In woods again, if the loose leaves in autumn are removed, the whole surface will be found strewn with castings.
—CHARLES DARWIN, The Formation of Vegetable Mould, 1881
I ARRIVED IN MINNESOTA on a clear, chilly day. It was May, but there was still snow on the ground. I met researchers Lee Frelich and Cindy Hale at the university, and they agreed to show me the forest where earthworms were causing such a problem. It took less than an hour to get there from campus, and on the way Cindy told me that most people react with genuine surprise when they find out what she’s working on.
“People always tell me, ‘I thought earthworms were good for the soil,’ and I tell them that they are, in some settings. But these European worms have invaded a forest that evolved without them over the last ten thousand years. Remember, this whole area was covered by glaciers until the end of the Ice Age. When the ice melted, there wouldn’t have been any worms.”
Cindy didn’t set out to study worms. She began her work as a forest ecologist, and in the process she and her colleagues noticed something changing in Minnesota’s forests. The understory was dying. Ferns were disappearing, wildflowers had all but vanished, and young tree seedlings couldn’t take root. Forestry experts couldn’t figure out what was happening, but they knew the forest couldn’t survive without this critical understory of small plants.
Lee, her doctoral supervisor, recalled how puzzling the situation was at first. “A couple plants really took over in some of the forests and replaced all the ferns and wildflowers that used to grow there. Pennsylvania sedge, which looks like grass, and one other plant, jack-in-the-pulpit, started to carpet the forest floor. People used to call me all the time asking what was going on in our forests. Nobody knew.”
“Then somebody published an article about the changes that take place in the ecology between urban and rural areas,” Cindy said. “It mentioned, in a kind of offhand way, that increases in earthworm populations might be causing changes in understory plant populations of New York forests. That’s when it finally occurred to us to go out into the forest with a shovel and dig.”
What they found was nine species of worms, including Lumbricus terrestris and other exotic species. Darwin noted how clever the nightcrawler was when it came to pulling leaves and pine needles into its burrow. But even he failed to realize how this efficiency, this special skill, could be destructive on such a large scale.
Earthworms, the Minnesota research team has learned, can —and do—consume the entire leaf fall of a forest in a single season. Small plants and tree seedlings flourish in the damp, sweet-smelling, slowly decaying layer of forest floor. This layer, the duff, is built up over many years. It contains leaves and other organic matter in all stages of decay. Many of the native plants that once flourished in the forest produce seeds that have intricate germination strategies. A seed might take two or three years to germinate, going through a complicated cycle that depends on this spongy duff layer. Now that the forest floor is bare, most small plants have simply disappeared.
“We’ve seen a loss of eighty to ninety percent of all understory plants in some areas,” Cindy said. “That’s where we find the most earthworms. They just expand their population to fit the available food source. They multiply until there are enough of them to eat all the leaf litter on the soil’s surface. And the ten or twenty percent of plants that do survive? The deer get those.”
I could hardly believe what she was telling me. I thought about the redwood forests back in California. I couldn’t imagine those forests without ferns, native columbine, moss. “It’s just so counterintuitive to think of earthworms as pests,” I said. “Does it ever sound strange to hear those words coming out of your mouth?”
“Not anymore.” She’d been studying this problem for four years. “But yeah, that’s what I have to remind everybody. Earthworms are great in a compost pile. They’re wonderful for agriculture. They do till the soil. They do add nutrients. They do all the wonderful things everyone has always believed them to do. But when they move into a forest that has evolved without earthworms, they can actually have negative effects on the native plants.”
I told her what I knew about the earthworm migration from Europe: that they arrived in potted plants, ship ballast, even in cocoons on the soles of shoes. How did they get into Minnesota’s forests? Have they just slithered in that direction over the years?
“Think about it,” Cindy said. “You’ve been at the lake fishing all day. You still have a few bait worms left when you’re finished. What do you do with them?”
“Dump them in the soil,” I said slowly, feeling a little uneasy as I thought about the bait stand worms I’ve dumped in my own garden.
“Exactly. That’s how they’re getting in. That, and ATV tires that have mud caked in them, and fill dirt people bring in when they build a cabin, and potted plants for landscaping projects. People are bringing them in. They couldn’t move this fast on their own.”
Lee added, “Wilderness managers have known for a long time that you shouldn’t just bring a new species into an environment like this. There’s a threshold effect that can happen in an ecosystem. It can tolerate just so much change, then it snaps like a rubber band. There are wilderness areas in Michigan where they’ve banned live bait since 1965. They might not have known exactly what the worms could do to the forest, but they knew better than to bring exotic species in where they could reproduce unchecked.”
