Caring for and Healing the Earth

Alien Animals

Worm Invasion Threatens Goblins

by Tyler Smith
from the Wood Duck, the Hamilton Naturalists Club Journal, September 2003 (Vol. 57, No.1)

Goblin-eating worms? 11 sounds like a scene from Tolkien's Lord of the Rings, but it's actually the conclusion of a scientific study published by forest ecologist Michael Gundale.

The worms in this story aren't the 15 metre-long, fire-breathing variety they're more likely to be found dangling from a fishhook than devouring fair maidens. And the goblins? They're an endangered plant species known as the Goblin Fern (Botrychium mormo).

Plant-killing earthworms seem a little absurd. 150 years of research has shown that worms are good news for gardeners. Worms eat organic matter ( dead leaves, decaying roots etc.) and turn it into nutrient-rich plant food. The soil we use to grow fruits and vegetables is liter ally worm manure. How could these little soil-builders be hurting plants?

New research conducted in northern Minnesota by Gundale suggests that earthworms are making life difficult for native plants. The problem is that these plants evolved in a worm-free environment. If there ever were any worms native to Canada and the northern United States, they were killed off during the ice ages. It's a little-known fact that the earthworms that now populate our soil are recent immigrants, introduced by European settlers in the last 500 years.

"Plants in the northern half of the continent have adapted to life without worm" said Gundale, who completed his study as a graduate student at Michigan Technological University. The results were published in the journal Conservation Biology last December.

They are accustomed to poor soils and the slow breakdown of organic matter. Some of these plants may have appreciated the introduction of earthworms. Sugar Maples likely grow a little faster as a result of the nutrients worms add to the soil. But Gundale's study shows that earthworms destroy the habitat of plant species like the Goblin Fern.

The Goblin Fern is a real botanical mystery. It was not discovered until 1981. The northern forests where Goblin Ferns live are well-studied, but the Goblin Fern is easy to miss. It spends most of its life hidden underground. When it does appear, it produces a single small leaf. In bad years it might not emerge at all.

Most plants don't have the luxury of sitting out an unfavorable season. Normal plants depend on sunlight for their energy , and they have to soak up as much as they can every year in order to survive. The Goblin Fern is not a normal plant, however. Young Goblin Ferns, called gametophytes, get their energy from soil fungi, so they don't need sunlight at all. Mature Goblin Ferns produce green leaves capable of photosynthesis, but some botanists suspect that even then they might be siphoning energy from fungi in the soil around them.

Gundale noticed that Goblin Ferns were most abundant in areas with a thick layer of dead leaves, or "duff," on the ground. This may be because the moist, rotting leaves are the perfect home for the fungus that supplies the plants with their energy. Could the invasion of duff-eating earthworms cause the extinction of the Goblin Ferns?

To answer this question, Gundale surveyed several dozen Goblin Fern populations, and determined which were increasing and which were decreasing. He also measured the duff at each location. And of course, he had to count the worms. Usually, worm biologists do this by pouring formalin, a poisonous liquid, on the ground. The worms then crawl to the surface to escape, where they are easy prey for the waiting biologist. This method isn't appropriate when you're working with an endangered species, however. Formalin is as toxic to Goblin Ferns as it is to worms. Gundale had to resort to a non-toxic alternative: hot yellow mustard powder diluted in a gallon of water. "I couldn't even look at a jar of mustard for months afterwards," he said.

The sacrifice was worth it. Gundale's study showed that Goblin Fern populations are declining in forests with thinner duff, and that the duff is thinner because of the invading earthworms. It's a remarkable discovery. "We've known for a while that earthworms alter the soil of natural habitats, but this is the first study to show that earthworms have a direct impact on plant growth," Gundale explained.

The implications are potentially devastating. While earthworms are destroying the growing conditions required by Goblin Ferns, they may be making our forests more susceptible to invasion by exotic plant species. "Weedy species generally have higher nutrient requirements," said Gundale. The invasion of natural habitats by earthworms may enable aggressive exotic plant species like Garlic Mustard and Norway Maple to get established. Gundale speculates that the earthworms could even disrupt the reproduction of native plant species with seeds adapted to germinate in darkness beneath the leaf litter .

We do not yet know how widespread the impact of earthworms will be. For the Goblin Fern, the outlook is not good. Earthworms may well be eating them out of house and home. It is endangered in Michigan, and its status is being monitored in Wisconsin and Minnesota.

Terrifying as they are, Tolkien's fire-breathing worms are forever confined between the covers of a book. Gundale's worms, on the other hand, are loose in our own world, and it's going to take a clever bit of magic to protect our forests from them.


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