW: Did they come over with the settlers? Who brought these earthworms from Europe?

Mr. GROFFMAN: Probably they came with settlers who brought plants that they wanted to grow, and also probably came in the ballast of ships, like many invasive species. When we have ships, you need something to balance the weight.

FLATOW: But don't they drown in the water, earthworms? They come out in the water.

Mr. GROFFMAN: Well, they do drown. They do drown in the water, so sometimes they used soil as ballast just to create weight. So let's just fill the hold of the ship with some dirt, and there were worms in there.

FLATOW: So let me see if I understand this. You're not concerned about the foreign earthworms that were brought over. You're concerned about the northern migration of the nativeā€¦

Mr. GROFFMAN: No, no, no. We see very little northern migration of the natives. What we see are the exotic speciesā€¦

FLATOW: Who are attacking the forest floors.

Mr. GROFFMAN: Pretty much. You could use the word attack. That's maybe a little strong, but they certainly are colonizing the forest floor and transforming it into something else.

FLATOW: Talking with Peter M. Groffman, senior scientist at the Institute of Ecosystem Studies in Millbrook, New York, on TALK OF THE NATION: SCIENCE FRIDAY from NPR News.

Wow, you know, this is something new to worry about now.

Mr. GROFFMAN: It's global worming.

FLATOW: Is it?

(Soundbite of laughter)

FLATOW: Tell us why it's global warming.

Mr. GROFFMAN: Global worming.

FLATOW: Oh, global worming.

(Soundbite of laughter)

Mr. GROFFMAN: And global warming could be accelerating it, could be accelerating the movement. In particular, in the last couple of years we've seen - you know, the nightcrawler has been here for a long time. In the last 10 or 20 years or so, we've seen Asian species of earthworms that seem to be spreading very rapidly. And based on what people know about their native range, they shouldn't be moving up into northern New York and in Minnesota and other places like that where we see them. And it is possible that if we're having warmer climate, that it is facilitating the invasion of these species, but they probably would be colonizing these soils.

FLATOW: Let me see if I can get a phone call. Diane(ph) in Florida, hi.

DIANE (Caller): Hi. This is Diane Livingston(ph) in Jacksonville, Florida.

Mr. GROFFMAN: Hi.

FLATOW: Go ahead.

DIANE: And let's see.

FLATOW: We only have a couple of minutes, Diane.

DIANE: Okay. I established a piece of work, a National Science Foundation piece of work using earthworms, and your professor was talking about the value of the earthworms in the forest.

FLATOW: Right.

DIANE: Well, they are carbon zinc. They provided carbon zinc, a very useful zinc, converting the leaves into fertile humus. And that fertile humus is the basis of our ecology, our ecosystems, and it's absolutely essential for our ecosystems, for the health of it, and roots will follow this humus.

FLATOW: Let me ask Peter Groffman for a reaction. Peter?

Mr. GROFFMAN: That's a great question, and it's actually one of our main research topics is exactly on the effects that earthworms have on organic matter or humus in the soil. I do have to mention, our work has been very generously funded by the National Science Foundation.

But so one of the most interesting findings from our work has been that, yes, the earthworms take those leaves and they mix some of it in with the soil, but they also eat a lot of those leaves and convert it to carbon dioxide and send it back to the atmosphere.

And so what we've seen in these forests that are invaded by earthworms, they have less humus than forests without earthworms because it's almost like tilling the soil. You're mixing the organic matter, you're stimulating the microorganisms that are degrading the organic matter. And so we found that in the forests invaded by earthworms, they were losing - 25 percent of the organic matter in the surface soil was reduced.

FLATOW: Dr. Groffman, what do we do about this creeping threat here?

Mr. GROFFMAN: There's really not much you can do to remove earthworms from a site where they're well established. You could apply soil pesticides. Pesticides have been known to kill earthworms accidentally in many settings. But there are many side effects of using pesticides, and I would suggest that probably we're not going to be applying pesticides to remove earthworms from forest soil.

FLATOW: Could we bring in predators like robins or birds?

Mr. GROFFMAN: So in - that's our current focus - one of the focuses of our current research is earthworms and their predation by salamanders, who are actually a very charismatic predator in the forest floor.

Mostly, and this is I think common thinking in invasive-species management -it's very difficult to remove invasive species once they're well established from a system. It's hard to remove them without dramatic side effects. So we focus much more on preventing their spread. And there has been some really excellent work, in particular the group in Minnesota, this Minnesota Worm Watch group, which has been basically education and outreach to get people to be more careful with earthworms.

So when people fish, you know, be careful with your bait. And when you're using earthworms in your compost pile, you should be more careful in your compost pile. And so there's been - I think the awareness is starting to improve through education and outreach that people need to be a bit more careful with earthworm.

FLATOW: All right. We're be earthworm wary. Thank you very much, Professor Groffman, for taking time to be with us.

Mr. GROFFMAN: Oh, it was a pleasure to be here.

FLATOW: Peter M. Groffman, senior scientist at the Institute of Ecosystem Studies in Millbrook, New York.

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