Invasive Species of Alien Earthworms Conquers North America

Scientists found that several invasive species of alien earthworms have been busy taking over the soils of North America.

Alien Earthworms

It is a little-known fact that almost all of the earthworms in the majority of the United States originated overseas. Hence, the term alien earthworms. Native earthworms were nearly exterminated by glaciers from a Pleistocene ice age more than 10,000 years ago. Few made it farther south. Today, however, almost all earthworms found north of Pennsylvania is introduced species.

Beginning in the 1600s with the first European settlers, new earthworms started migrating to North America. They crossed over in dry ship ballast or root balls. Second colonization was taking place under the feet of the British, Spanish, French, and Dutch as they colonized the American continent.

European earthworms grew abundantly in the topsoil of gardens and forests. If there were any native earthworms, they remained further underground.

Many of the earthworms that are most frequently found in gardens today, such as the well-known red worm Lumbricus rubellus, have Old World roots.

Love Them or Hate Them

Most gardeners would love having earthworms in their garden soil.

Ironically, the characteristics that make earthworms beneficial for gardens are also what make them hazardous for forests. By aerating the soil, earthworms improve the availability of nutrients to plants like flowers and vegetables.

Forest plants have developed alternative methods of obtaining nutrients from the ground. They may favor invasive plants that later take over the understory when earthworms transform them into simpler-to-access forms. They have also been known to put birds and orchids in danger during this process. They can also pose a threat to people by causing irrigation ditches to collapse and accelerating erosion.

SERC ecologists Melissa McCormick and Dennis Whigham, along with Purdue graduate student Yini Ma were joined by Katalin Szlavecz from Johns Hopkins and Timothy Filley from Purdue in uncovering the secrets of these alien earthworms. The team monitors how the aboveground and belowground worlds interact.

Earthworms and Climate Change

They've found that younger forests tend to be the most favorable for earthworms. Earthworms enjoy the leaf litter that is produced by the trees there, such as the tulip poplars and sweet gums. In older forests, where the oak, beech, and hickory leaf litter is less tasty, they are much less common. This means that by removing old forests, humans unintentionally increase the ground's susceptibility to earthworms and other invasive species.

However, one important issue may allow earthworms to make up for it: climate change. Carbon is present in 2.5 trillion tons of soil on Earth. Researchers believe that the churning and chewing of the soil by earthworms has some effect on the soil's capacity to store carbon. But it's unclear whether they're beneficial or harmful.

Materials eventually decompose and release CO2 into the atmosphere. Earthworms seem to hasten the process in the short run. They accelerate the decay of wood and leaf litter, causing microbes to release more CO2 than they otherwise would. However, the team believes they might have the exact opposite effect in the long run. Some of the soil that earthworms eat is expelled in the form of casts, which are hard pellets. Since these casts take longer to decompose, the carbon they contain may stay in the ground for longer.

In other words, the invasion of alien earthworms could endanger the entire planet as well as specific forests. Or, at a crucial juncture, they might emerge as unexpected allies.