Exotic Earthworms
DB: This is Earth and Sky. In your garden, earthworms do good things for the soil.
JB: But in the sugar-maple forests around the Great Lakes, they’re changing the natural balance of the forest ecosystem. Spring wildflowers are disappearing in many places. So are the maple seedlings that produce the next generation of trees.
DB: There are no native earthworms in the northern half of North America. Ten thousand years ago, this area was covered by a vast glacier – all native worms died under the ice. Since then, plant species evolved to depend on a deep layer of decaying leaves that takes up to five years to decompose.
JB: But, in the last century, about a dozen species of European earthworms were brought to the area. These worms have been good for farms and gardens, but not for forest soils. The worms eat the leaf litter that forest plants depend on. When worms move in, the forest floor loses about three-quarters of its plant species.
DB: The worm invasion moves slowly – It expands by just a few meters each year. But the worms get accidental help from fishermen who dump their bait at the end of the day, and from cars and trucks that carry soil and worm egg cases in their tire treads. For more information, check the links at earthsky.com. Special thanks today to the National Fish and Wildlife Foundation, and to the U.S. Forest Service. We’re Block and Byrd for Earth and Sky.
The following individual(s) were interviewed for today’s show. Our thanks to:
Cindy Hale
The Natural Resources Research Institute University of Minnesota-Duluth
Duluth, MN
The following articles were used in preparing this script:
Berlinger, Noirman. “A bad case of worms: Slippery interlopers threaten Minnesota’s northern hardwoods.” Minneapolis Star Tribune, August 21, 2000; News page 1A.
Samuels, Sam Hooper. “Alien earthworms’ offspring thrive and alter soil in U.S.” The New York Times August 29, 2000; Section F, page 1.
Web sites to visit:
Cindy Hale’s ““Minnesota
Worm Watch”“:http://www.nrri.umn.edu/worms/default.htm web site. Help the scientists collect data on exotic earthworms!
A press release about Cindy Hale’s presentation at the “htm” >2000 meeting of the Ecological Society of America”:http://www.academicpress.com/inscight/08082000/grapha.
website of the Chippewa National Forest, where the worm research was conducted
A report on “html” >exotic earthworm invasions”:http://www.ecostudies.org/research/reports/grofrep2. by scientists at the Institute of Ecosystem Studies in Millbrook, NY.
“The “Whyfiles” website”:http://whyfiles.org/shorties/063worm_infest/
Author’s notes:
Biologists first became aware of the earthworm invasion through anecdotal reports that in some tracts of forest, the carpets of trillium that used to spring up in spring were gone.
“John Kotar at the University of Wisconsin uses understory vegetation as away to classify forest types,” says Cindy Hale. “He was telling me he was finding place where he EXPECTED to see a certain mix of plants, but the plants just weren’t there. It was a mystery.”
In 1996, Hale was a grad student in search of a doctoral dissertation. She attended to a research conference in Minnesota and went out on a field trip with Dave Shattus, the soil scientist at the Chippewa National Forest. Shattus took the group to two places in the forest: one where the understory was gone, and another where the forest looked normal. He explained that he’d found worms in the bare zones, but NOT in forests where the plant community was intact.
“I was blown away by the differences between the two places,” says Hale. She’s been investigating the worm invasions ever since. “Three years later, we’re realizing this is a BIG issue,” she says. “In the Chippewa and other Great Lakes forests, we have a great opportunity to document these invasions. We literally have a visible, leading edge of invasion moving into the forest at about three to five meters a year. It’s very discrete- you can walk 150 meters and go from an area with lush native understory to an area with expanses of exposed, eroded mineral soil and virtually NO understory vegetation.” Hale says, the forest floor plants reply on a thick, spongy layer of decaying leaves (called “leaf litter” or “duff”) for their survival. “The typical forest floor plantstrillium, Solomon’s seal, wild ginger, spikenard, bloodroot, sweet cicely, and many fern specieshave very complex seed-germination and plant establishment strategies,” she says.
They make take two or three years to germinate instead of germinating in a single spring. During this prolonged germination period, they need the protection of the moist, thick leaf litter. “What the worms do when they move into these forests is very very quickly consume the leaf litter and mix it into the mineral soil below,” says Hale. “So you get the physical loss of the rooting zone. Plus for any plants that DO manage to germinate, they’ve lost their rooting zone. Normally, their roots penetrate the leaf litter but don’t extend very far into the mineral soil that’s below it.
Another part of the story is that the effects of the worm invasion seem to be influenced by effects of deer grazing. “We’ve found a handful of sites that have been heavily invaded by worms but have recovered,” says Hale. Almost without exception, these places are isolated
from deer, which graze on forest floor plantsthey’ve been on an island or a
peninsula. So we’re experimented with deer exclosures. We think that in the absence of deer grazing, about 20 precent of the plants survive the worm invasion and can eventually recolonize the area. But in the presence of deer grazing, we think they get driven to extinction.” Everyone asks Hale whether there are practical earthworm control measures. She says many kinds of pesticides, herbicides, and fungicides do kill earthworms, the problem is that they kill beneficial soil organisms as well. Also, the invasion is happening across millions of acres, making any kind of mechanical or chemical control measure impractical. One idea is to import a species of flatworm from New Zealand that preys on earthworms (it’s wreaking havoc right now on the worms in Scotland’s farms) but with biological controls there’s always the risk that the control agent will itself turn out to be a pest.
Right now, says Hale, the one helpful thing people can do is to NOT dump their bait when they go fishing. This is the key way that the worms spread into new areas. “If you don’t dump your bait, that could literally save us hundreds of years in which to find a solution to this problem,” she say.
Additional Teacher Resources
Michigan Department of Agriculture: Earthworms and the Soil Foodweb
The soil foodweb is the set of organisms that work underground to help plants grow. There are billions of organisms that make up the soil foodweb. These include bacteria, fungi, protozoa, nematodes, arthropods and earthworms. Each type of organism plays an important role in keeping the soil healthy for all living things. This publication takes a look at why earthworms are so important to the earth’s soil.
The Ecological Society of America: Non-native invasive earthworms as agents of change in northern temperate forests
This ?heady’ report discusses how the biological invasion of earthworms in northern forest threatens to alter ecosystem structure and functions, especially when they change the habitat of other species, alter the availability or transformation rates of key resources, or compete with or replace native species. This report is more suited for secondary students.
