Alien Earthworms' Offspring Thrive, and Alter Soil, in U.S.
By SAM HOOPER SAMUELS
nglers, take note: The night crawler on
that hook is an invader. According to recent
research by an earthworm ecologist, diggers could tear up approximately the upper
half of the North American continent and
almost never find an earthworm of true
North American descent.
"Basically, anything you can buy in a bait
store in most of the country is European,"
said the ecologist, Dr. Sam James, a professor of life sciences at Maharishi University
of Management in Fairfield, Iowa.
Biologists have a term for many species
that wander into nonnative territory and
prosper: weeds. The area in question, which
includes Canada, New England, some mid-Atlantic states and much of the upper Midwest, is entirely populated by weed earthworms, Dr. James says. The rest of the
continent contains a mixture of weeds and
native earthworm species.
While the ancestry of one's
local earthworm might not
seem like a big deal, scientists say otherwise. The encroachment of the nonnative
species, for example, is having a profound effect on soil
structure and can throw ecosystems off balance.
Dr. James, who earned his
doctorate in ecology at the
University of Michigan, has
been fascinated with earthworms for years.
numerous soil samples and other data, he
has concluded that the species dividing line
begins in New Jersey, at about 40 degrees
latitude, then snakes its way westward, dipping a few degrees south in
the Midwest, then rising to
just below 50 degrees latitude
as it makes its way to the
Pacific in Washington.
The most probable reason
for the divide, he says, is geological.
"Look along that
line," Dr. James said.
"That's where the last glaciation stopped, the furthest
advance of the ice in the last
glacial period, which was
15,000 to 20,000 years ago. To
the north of it, you generally
don't find native earthworm species. To the
south, you can find them."
During that last period of glaciation, when
areas that are now temperate were covered
with vast sheets of ice, earthworms were
among many of the creatures that were
wiped out, he explained. For some distance
below that line, permafrost set in and
pushed the earthworms even farther south.
In the 150 to 200 centuries that followed, they
simply have not made their way back again.
"Earthworms have been very slowly diffusing over the north," Dr. James said.
"They aren't like an organism that can look
over the rise of the next hill. They can't
disperse like trees. They are way, way behind."
Dr. James found support for his earthworm findings, which have not been
published, in a Dutch study that
measured the speed at which earthworm populations enlarge their territory. The Dutch researchers placed
earthworm colonies in a field that
was free of worms. After one year,
the colonies had radiated outward by
about 10 meters -- 32 feet -- in each
direction. The worms were a European variety, and especially good
"If you take 10 meters per year as
a high-speed earthworm infiltration,
and multiply that by 20,000 years, it's
not very much," Dr. James said.
"It's 200 kilometers, or about 120
That means it has taken the native
earthworms of North America more
than the duration of human history to
expand their territory by the length
of a small state.
Yet there are earthworms north of
the line, and Dr. James has identified them as European and Asian
immigrants. These were brought inadvertently by humans.
"It was the horticultural trade,"
said Dr. James. "People moved
trees, probably in the colonial period,
by bringing tubs of trees with them.
Plants were moved in their soil: apples, rosebushes, any woody plant."
And along with the soil came
While scientists have long known
that invader species of earthworms
exist in North America, Dr. James
appears to be the first to find that
native worms were absent north of
the line of glaciation.
Dr. Diane Debinski, an associate
professor of animal ecology at Iowa
State University who specializes in
biodiversity studies of vertebrates
and invertebrates, said: "This is
news because there are so few people
who study invertebrates that are not
pest species. There are a lot of taxonomic groups that are not studied
because they are not perceived as
being of value to humans. So I'm not
surprised that we didn't know this
Worms are such poor travelers
that their presence or absence can be
used to tell geological history.
"They're indicators of plate tectonic
movement," Dr. James said.
Some geologists use worms as
markers to track the excruciatingly
slow drift of land masses. They
know, for example, that Puerto Rico
was once in the Pacific and passed
between North and South America
en route to its present location in the
Caribbean. Dr. James is confident
that the island touched what is now
Colombia along its way, because of
the similarity of Puerto Rican earthworms to Colombian earthworms.
"This group of earthworms on Puerto Rico must have arrived on ancient
Puerto Rico by ancient Puerto Rico
coming into contact with ancient Colombia," Dr. James said.
What difference does it make
whether alien worms are on American soil? A lot, scientists say.
For one thing, the invasion is still
in progress, with alien worms moving into areas that have long been
In a wooded area with worms, the
ground is covered by a thin layer of
the previous year's fallen leaves. Below that is a potting-soil-like mixture
left behind by worms that have eaten
those leaves and passed finely broken down organic matter. But in
worm-free zones, like those in Michigan or Minnesota, that layer of leaf
litter is much deeper.
"It's probably a good four inches
thick, and it gets progressively older
as you get deeper," Dr. James said.
"If you walk out to where the earthworms haven't penetrated and you
kick aside leaves, you just find more
He added, "There are various organisms that live in this mat of
leaves that are probably very important for decomposing them."
But once the worms move in, they
devour that leaf litter and alter the
landscape below the surface.
While that may sound like a minor
environmental event, ecologists are
always wary when human intervention, like the inadvertent transfer of
earthworms, changes the balance of
Hawaii, for example, was historically earthworm free, until it started
drawing settlers from all parts of the
globe. "It has now got a vast diversity of introduced tropical and European earthworms," Dr. James said.
The trouble is that humans also
introduced another species to Hawaii: pigs, which now run wild.
The pigs preceded the worms.
They made an easy living in the
Hawaiian woods, finding enough food
on the ground that they did little
rooting below the surface. Once the
worms arrived, though, the pigs developed a taste for them and are now
tearing up the remaining natural forest in search of worms.
Working in Iowa has placed Dr.
James in an especially good position
to observe the North American invasion, since the line of glaciation
passes through the state.
But in his travels, he has mapped
worm distribution throughout the islands of the Caribbean, found European earthworms at 2,000 meters
elevation in Costa Rica and spotted a
South American species in the Los
His next project is an earthworm
survey of the Philippines, to be paid
for by a National Science Foundation
"There's an earthworm invasion
going on in the scenic rice terraces in
the mountains of the Philippines,"
Dr. James said. "They're making
Swiss cheese of their mountains."