Foreign pests hitched their first ride to North America aboard ships carrying early European settlers, and many quickly developed an appetite for the continent's crops and trees.
European earthworms lurking either in the settlers' potted plants or in the ballast that steadied their ships on the ocean crossing were among those initial invaders.
The slimy intruders found a smorgasbord in the vast northern forests where glaciers had killed off all native worms during the last ice age, said Cindy Hale, a natural resources research associate at the University of Minnesota in Duluth.
The foreign worms devoured thick leaf litter and topsoil that took thousands of years to form, reducing populations of native plants and animals that long relied on this rich mulch. And those worms are still on the move, joined periodically by new foreign arrivals.
Hale has spent nearly 15 years studying some of the thousands of small pockets of infestations that are slowly encroaching on remote, wormless woodlands in Minnesota.
Her big worry these days is a wave of Asian earthworms moving west from the Eastern seaboard — ravenous worms that could prove devastating for forests and even pose a threat to U.S. agriculture.
"They're very good composters because they can live in very high densities, and they have a very high metabolism, but that's also what makes them very destructive," she said.
Mac Callaham, an ecologist with the Forest Service's Forestry Science Laboratory in Athens, Ga., calls the Asian worms an "insidious" menace. But unlike other destructive invaders, such as a fungus believed to have originated in Asia that decimated Appalachia's American chestnut trees, the worms' impact is not as obvious as stands of dead trees.
Last December, researchers from three universities and the Department of Agriculture reported that about 450 foreign insect species have entered what's now the U.S. over the past four centuries, periodically reshaping the continent's forests.
Only 18 of those pests arrived in the period from 1635 to 1859. The rest poured into the country from 1860 to 2007 as global trade and modern air travel surged, said Betsy Von Holle, an assistant professor of biology at the University of Central Florida.
She and her colleagues also found that "high-impact" insects capable of inflicting serious harm to forests arrive every two to 2½ years. One high-impact pest, the Asian insect known as the emerald ash borer, has killed millions of ash trees in the Midwest and Canada since 2002.
"Entire forests are being wiped out," Von Holle said.
Today's invasive species often arrive in the same way as those first pests.
Jamie K. Reaser, a biologist and environmental consultant, says she's heard stories for years about seaport crews who open up the holds of ships and take a step back as clouds of insects trapped inside fly off to freedom.