An Emerging Ecological Disaster

Ah, the mystery has been explained. Why, that is, earthworms are absent from North American forests. Earthworms are fascinating creatures, and their reputation as hard-working facilitators of soil management is legendary. Their activities were said to have been of immense fascination to Charles Darwin, who devoted much of his research and leisure time to observing them, after the publication of his ground-breaking work on the Origin of Species.

Gardeners know their gardens will be healthy, the soil maintained in excellent condition by the presence of earthworms. But in the forests of North America they have been absent. And the wonder of it is why? But then, one discovers that worms were destroyed on this Continent during the last Ice Age. As the massive sheets of crushing Ice retreated, scraping everything before them, everything suffered.

But then, gradually, it took 15,000 years and forests returned and the animals that inhabited them were restored to their habitat. The details elude yet, as they must with that interim space of unwritten natural history unknown. As the forests returned their ecosystem consisted of the detritus of the seasons slowly decomposing with the assistance of fungi and the process of regeneration is ongoing.

While worms fertilize and aerate soil in gardens and fields tilled for agricultural use, in forests they interfere with a different kind of natural process, altering the soil in a manner that destroys the existing system of nutrients and moisture; that organic layer on the forest floor; leaf litter which takes up to 3 years to decompose naturally becomes drawn into the earth by worms when they enter forests.

In the natural state consisting of decomposing detritus, with an underlay of organic and mineral soil moisture is retained protecting roots of trees and shrubs; the earth breathes, prevents erosion, deters pathogens and promotes seed germination, according to Pennsylvania horticulturist Dennis Burton.

"A nutrient balance has evolved in this stable system between the vegetation above ground and the enormous biosphere in and below the leaf litter. When that system loses its leaf litter is is like puncturing your skin. Erosion follows and nutrients bleed quickly from the soil."

So if worms were not present on this Continent, where did they appear from? Europe. Worms were carried into North America in the root balls of plants from Europe and Asia. Common enough for centuries in cities and on farms they are now steadily moving into forests. Stuck to mud on the tires of undersides of logging trucks and all-terrain vehicles, and by left-over fishing bait being dumped near streams and rivers.

"Anywhere there's human settlement, there are earthworms", explains Scott Loss, a post-doctoral research fellow at the Smithsonian Bird Migration Center. "You pretty much can't find a place without earthworms within 50 miles of a big city." Common around boat launches, cabins and logging roads increasingly, wherever humans unknowingly take them in their travels.

Their presence impacts more than we realize. The small brown songbird called an ovenbird builds its nests on the forest floor, covering eggs with a pile of leaf litter. When that litter is absent, the population drops. "In the forested areas it's one of the most common birds in North America." A bird that inhabits northern parts of the Continent, Central America and the Caribbean.

Their survival is threatened as the young are less protected by thick leaf litter from predators relishing the eggs. The bug population, reliant on leaf litter for their existence is also experiencing a decline in the presence of worms invading forests. Who would ever have imagined that earthworms, those industrious workhorses of the urban garden now represent an ecological disaster?

And this: once established, there is no way to kill or to remove them.