Earthworm invaders may be stressing out some maples | Science News for Students

Earthworm invaders may be stressing out some maples

Worms are only great for soil when ecosystems have evolved with them
Nov 7, 2017 — 6:45 am EST
earthworm soil

Where worms are ecological newcomers, such as in the U.S. Upper Midwest. There invasive earthworms seem to be causing problems.


Earthworms have a reputation for being good for the soil. They recycle nutrients in the ground and keep everything mixed. But where there have been no earthworms for thousands of years, incoming wrigglers can wreak havoc. They can trigger a cascade of problems throughout the local food web. Now comes evidence that earthworms in the U.S. Upper Midwest may be stressing the region’s sugar maples.

There are native earthworms in North America. Those are not, however, found in regions that were covered by glaciers during the Ice Age. Once that ice melted, living things returned. Earthworms don’t move quickly, though. So even after 10,000 years, they’ve only made small inroads into the North on their own.

But people have inadvertently encouraged their settlement. Sometimes they’ve dumped leftover fishing bait at worm-free sites. Or soil stuck to their cars or trucks may accidentally have brought along worms or eggs. Today, earthworms can be found living as far north as the boreal forests in Alberta, Canada.

maple branches
The dead branches on this sugar maple indicate this tree is under stress.
Courtesy of T. Bal

Earthworms “are not really supposed to be in some of these areas,” says Tara Bal. She’s a forest-health scientist at Michigan Technological University in Houghton. “In a garden, they’re good,” she notes. They help to mix soil. But that isn’t a good thing in a northern forest where soil is naturally separated into layers and where nutrients tend to be found only in the uppermost layer near the leaf litter.

Those unmixed soils are “what the trees have been used to,” Bal explains, including the sugar maple. Its shallow roots look for shallow access to nutrients. As worms mix up the soils, they move some of that nutrient-rich layer away.

Bal didn’t start out studying worms in northern regions. She and her colleagues were brought in to address a problem that sugar-maple growers were experiencing. Some of the trees appeared to be stressed out. They were experiencing what’s called dieback. This is where whole branches die, fall off and regrow. This is worrisome because if enough of the tree dies off, “it’s a slow spiral from there,” Bal says: The whole tree eventually dies.

To investigate, the researchers collected data on trees in 120 plots in Michigan, Wisconsin and Minnesota. They looked for anything that might be affecting them, from soil type to slope to insects. And they compared trees on growers’ land to those on public land. They thought the problem might trace to how the trees might have been managed.

What “kept coming up over and over again,” Bal says, “was the forest floor condition.” And that, she concludes, “is directly related to the presence of earthworms.”

While they didn’t go out to look for the worms, they could see signs of them in the amount of carbon in the soil and in changes in the ground cover. Wildflowers, for instance, were replaced by grasses and sedges.

The researchers described their new findings early online in Biological Invasions.

Bal and her team can’t say what this means for the production of maple syrup and your morning pancakes. It might mean nothing at all. But “worms are ecosystem engineers,” she notes. “They’re changing the food chain.” Everything from insects to birds to salamanders could be affected by the arrival of worms.

Even if the sugar maples take a hit, though, there could be an upside, Bal notes. These trees are often grown with few other types of trees around. Such a grove is naturally less resilient to climate change and extreme weather. So replacing some of those sugar maples with other trees, she says, might result in a healthier, more resilient forest in the future.

Power Words

birds     Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.

boreal     An adjective referring to something in the far North, especially lands dominated by pine forests.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

climate change     Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

colleague     Someone who works with another; a co-worker or team member.

data     Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

earthworm     A type of worm that lives in the soil. As it moves through soil, an earthworm creates burrows. These allow air and water to move more readily through the soil. The worms feed on decaying organic matter, which helps improve soil fertility.

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.

engineer    A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

food web     (also known as a food chain) The network of relationships among organisms sharing an ecosystem. Member organisms depend on others within this network as a source of food.

forest     An area of land covered mostly with trees and other woody plants.

glacier     A slow-moving river of ice hundreds or thousands of meters deep. Glaciers are found in mountain valleys and also form parts of ice sheets.

Great Lakes     A system of five interconnected lakes — Superior, Michigan, Huron, Erie and Ontario — the Great Lakes constitute the largest freshwater source in the world (based on surface area). They hold an estimated 6 quadrillion gallons of water, or about a fifth of the world's fresh surface water. To give some perspective on that amount, the lakes' water would, if spread evenly, cover the 48 touching U.S. states to a depth of about 2.9 meters (9.5 feet) deep.

ice age     Earth has experienced at least five major ice ages, which are prolonged periods of unusually cold weather experienced by much of the planet. During that time, which can last hundreds to thousands of years, glaciers and ice sheets expand in size and depth. The most recent ice age peaked 21,500 years ago, but continued until about 13,000 years ago.

insect     A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

litter     (in biology) Decaying leaves and other plant matter on the surface of a forest floor.

native     Associated with a particular location; native plants and animals have been found in a particular location since recorded history began. These species also tend to have developed within a region, occurring there naturally (not because they were planted or moved there by people). Most are particularly well adapted to their environment.

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

recycle     To find new uses for something — or parts of something — that might otherwise be discarded, or treated as waste.

sedge     A grass-like plant that tends to grow in damp soils, such as those along rivers. It has tough, narrow leaves, strong stems and flowers that are not showy.

weather     Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.


Journal:​ ​​T.L. Bal et al. Evidence of damage from exotic invasive earthworm activity was highly correlated to sugar maple dieback in the Upper Great Lakes region. Biological Invasions. Published early online July 26, 2017. doi: 10.1007/s10530-017-1523-0.