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Public Radio's Environmental News Magazine (follow us on Google News)


Air Date: Week of

From the Chinese mitten crabs infesting San Francisco Bay to Zebra mussels clogging waterways in the Midwest, producer Clay Scott travels across the country and brings us this survey of foreign, invasive species.


ROSS: In North America, there are about 50,000 non-native plants, fish, birds, insects, and other organisms, and invasive species continue to arrive almost every day. The so-called exotics are the second leading cause of habitat destruction. One study puts the cost of fighting this biological invasion at about 137 billion dollars a year. It's a problem so huge and ubiquitous that we often don't really see it. Reporter Clay Scott recently traveled across North American and has this report.

SCOTT: Take a drive across the United States and try to find a habitat entirely free of non-native plants or animals. It's not easy to do. Almost everywhere you go you'll find the environment has been affected, in some cases drastically, by invasive exotic species.

Take San Francisco Bay. It's been called the most invaded bay on earth. Ecologist Andy Cohen of the San Francisco Estuary Institute gave me a brief tour of the Bay, or rather, of a single berth at the Berkeley Marina. On his hands and knees, reaching under the dock, he pulled a plastic boat bumper halfway out of the water.

COHEN: As we look down here, we've got these two sea squirts. They're exotic. There's one or two species of colonial sea squirt that are also exotic. There's a sponge here from the Atlantic, bryozoan from the Atlantic, another bryozoan that's sub-tropical and exotic. About eight different organisms, all of them, or almost all of them, from some other part of the world.

SCOTT: So would you say that the exotics are dominating here?

COHEN: They're very much dominating here. Virtually everything I see here is an exotic.

SCOTT: More than 200 non-native species live in San Francisco Bay. They've been showing up here for a long time. A century ago these ecological stowaways hitchhiked on the hulls of wooden ships. But with the increase in global trade the biological invasion has accelerated sharply. One of the more destructive newcomers is the Chinese mitten crab, first noticed here in 1992. Within six years its population had exploded. Millions of crabs have now made their way up the streams and rivers that feed into the Bay, where they clog drinking water systems and compete with native crabs. The problem of invasive species like the Chinese mitten crab, says Andy Cohen, is one with uniquely far-reaching consequences.

COHEN: When you introduce an exotic organism which has the potential to breed and reproduce-- and indeed, that's why we care about them-- the impact can end up over a very large area, much larger than any chemical spill could be, and it can be there forever. And rarely, in fact, will we be able to turn back the clock.

SCOTT: Half a continent away is the Mississippi River, another rapidly eroding eco-system. Here, too, much of the blame can be laid on invasive species, including several varieties of carp introduced by sports fishermen. But today, the biggest concern is the rapid spread of the zebra mussel, the tiny Eurasian invader that has already wrought havoc in the Great Lakes.


SCOTT: In Muscatine, Iowa, below lock-and-dam number 16, the current of the mighty Mississippi has slowed to a stagnant crawl. Here, the dredged-out bottom is zebra mussel heaven. State biologist Kevin Hanson is on the river, monitoring the impact of the invasion on the native mussel beds. There are more than 40 indigenous varieties here, mussels with names like monkey face, pink heel splitter, ladyfinger, pigtoe, three ridge, pimple back. Using a contraption called a brail, a kind of loose-tined steel rake fixed to a rope, Hanson drags the river bottom. Each time he pulls the brail up there are mussels attached to its tines. The bad news is there are mussels attached to the mussels.

HANSON: Another one, a couple of them. There's one of them, or they're two. Zebra mussels on this again. This looks like a little three ridge here. That looks like another pimple back.

SCOTT: And they both have zebra mussels on them?

HANSON: Have zebra mussels, yeah. See down here where there's hardly any abyssal threads? This is where they were probably buried.

SCOTT: Every single native mussel we find is encrusted with dozens of zebra mussels, weighing them down, literally choking them. The zebras have been in the Mississippi less than 10 years. Within another 10 it's predicted they will have occupied all suitable habitat in the Mississippi drainage: one-and-a-quarter million square miles. Hanson is most concerned with the endangered Higgins eye mussel.

