ANGLERS TAKE NOTE: The night crawler at the end of your hook is an invader. According to recent research by an earthworm ecologist, you could dig up approximately the upper half of the North American continent and almost never find an earthworm of true North American descent.
"Basically, anything you can buy in a bait store in most of the country is European," said the ecologist, Dr. Sam James, a professor of life sciences at Maharishi University of Management in Fairfield, Iowa.
Biologists have a term for many species that wander into nonnative territory and prosper: weeds. The area in question, which includes Canada, New England, some mid-Atlantic states and much of the upper Midwest, is entirely populated by weed earthworms. The rest of the continent contains a mixture of weeds and native earthworm species.
The dividing line begins on the East Coast in New Jersey, at about 40 degrees latitude, then snakes its way westward, dipping a few degrees south in the Midwest, then rising to just below 50 degrees latitude as it makes its way to the Pacific in Washington.
"Look along that line," said James. "That's where the last glaciation stopped, the furthest advance of the ice in the last glacial period, which was 15,000 to 20,000 years ago. To the north of it, you generally don't find native earthworm species. To the south, you can find them."
During that last period of glaciation, when areas that are now temperate were covered with vast sheets of ice, earthworms were among many of the creatures that were wiped out, he explained. For some distance below that line, permafrost set in and pushed the earthworms even farther south. In the 150 to 200 centuries that followed, they simply have not made their way back again.
James found support for his earthworm findings, which have not been published, in a Dutch study that measured the speed at which earthworm populations enlarge their territory. The Dutch researchers placed earthworm colonies in a field that was free of worms. After one year, the colonies had radiated outward by about 10 meters -- 32 feet -- in each direction.
Yet there are earthworms north of the line, and James has identified them as European and Asian immigrants. These were brought inadvertently by humans.
"It was the horticultural trade," said James. "People moved trees, probably in the colonial period, by bringing tubs of trees with them. Plants were moved in their soil: apples, rosebushes, any woody plant. Lilacs were probably brought over in pots."
And along with the soil came earthworms.
While scientists have long known that invader species of earthworms exist in North America, James is the first to find that native worms were absent north of the line of glaciation.
Worms are such poor travelers that their presence or absence in an area can be used to tell geological history. "They're indicators of plate tectonic movement," James said.
Remarkably, worms can be used as markers to track the excruciatingly slow drift of land masses. Geologists know, for example, that the island of Puerto Rico was once in the Pacific and passed between North and South America en route to its present location in the Caribbean. James is confident that the island touched what is now Colombia along its way, because of the similarity of Puerto Rican earthworms to Colombian earthworms.
"In the grand scheme of things, what difference does it make whether alien worms are on American soil? A lot, scientists say.
For one thing, the invasion is still in progress. There are still some areas that have no earthworms. And there, the gradual penetration of alien worms is having a profound effect on the soil structure.
In a wooded area with worms, the ground is covered by a thin layer of the previous year's fallen leaves. Below that is a potting-soil-like mixture left behind by worms that have eaten those leaves and passed finely broken-down organic matter. But in worm-free zones, like those in Michigan or Minnesota, that layer of leaf litter is much deeper.
"It's probably a good 4 inches thick, and it gets progressively older as you get deeper," James said. "If you walk out to where the earthworms haven't penetrated and you kick aside leaves, you just find more leaves. Then you can't kick them because of fungi and tree roots.
"You can cut out almost a block of tree litter. And there are various organisms that live in this mat of leaves that are probably very important for decomposing them."
But once the worms move in, they devour that leaf litter and alter the landscape below the surface.
While that may sound like a minor environmental event, ecologists are always wary when human intervention, such as the inadvertent transfer of earthworms, changes the balance of nature. In Hawaii, for example, an earthworm invasion is having a devastating effect. "Hawaii has been historically earthworm free, because of its isolation," said James.
But Hawaii has drawn human settlers from all parts of the globe. With people came plants, and with plants, worms. "It has now got a vast diversity of introduced tropical and European earthworms," he said.
This might not have made a major difference, except that Hawaii is also populated by another species introduced by humans: pigs, which now run wild.
The pigs preceded the worms. They made an easy living in the Hawaiian woods, finding enough food on the ground that they did little rooting below the surface. Once the worms arrived, though, the pigs developed a taste for them and are now tearing up the remaining natural forest in search of worms.
Working in Iowa has placed James in an especially good position to observe the North American invasion, since the line of glaciation passes through the state.
But in his travels, he has mapped worm distribution throughout the islands of the Caribbean, found European earthworms at 2,000 meters elevation in Costa Rica, and spotted a South American species in the Los Angeles basin.
His next project: an earthworm survey of the Philippines, to be paid for by a National Science Foundation grant.
"There's an earthworm invasion going on in the scenic rice terraces in the mountains of the Philippines," James said. "They're making Swiss cheese of their mountains."
Sam Hooper Samuels wrote this article for the New York Times, where it first appeared.