Cindy and Lee have encountered their share of skeptics. “After all,” Cindy said, “We’re working against generations of—well, of common knowledge! It just makes sense that worms are good for the soil. But when I take people out to the forest, and they see what’s happening, I can make believers out of them.” Lee told me that when he first applied for funding to do this research, he met a lot of resistance. “Earthworm researchers are working so hard to prove all the benefits that worms bring to the soil. Nobody wanted to fund research that showed they might be a pest.
“A couple years ago I went to an ecological conference on hardwood forests,” Cindy added. “I sat in the back of every presentation and at the end I raised my hand and asked, ‘Do you have worms in your forest?’ I must have sounded nuts. But you know what? Nobody knew. I got one call from a guy who worked in the Department of Resources in New Hampshire. He’d been telling everybody for years that they should try to keep worms from being imported into the forest. Nobody believed him. There just aren’t a lot of people looking into this yet.”
Then she said something that no earthworm scientist had said to me. “They can be so beneficial, or so destructive,” she said. “They are literally ecosystem engineers. They are at the very base of the ecosystem. Their actions drive everything else that happens. And yet there are a lot of ecologists out there who pay no attention to earthworms at all.”
WE ARRIVED AT THE Wood-Rill forest, a 150-acre preserve outside the Twin Cities. Buds were just beginning to appear on the maple trees. Over the weekend an unexpected snowstorm had left several inches of snow in the forest. I wasn’t surprised that the forest floor was still brown and bare, but Cindy and Lee told me that a carpet of young green plants should have sprouted underfoot by now.
“You’re not used to seeing this forest,” Cindy said, “but let me tell you: this is shocking to us. It looks like the forest’s been nuked. There’s no trillium or wild ginger, no false Solomon’s seal, no bellwort. And look at this.” She kicked aside a layer of brown leaves on the forest floor. “These leaves fell last autumn. Usually there would be layers and layers of leaves from earlier years underneath. The forest floor used to be so spongy, it felt like you were walking on a mattress. Now look what’s happened.”
I knelt down and peered at the black earth under the leaves. It looked exactly like the castings I collected from my earthworm composter. “This is nothing but worm castings!” I said, and she nodded. I scraped the ground with a stick and turned up a tiny pink worm.
“Probably Aporrectodea rosea,” she said. “It’s a little endogeic worm. That’s one thing we’re just starting to figure out—what role each species plays in this invasion. The epigeic worms seem to come in first and eat all the rotten leaves. Then we start to see soil-dwelling endogeic worms, like this one, then the anecic worms like the nightcrawler—the ones that pull fresh litter into their burrows—come along behind them and take care of the stuff that hasn’t even started to rot. All these leaves will be gone by summer.”
I told Cindy and Lee that I couldn’t believe how fast the worms have moved in, or how many different exotic species she’d found.
“You’re not the only one,” Lee said. “When we applied for funding a couple years ago, we were nearly denied in the first round because the reviewers—all scientists—simply didn’t believe what we were saying. They just couldn’t believe that all these earthworms were exotic, or that they were capable of doing this kind of damage.”
“But look over there,” Cindy said. We were standing on a slight rise at the edge of the forest. Off in the distance was the brilliant green of a golf course. “They probably brought in a hundred acres of sod to build that course,” she said. “Imagine all the worms in that sod. And I buy worms at the bait stand for my research. One time I found four different species of worms in one container of fish bait. They’re coming in from everywhere.”
Lee and Cindy are only just beginning to document the extent of the problem. They’ve got graduate students doing field experiments that are not too different from the experiments Darwin did. One student sorts the fallen leaves, placing sugar maple, basswood, and oak leaves in separate piles on the ground, secured by wire cages. He visits the site regularly to monitor the food preferences of the worms. At a “leading edge” site near Duluth, a place where worms have not fully invaded the forest, plant life is carefully monitored so that data can be gathered on changes in growth and germination as worm populations advance. And a series of experiments tracks the food preferences of individual worm species living in pots of soil with particular leaves added as food.
After we’d been walking awhile, we came across a high wire fence in the forest. “This is one edge of the deer excluder,” Lee said. “We realized that some plants do survive the worm invasion—maybe twenty percent. But those plants are eaten by deer. Oddly, deer may be the key to all of this. If we can keep the deer out, the plants might just have a chance.”
The forest didn’t look any different inside the deer excluder. “Is it working?” I asked.
“Look at the young trees out here.” Among the bare trunks of mature trees I started to pick out a few saplings only five or six feet tall.
“Those are about twenty-five years old,” he said. “There’s nothing younger than that around here. The deer have gotten all of them. But just inside the fence, we’ve got one- and two-year-old trees.”