HANSON: Because of the zebra mussel, almost every species of mussel on the river is endangered now. And the Higgins eye, to a certain extent, we're fearing that it is a losing battle.

SCOTT: A losing battle because the zebra mussels are already far too well established to eradicate. They can't be controlled with chemicals that would harm native species, as well. The real root of the problem, say many biologists-- the reason that zebra mussels have taken hold in the river systems of the eastern U.S.-- is that rivers like the Mississippi and the Ohio were already degraded by damming and channeling, creating exactly the kind of environment the zebras flourish in. As Jason and Roy Van Driesche write in the book "Nature Out of Place," the zebra mussel invasion is as much a symptom as a cause of ecosystem degradation. There are no easy fixes in the fight against zebra mussels and other invasive species, but there have been success stories.


SCOTT: On the Great Plains, ranchers are making headway in the fight to control leafy spurge, a European weed so noxious to cattle that simply being near it gives them painful sores on the eyes and mouth, a weed that has taken over millions of acres of range and grasslands. Not long ago, the fight against spurge was considered nearly hopeless. Even with expensive herbicides it was doubling its acreage every ten years. In North Dakota's Ward County, the problem was especially bad until recently. In the last six years, the county has cut its spurge acreage nearly in half without the use of chemicals.

FICKE: Right here, there's a larva of a spurge beetle that's just starting to come out of the spurge root and going to go on to the pupa stage, or they're going to come out and go right into the soil.

SCOTT: Derrill Fick is a Ward County Weed Control Officer and an expert on leafy spurge. Every year he spends weeks monitoring and collecting tiny insects called flea beetles, imports from Europe that feed on leafy spurge. On an experimental plot of spurge infested land near the town of Minot red flags mark where the beetles were released two years ago and the results are dramatic. On one side of the plot, the yellow flowered plants stand three feet tall. On the other, long-suppressed native grasses have begun to push through the dead brown stalks of spurge.

FICK: It was so thick you couldn't walk through it. And it had been there for 20-some years before that. So, once the beetles had worked in there and cleaned it out, instantly the next year the grass came back knee-high and the spurge was down low.

SCOTT: It's the flea beetle larva which does most of the damage to the spurge, burrowing its way into the roots and feeding there until the plant withers and dies. The introduction of one alien species to prey on another is known as bio-control, something that has often backfired in the past. In Hawaii, for example, in the 19th century, mongooses imported to kill rats in the sugar cane fields wound up decimating the native bird population.

The flea beetles have been effective because they are host-specific. They eat nothing but leafy spurge. This year the Ward County Weed Board passed out more than 20 million flea beetles at no cost to local ranchers. But catching the insects-- so tiny that 2500 fit into a film canister-- is time consuming and labor-intensive. And even Derrill Fick says it's unrealistic to expect to wholly eradicate the weed. The yellow flowers which homesteaders once thought pretty enough to decorate prairie cemeteries with are likely to remain a permanent fixture of the western landscape.


SCOTT: Leafy spurge, tumbleweed, cheat grass-- species that have been with us so long and are so well entrenched that we often no longer think of them as invasive and foreign. Staying with a friend recently in Idaho, I noticed a painting on his wall, a typical cowboy scene. Then I looked at it again. The cowboy, his horse, the longhorn steers he was driving, the tumbleweed, and in the foreground, the covey of chukar partridge hiding in a patch of cheat grass. It was a classic western landscape, yet nothing in the picture was native. Then I wondered, is something inherently more valuable just because it's native? And in an age where millennia-old biological niches are rapidly being overrun, is the native versus non-native distinction losing its validity? Back in San Francisco, I asked Andy Cohen.

COHEN: On a species by species basis, we can sort of make the good animal/bad animal judgment. But to the extent that we value native ecosystems, we value having that kind of native diversity, then every organism that is brought in from elsewhere that becomes established changes that and erodes that naturalness and nativeness of the system to some degree. And to that extent, every introduction represents some kind of loss.

SCOTT: Many ecosystems, says Cohen, will never be pristine again, they'll never be exactly like they were in the past. But while invasive species will continue to have an impact on our environment, he says, we do still have the power and the responsibility to try and shape that impact. For Living on Earth, I'm Clay Scott in San Francisco.




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