European explorers and settlers didn’t arrive in the "New World" alone. Alongside them came cattle, sheep and horses, as well as crops like sugarcane, wheat, bananas and coffee. And hidden in the ballast soil and rocks, and tucked within the root balls of imported plants were stowaways the Europeans knew nothing about: earthworms.

These soft-bodied invertebrates—creatures often regarded as agriculturally beneficial—would quietly transform the North American continent in ways neither Christopher Columbus nor his successors could have anticipated, explains Charles C. Mann in 1493: Uncovering the New World Columbus Created.

For thousands of years before European contact, much of northern North America was earthworm-free—apart from one surviving native species, Bimastos rubidus, which existed in such low numbers that it had minimal ecological impact, says Lee Frelich, director of the University of Minnesota Center for Forest Ecology. The rest of the native worm population was wiped out by the glacial advance of the last Ice Age that swept over what is now Hudson Bay, New England and the Great Lakes region, stretching as far south as Illinois.

Without these wriggly ecosystem engineers, North American hardwood forests evolved undisturbed for 10,000 years, developing a thick organic covering called the duff layer, made of partially decomposed leaves and freshly fallen leaf litter that gathered on the surface. 

“In the areas where there were glaciers, that organic layer kept growing, sometimes as high as a foot,” explains Josef Görres, a professor in ecological soil management at the University of Vermont. “That fluffy soil layer retained moisture, preserved seeds sometimes for many years, and allowed germination of understory species like the sugar maple.” 

That all changed with the advent of European exploration and colonization, with explorers like Henry Hudson, whose ships first sailed up the Hudson River in 1609, laying the groundwork for Dutch colonization of the Hudson River Valley, and the Pilgrims of the Mayflower, who anchored at Plymouth harbor in present-day Massachusetts in 1620 and established the first permanent European settlement in New England. “These could have been the first introductions of European worm species further north,” says Frelich.

While wooden sailing vessels of the time were inherently buoyant, their tall masts made them extremely top-heavy, so soil and stone ballast—prime opportunities for earthworms to hitch a Transatlantic ride—were commonly added to maintain stability. Indeed, five tons of jagged, football-sized boulders were discovered in a 17th-century Dutch shipwreck near Amsterdam, and stone ballast, too, was found in the Sparrow Hawk, an English pinnace that ran aground near Cape Cod, Massachusetts in 1626.  

“However, for the cold temperate and boreal parts of North America it is more likely that earthworms were brought and introduced over large areas of the landscape by the much more numerous European settlers that arrived in the late 1800s and early 1900s, who also settled over very large land areas,” Frelich says.

Initially brought over by ship ballast, these worms and their cocoons would have traveled west across the continent, on the hooves of horses and the wheels of wagons as nascent logging, mining and farming industries lured Swedes, Finns, Germans and Norwegians to settle in Michigan, Wisconsin, Minnesota and beyond. “The European settlers introduced a whole suite of species,” says Frelich. 

Earthworms continue to expand their range—particularly northward. “Use as live fishing bait has spread them into remote areas in the north,” Frelich says, noting that most forests with mature invasions now host around eight to 10 species of European earthworms. This includes the most commonly recognizable species, the nightcrawler (Lumbricus terrestris), which, not so coincidentally, also has the largest ecosystem impact.

The ecological consequences of invasive European earthworms have been profound. Worms eating their way through the duff layer have led to warmer, drier soils, causing nutrients to leach away, reducing soil fertility, says Frelich. Populations of certain rare plant species including the small fern Botrychium mormo and some native orchids, have declined, and there’s been a large shift in native understory communities as tree seedlings—accustomed to the soft duff layer—have struggled to take root in the hard soil the worms leave behind. Ground-nesting birds like the ovenbird have also suffered as the forest floor becomes less hospitable.

“In the long term, we’re wondering whether the earthworms will have an effect on the tree canopy," says Görres. "If all the sugar maple seedlings are consumed, for instance, there won’t be any more maple trees. In 100 years, there may not be any maple syrup from these forests.” 

But not all consequences of invasive European earthworms have been negative—farmers have observed certain advantages in crop fields. The burrows created by nightcrawlers can help channel water into deeper soil layers, making moisture available to crops during dry spells. Additionally, earthworms can reduce the compactness of agricultural soil and may help plants grow by quickly releasing nutrients.

Meanwhile, scientists are now raising concerns about a second wave of invaders—this time from Asia. These species, which appear to have similar effects to their European counterparts, might have the potential to reshape forest ecosystems even more dramatically.

The earliest known arrivals of these earthworms, commonly referred to as jumping worms (genus Amynthas and Metaphire), are believed to have reached California in the 1860s, shortly after Commodore Matthew Perry opened trade with Japan, suspects Görres. As Japan ended its 200-year policy of national seclusion, it triggered a biological ripple effect. Görres notes that the jumping worm's spread may have been accelerated by Japan’s 1910 gift of cherry blossom trees to Washington, D.C.—a shipment that unfortunately arrived infested with insects—and yes, worms.