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Gone With the Worm: Invasive Earthworms Irrevocably Alter Kenai Peninsula Forests

by | November 9, 2021

Most people believe earthworms help in the garden, which is generally true. Worms mix the soil, improve aeration, and convert dead plant material into nutrient-rich worm castings (worm waste, excreted from the digestive tract). This is great for most garden plants. When we bring earthworms into the woods of Southcentral Alaska, the worms do the same kinds of things but with unintended consequences.

Since 2018, a team of researchers from the University of Minnesota led by Dr. Kyungsoo Yoo has been studying earthworm invasions in Alaska to learn how non-native earthworms affect our forest soils. They found that introduced nightcrawlers could reach remarkably high abundances of up to 1,300 pounds per acre on the Kenai Peninsula! Compare that to a moose population density of one 1,300-pound moose for roughly every 500 acres of boreal forest – in some areas, earthworm biomass might exceed that of moose by almost 500 times!

Researchers examine the forest floor in search of invasive earthworms. One researcher records data while the other digs with a shovel and their hands.
University of Minnesota researchers Tyler Baumann and Adrian Wackett collect earthworms from a gully off of China Poot Street in Homer, September 13, 2019. Photo by Matt Bowser/USFWS

These dense populations of worms rapidly consumed leaf litter, leaving plant roots exposed above the new, lowered ground surface. The nightcrawlers also mixed the soil, bringing organic material down and mineral soil and castings up to the surface.

Once you have seen this kind of change, you can recognize nightcrawler infestations with a passing glance: the familiar carpet of leaf litter under birches and cottonwoods replaced by a barren surface of crumbly worm castings and all except the most recently fallen leaves consumed.

Tree roots are exposed above the soil line where invasive worms have lowered the soil level. A small, silver knife sits next to the roots to show the scale of soil depletion.
Roots of an aspen tree exposed due to earthworms removing the decomposing leaf litter in which the roots had grown. Vicinity of Fish Lake, September 18, 2020. Photo by Matt Bowser/USFWS

These changes might be beneficial for some plants like grasses and invasive weeds. However, native ferns and herbs, which are adapted to grow in slowly decomposing leaves, tend to decline. Over time, seed germination and seedling survival of native trees are impacted by the worm-altered soil.

Invasive earthworms clearly affect forests, but we know less about what this might mean for fish. Young salmon readily eat earthworms that end up in the water, but the vast majority of earthworms stay on land, doing what they do there – alter the forest ecosystem. It may be that the most important ways that earthworms touch the lives of salmon have to do with water moving through or over the soil. Because worms remove the protective layer of leaf litter, worm-infested soils become more susceptible to erosion. And increased erosion can potentially increase the amount of sediment and nutrients that end up in the water.

A close-up photo of an area infested by invasive worms shows worm damage. Only a few leaves and small sticks cover crumbly worm castings in an area that would, in the absence of worms, be covered in a thick layer of leaves.
The forest floor in a nightcrawler infestation near Fish Lake off of Swanson River Road, Kenai National Wildlife Refuge, September 18, 2020. The worms have consumed all leaf litter, leaving only worm castings, the most recently fallen leaves, and woody debris. Photo by Matt Bowser/USFWS

The good news is that, on the Kenai Peninsula, nightcrawlers and most other non-native earthworms generally live close to houses and farms – our forests remain mostly free of them. Earthworms do not get very far on their own, only dispersing tens of feet per year at most. At that rate, it will take hundreds of years for the worms to spread around the peninsula.

That is, without the help of humans.

We mostly see nightcrawlers showing up in new locations at popular fishing sites, where anglers apparently discard unused live bait onto the ground before they head home at the end of the day.

Unfortunately, once the non-native worms become firmly established, we currently have no way to remove them. Any changes they make cannot be undone. The only way to limit the damage caused by invasive worms is to prevent them from being introduced to new places in the first place.

A close-up photo of a the palm of a researcher's hand, holding a few pink- and red-colored worms, shows what the invasive worms look like. The worms are wrapped around a clump of wet mud.
Red marsh worms from near China Poot Street in Homer, September 13, 2019. This invasive species is one of the most common earthworms in the Homer area. Photo by Matt Bowser/USFWS

Recently, a few species of Asian earthworms – commonly called Asian jumping worms – have spread rapidly over much of the lower 48 states, from the eastern U.S. to Canada and west to Oregon. Where they have invaded, these new arrivals have consumed leaf litter even more quickly than other non-native worms. 

Luckily, we have not found Asian jumping worms in Alaska yet. Though, they could arrive any time in potted plants, soil, or fishing bait. We do not know if these worms could flourish in the climate of Southcentral Alaska, but it may be best to deny them the opportunity.

You can help prevent the spread of non-native earthworms and nightcrawlers. Never transport infested soil or compost to worm-free areas, and always dispose of live bait properly – in the trash.


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