Sure enough, when I looked closer I realized that young trees, each little more than a few twigs with fragile leaf buds at the end, were sprouting. The forest still didn’t look lush and green, but to an ecologist, this was real progress.
On the way back to the car, I asked Cindy about the animal population of the forest. She had described the forest floor as a crucial component that affects everything else. Do other animals depend on the forest floor for their survival, animals that the earthworms have displaced?
“Sure,” she said. “As worms come into the forest, we see a shift from voles and shrews to mice. There are all kinds of frogs and other amphibians that live in that duff layer. And there’s even a ground warbler that nests in the forest floor. It’s called an ovenbird because its nest looks like a little oven. We’re only just starting to look at that.”
It’s hard to believe that a creature as small as an earthworm could push ground-dwelling birds and animals out of a forest, but this is exactly what they think is happening. That’s not all: insects that live in the duff layer—including microscopic creatures such as springtails—may be disappearing before they have even been identified and described. The change in soil texture could lead to erosion, especially in the summer when water runs across hard, bare ground in sheets. Even the composition of the soil can change; the presence of earthworms can lead to an increase in bacteria and a decrease in fungi populations, which could in turn affect which plant types proliferate and which struggle or fail entirely.
I looked up at the bare branches high above me. How could an earthworm push something as enormous as a tree out of the forest? But there was no doubt that they were accomplishing something out there in the woods, in their slow, methodical way. I thought about the logging protests in the redwood forests back home, and the tree sitters living high in the canopy. The fight to save those forests happens aboveground. It is a battle for the part of the forest that we can see: branches, leaves, tree trunks. But the fate of this forest in Minnesota lies entirely with the part of the forest we can’t see: the dark underground.
I ASKED LEE AND CINDY what they thought should happen next. You can’t put up a fence to keep out earthworms. What do they want the public to do?
“We’ve got to educate wilderness managers and the fishing community,” Lee said. “They’ve either got to ban live bait altogether or at least stop dumping their leftover worms on the ground at the end of the day. We need to manage the deer population, since they’re grazing on the few plants that do survive. There are some forests in southern Minnesota that are completely invaded by earthworms, but they’ve been able to keep the deer count low, and the understory’s managed to survive.”
“What about farmers?” I asked. “You’ve got farmland adjacent to forests. The farmer wants to cut back on chemicals and build up the population of earthworms in the soil. What should they do?”
“Frankly,” Lee told me, “the forested areas near farms are already invaded by worms. There’s no reason for farmers to cut back on their use of them. It won’t make any difference now anyway. But even in those areas, there are things we can do. We can replant the understory, but we’ve got to learn a lot about our native plants first. Cindy tried to grow some seeds for her research and found out that they have to go through two or three winters before they germinate.”
“You haven’t got that kind of time,” I said.
“Not if you’re a graduate student trying to finish your dissertation,” Lee said. “She decided to grow columbine instead. They’ll bloom in one season.”
“Look,” said Cindy, “worms by themselves only travel a few meters a year. You do the math—it would take them about a hundred years to travel a quarter-mile. If we can keep people from bringing them into the forests, we’ve got some time. There’s plenty of forest land that is still worm free. We can keep it that way for quite a while.”
But Cindy knows what she is up against. “The U.S. imports millions of dollars’ worth of earthworms every year from Canada. That’s a lot of worms. And I’m not opposed to earthworms generally. I’ve got two worm composters at home myself, but I try not to add any worms to my garden soil. I tell people to take their castings, put them in one of those freezer bags, and put them in the freezer for at least a week before they add it to their garden soil. It won’t hurt the soil microbes, but it will kill all the worms.”
I couldn’t help but picture the look on my husband Scott’s face if he saw a bag of earthworm manure in the freezer. “Do you have earthworm castings in your freezer right now?” I asked.
“I do,” she said. “It works great. You should try it.”
I told her I’d think it over. The fact is, I know there are European worms in my soil already, and at this point, I’m not worried about adding more.
BACK HOME, I LEANED against the window in my study and thought about what Lee had said. There’s no reason for farmers to feel guilty about the non-native earthworms living in their fields—those vigorous European species have traveled, alongside humans, to just about every square mile of land in this country and in many places around the world. This shift in the earthworm population has already happened; it would be futile for a farmer or a gardener like me to be concerned about a few more European worms making their way into the soil. Still, as I looked out over the grid of streets that form downtown Eureka, and Humboldt Bay beyond that, and finally looked up to the redwood forests that rise along the hills around the bay, I couldn’t help but wonder where, in the fragile balance of human needs and ecological concerns, the earthworm might take us next in its unintentional tipping of the scales.