Earthworms effect on the forests: Maybe not so
Aldo Leopold, a famous conservationist, once wrote: "One of the
penalties of an ecological education is that one lives alone in a
world of wounds." This whole idea puts nature writers in an odd
position. On the one hand, it's our job to raise the alarm when we
see something amiss, but on the other, we run the risk of spending
so much time dwelling on nature's wounds that we end up giving
people the impression that everything has gone to hell, which of
course it has not.
So what to make of earthworms? We've been told for years that
worms are good. Darwin was a great admirer. They make our gardens
grow. But as invasive plant and animal awareness grows, we're now
being told they're invasive animals that have the potential to
destroy whole forest ecosystems.
It is true that earthworms are not native in the Northeast,
though neither are apples or honeybees. Conventional ecological
thinking holds that our native earthworms got smooshed by the last
glacier, and that the earthworms here today are European immigrants
that arrived with the first settlers and have continued to migrate
ever since. When you're done fishing and you dump your surviving
nightcrawlers (Lumbricus terrestris) out on the bank of a creek,
you're contributing to (or establishing a new) non-native
population. And, of course, introducing a new variable into an
equation causes a disruption.
So all this we know. What ecologists are still trying to get a
handle on is what, exactly, worms are doing to the forest. And
there are conflicting reports. Tim Fahey, a forest ecologist at
Cornell University, has been studying New York's 20 species of
exotic earthworms for years (Vermont and New Hampshire have between
15 and 20 species), and some of what he's found is encouraging.
Earthworms don't move quickly - maybe five or ten meters a year.
And in the area where he's studied, Fahey's observation is that the
worms aren't really invading anymore. They exist in rich soil sites
in the forest, adjacent to farm fields and roads and fishing ponds;
they aren't found in areas with acidic soils. Barring major
ecosystem change, he guesses that they're pretty much where they're
going to be.
Where earthworms exist in large numbers in a forest (large would
be about 150 per square meter), they destroy the layer of leaves
that blankets the forest floor. This can be bad. There's evidence
in some places that native plants, like wild sarsaparilla and
bellwort, are negatively affected, and the exposed dirt may make it
that much easier for an invasive plant like garlic mustard to get a
foothold. But in other places the woods seem fine. I went for a
worm walk with Fahey and a conservation group in eastern New York,
and we found earthworms in an ecosystem that supported a good
diversity of trilliums, wild ginger, jack-in-the-pulpit, and other
native forest herbs. If we hadn't doused the ground with
mustard-laced water, an act that prompted the worms to race to the
surface, we might not have even known they were there.
Now the fine print.
Earthworms do seem to affect the amount of carbon a forest can
sequester, something to consider as atmospheric carbon dioxide
levels continue to rise. Fahey says that in one recent study, wormy
forest soil contained about a third less carbon than non-wormy
Also, earthworms eat tree roots - as much as one fourth of a
tree's roots per year - and break up the beneficial mycorrhizal
relationship the roots have with fungi in the soil. Fahey hasn't
seen this kill a tree, but it's one more stress they have to
And there's a new crop of Asian worms in the genus Amynthas that
have ecologists concerned - one's been dubbed the "crazy
snakeworm." These new worms are reportedly much more aggressive
than our lumbering lumbricus species, though they've yet to become
The take home seems to be that earthworms are a non-native
species that affect forest ecosystems. There's nothing we can do
about it, but next time you go fishing you may as well dispatch
your bait instead of dumping it and contributing to the problem.
And Asian snakeworms may be on the way. Or maybe not. If the
ambiguity here is terribly unsatisfying, and why wouldn't it be,
I'll leave you with Fahey's worm conjuring recipe. Mix 20 grams of
Coleman's hot powdered mustard with a gallon of water. Dump on
ground near nightcrawler holes. Wait for worms to come to surface
(about 30 seconds). Collect. Go fishing.
Dave Mance III is the editor of Northern
Woodlands magazine. The illustration for this column was drawn by
Adelaide Tyrol. The Outside Story is assigned and edited by
Northern Woodlands magazine and sponsored by the Wellborn Ecology
Fund of New Hampshire Charitable Foundation: