Serving Northern St. Louis County, Minnesota

Study cites impact of region’s worst invasive: earthworms

Spread of non-native worms is causing severe effects to forest soils, worsening drought and water quality

Marshall Helmberger
Posted 10/9/19

REGIONAL— Concern about invasive species is widespread these days, and with good reason. Invasive organisms, from ash borers to zebra mussels, can have significant consequences for both terrestrial …

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Study cites impact of region’s worst invasive: earthworms

Spread of non-native worms is causing severe effects to forest soils, worsening drought and water quality

Posted

REGIONAL— Concern about invasive species is widespread these days, and with good reason. Invasive organisms, from ash borers to zebra mussels, can have significant consequences for both terrestrial and aquatic ecosystems.

Yet one of the most harmful and pervasive of the invasive species lives all around most of us, affecting the ground on which we walk, the trees that grow around us, and even the quality of the water in our lakes. And, in most cases, we don’t even think of them as invasive species or recognize the dramatic changes they’ve brought to the region.

Earthworms, angleworms, or night crawlers may seem like part of the natural environment here, given that in most parts of northern Minnesota, these invasive worms are now found almost everywhere. But that wasn’t always the case. The spread of these invasive worms has happened gradually over the past century, facilitated mostly as worms became popular fishing bait. The invasion has spread from hot spots around popular fishing lakes to eventually encompass most of northern Minnesota.

Lee Frelich has reason to know about the impact of invasive earthworms. He’s a professor of forest ecology at the University of Minnesota and the lead author of a recently published article in the journal, Frontiers in Ecology and the Environment, that documents the extent to which the spread of non-native earthworms is fundamentally altering northern forests.

According to the study, European earthworms now inhabit more than 80 percent of Minnesota, Wisconsin, and Michigan, and the presence of these invasive worms is creating a cascade of impacts, from increasing the susceptibility of forests to drought to creating excessive runoff to diminishing water quality in lakes and streams.

Perhaps the most obvious change, says Frelich, is the almost total elimination of the deep duff layer that used to be commonplace in Minnesota forests. That’s the result of aggressive feeding by earthworms, which consume and digest this kind of organic matter. According to Frelich, prior to the introduction of non-native earthworms, the leaf litter that covered the ground in northern Minnesota’s forests was often 6-7 inches thick, representing the accumulation of as much as 20 years’ worth of organic material.

Frelich, who has studied areas that invasive worms have yet to reach, said the difference between the forest floor in infested and un-infested areas is dramatic. Where earthworms still are absent or rare, the ground is spongy. “It’s literally like walking on a memory foam mattress,” he said.

That layer of duff plays a significant role in many aspects of the forest ecosystem, according to Frelich. “The duff provides a lot of insulation, which means midsummer soil temperatures don’t get as warm. It also helps to retain moisture, like mulch in your garden.”

The loss of forest floor duff results in warmer soil temperatures and less retention of moisture, which makes forest soils more subject to drought. Frelich said the arrival of worms in any given location can usually be determined through tree core samples, which soon begin to indicate more drought stress. Frelich said he suspects, but has not yet confirmed, that the arrival of invasive earthworms was the primary cause of the massive birch die-off on the North Shore in the past 15-20 years, by making soils there warmer and more subject to drying.

The loss of duff can also impact water quality. According to Frelich, the duff layer has traditionally helped filter rain and snowmelt and slow runoff. Once the duff is gone, the soil becomes harder and more subject to heavy runoff. “The other thing that happens is there is more erosion of the soil and more leaching of important nutrients,” said Frelich. That includes both nitrogen compounds and phosphorus, which can quickly impact water quality by increasing the growth of unwanted algae. “Once the worms cover an entire watershed, it’s been shown it will impact water quality. And those impacts will worsen over time,” said Frelich.

And as nutrients in the soil are leached into nearby waterways, the availability of nutrients to plants in the forest is diminished, slowing the growth of many forest plants, including trees like the sugar maple by as much as 30 percent.

The duff also provided a hospitable seed bed for many tree species, like birch, sugar maple, or balsam fir, which appear to be declining in many locations as they’re replaced by species like red maple and white pine, which do better on hard, mineral soils than other species. The spread of red maple into northeastern Minnesota forests has been one of the most dramatic effects of the spread of invasive earthworms.

The disappearance of the duff has also affected the type of wildflowers found in northern Minnesota forests. According to Frelich, once-common forest wildflowers, like trilliums, are being replaced by sedges and grasses, which do better on the hard, duff-less soil now found on forest floors across most of northern Minnesota.

That’s why Frelich’s recent journal article refers to a “cascade” of effects from the introduction of invasive earthworms. “When you change the structure of the soil over whole landscapes, it will change ecosystem function in a lot of ways,” said Frelich. “It just cascades from one organism to the other.”

Given the widespread distribution of earthworms, the change to Minnesota’s environment is probably a permanent fixture, notes Frelich. Any effort to rid the soil of earthworms would likely impact other soil fauna, he said, causing even greater disruption to the environment.

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