3/3/2007

New Scientist

By Jessica Marshall

Freelance Writer

St Paul, Minnesota
DEEP IN the woods of Minnesota, an army of environmental do-gooders has gone bad. Very bad. Seemingly hell-bent on wrecking the forest floor, it is wiping out plants and wild flowers and leaving only hard, bare soil in its wake. If someone doesn't find a way to stop them, the forests of the Midwest will be doomed.

A similar sorry tale is being told all over the world, but the identity of the culprit will come as something of a surprise. Of all the creatures capable of destroying ecosystems and wiping out species, it is perhaps the least likely. It's the ecological equivalent of a gentle but murderous granny: it is the earthworm.

Earthworms have a reputation as environmental good-guys, churning and enriching the earth as they munch their way through soil and leaf litter. The trouble is that Minnesota shouldn't have any worms. Nor should anywhere else in the US and Canada north of a line that runs roughly west from Boston -- at least not since the end of the last ice age 10,000 years ago. These worms are invaders who hitched a ride with goods and settlers from Europe three centuries ago, and are now brought in as fishing bait.

In Minnesota the worms have a clear run at worm-free zones, but elsewhere in the world, worm wars are being fought between native species and exotic intruders. In the Carpathian Mountains of Romania, several species of native earthworm are being displaced by hardier invaders from other parts of Europe, probably brought in inadvertently through agriculture or deforestation. In the UK, native earthworms, including one of the species that's causing trouble in Minnesota, are under threat from two foreign flatworms, Arthurdendyus triangulatus from New Zealand and the Australian Australoplana sanguinea , which inject enzymes into their prey before eating them alive. Some Amazonian earthworms, meanwhile, are struggling against another invader, Pontoscolex corethrurus , which has spread from a southern region of the Amazon, probably in potted plants and crops.

These battles all have something in common: they are the result of taking species from one ecosystem and -- intentionally or otherwise -- releasing them where they don't belong. Alien species are one of the biggest threats to global biodiversity: without natural predators the invaders can spiral out of control and wreak havoc on an ecosystem. But worms? It just seems so unlikely.

"We all grew up learning that worms are universally good for the soil," says Andrew Holdsworth, who studied Minnesota's marauders under Lee Frelich at the University of Minnesota in St Paul. It turns out that this is not always the case.

Frelich first got on the trail of the invaders in 1997 when local millionaire Bruce Dayton called to ask why the wild flowers on his forest property at Wood-Rill, west of Minneapolis, had disappeared. "I said, 'I don't know, but I'll find out,'" recalls Frelich. Having heard of the damage introduced worms had done in New York City parks, he suspected a similar problem and went in search of the culprits.

Walking around the forest at Wood-Rill, which Dayton and his wife have since donated to the state, the evidence of worm activity was clear. The woods should be carpeted with a 10 to 15-centimetre layer of fluffy leaf litter, or "duff", home to a lush undergrowth of plants and tree seedlings. Instead there is bare, hard-packed earth with barely a plant in sight.

At least six alien worm species are at work here, but two are responsible for most of the damage. The leaf worm, Lumbricus rubellus , which is partial to leaf fragments, can chew the stuffing out of the duff mattress in a single season. Meanwhile, the night crawler, Lumbricus terrestris -- known as the common earthworm in the UK -- takes care of the top layer, pulling whole leaves inside its burrow to eat, and discarding the stems at its burrow's entrance like bones picked clean.

Digging the dirt

Frelich points to the spiky bouquets of stems sticking up out of burrow after burrow, only centimetres apart. The worms have already polished off their favourites, the basswood leaves, and most of the maples. Even the less-relished oak leaves will be gone before the leaves fall again. "Terrestris will never allow a new litter layer to get established," he says.

In Europe, where the species is native, L. terrestris and other earthworms play an important role in incorporating nutrients from fallen leaves into the earth. Their burrows aerate the soil and create handy channels for growing roots, water infiltration and gas exchange. Their casts form hotspots for nitrifying bacteria, which fix nitrogen into a form that plants can absorb. In Minnesota, though, the soil is aerated by soil insects, such as beetles, centipedes and millipedes, and other duff-inhabitants like salamanders or small mammals that move through the duff and keep it loose. "A more efficient system of aerating the soil is displaced when the earthworm invades," says Frelich. As a result it becomes denser, native plants and tree seedlings can't take root in the packed forest floor, and those that do are promptly polished off by burgeoning populations of deer.

In Europe, the battle between native and exotic earthworms has not yet wreaked the havoc evident in Minnesota's woods. But "from the point of view of biodiversity, the change is absolutely a negative", says Victor Pop of the Institute of Biological Research in Cluj-Napoca, Romania. Pop recently returned to sites he surveyed in Romania in the 1960s and 1970s to find many native species gone (Biological Invasions , vol 8, p 1219).

It's a similar tale in the tropics, where disturbances to the soil through deforestation or agriculture have destabilised native earthworm populations, allowing invaders like P. corethrurus to take hold and displace the natural decompactors of the soil.

Depressingly, there's little that can be done once the worms have moved in. Holdsworth tested the feasibility of removing them from the Minnesota woods by running an electric current through a plot of soil and recruiting volunteers to collect the worms driven to the surface. It had limited success. The worms' eggs remained, and the young population vigorously moved in to fill the void. There's been talk of bringing in the New Zealand flatworm, which preys on earthworms in the UK, but bringing yet another alien predator into the woods is a risky strategy. Such a voracious predator may turn out to have a taste for native, non-target species as well as for the earthworms. "We're not sure we want it," admits Frelich. For now, all that can be done is try to prevent more worms being brought in and control the deer population to give seedlings a fighting chance.

As ecologists like Frelich struggle to contain the war beneath their feet, parts of the south-eastern US are braced for a second invasion. This time it's a pencil-sized Asian species of earthworm from the genus Amynthas , which makes particularly good fishing bait. This enemy is big, has a voracious appetite for leaves, and, says Paul Hendrix of the University of Georgia in Athens, who is studying the problem, wriggles along so fast, "you can almost hear them moving through the litter". Local worms are advised to dig in and prepare to do battle.