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Not So Fast
Turns out – earthworms are NOT native to the Great Lakes
region. Any native North American earthworms that might
have been here were frozen out when glaciers covered the
Upper Midwest 11,000 - 14,000 years ago. Natural
re-colonization by earthworms happens VERY slowly, less
than a half-mile in 100 years. So there’s just no way
that native worms south of the glacial area, in the
southern US, could have gotten to the UP on their own
(they would have only moved a mere 40 miles or so).
All the earthworms you have come to know and love are
exotic. First brought here by European settlers in the
mid-1880s, they continue to be transported,
intentionally or unintentionally, by humans. Like when
unused fishing bait is dumped along the shore of a lake,
or when we transport compost and mulch, buy potted
plants from distant locales, or bring in firewood or
equipment that has dirt on it.
The widespread use of earthworms as fishing bait is
probably the leading cause of their spread: one need
only look at the advancing edge of the earthworm
invasion that radiates from lakeshores, fishing resorts,
and boat landings. Besides, all our common bait worms
are non-natives: night crawlers, Canadian crawlers, leaf
worms, and angle worms.
“Don’t they just drown when I dump them in the water?”,
some might ask. Earthworms can actually live in water
for many months because they “breathe” through their
skin. So unless those fish that wouldn’t come to the
worm on your hook suddenly decide to eat all your dumped
worms, chances are some not only survive, but thrive in
their new North Woods home.
Dandy Duff
For the past 10,000 years, the forests of the Great
Lakes region grew without earthworms. In the absence of
such a powerful detrivore (eater of dirt and dead
vegetation), natural decomposition of leaf litter was
controlled by fungi and bacteria. This decomposition
occurs at much slower rate than that of earthworms. As a
result, a thick, spongy forest floor, known as “duff”,
was able to develop.
In un-wormed areas (there’s still a few left), the
natural duff layer is typically 4- 5” thick, MUCH deeper
than in areas with worms. Dozens of native understory
plants need extra thick duff, including trilliums and
other spring flowers. Well-developed duff provides
protection from predation and insulation from
temperature and moisture extremes. The thick duff is so
critical that many understory plants and tree seedlings
root exclusively there, since it is where most nutrients
are found.
The loss of a forest duff layer also causes changes in
the diversity and composition of microscopic animals,
insects, fungi, and small mammals, which are the primary
food source for hundreds of species of forest birds,
other mammals, bigger insects, spiders, amphibians, and
reptiles. While earthworms can be a good food source for
some of these animals, the loss of cover means they are
more vulnerable to becoming prey themselves.
Researchers at the University of Minnesota and elsewhere
have documented dramatic changes in native hardwood
forest ecosystems when exotic earthworms invade. These
changes include losses of native understory plant
species and tree seedlings, changes in soil structure,
and declines in nutrient availability.
Winning the Worm War
Some large earthworm-free areas still exist, albeit
mostly in the most remote areas. Ottawa Forest botanists
routinely survey for worms during timber sale planning.
Standard contact clauses require contractors to clean
their equipment and tools before and after operations,
which reduces the risk of both worm and other non-native
invasive species transport.
You can help, too.
1.
The best way is to NOT dump bait worms. Put them in the
trash instead.
2.
Remove dirt/worms/seeds from your ATV, trailer, pickup,
etc. before heading to the woods.
3.
Don't dump compost, leaves, or garden waste in the
woods.
4.
Participate in ongoing worm research by joining an
existing study or conducting your own as part of the
Great Lakes Worm Watch. Visit
www.nrri.umn.edu/worms/team/index.html
or contact me for more information: 932-1330 x539;
mfullman@fs.fed.us.
I can honestly say I never expected to write an article
on earthworms, especially one touting their unnatural,
undesirable effects on our ecosystem. Something, it
turns out, that IS not supposed to be In the Woods. Hope
to see YOU there instead!
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