We talk about exotic species as if they are invaders hellbent on destruction. But that's not necessarily so.
Since they escaped from aquaculture ponds in the early 1970s, silver and bighead carp have spread up the Mississippi. In Minnesota, the bighead carp has been found in both Lake Pepin and the St. Croix River. Both species have swum up the Illinois River, which joins Lake Michigan through an artificial canal. Great Lakes states and environmental groups have sued to close the channel. (Last week the U.S. Supreme Court declined to take immediate action.) Moreover, ecologists have identified Asian carp DNA in the lake, suggesting that efforts to block their advance are already too late.
But the carp are only two among the thousands of foreign organisms that have washed up on our shores, from deadly smallpox to annoying dandelions. Included are nearly 200 fish species, from the detested common carp to the prized brown trout.
Now that the drumbeat against Asian carp is at full volume, let's consider the things we thought we knew about exotic species -- also known as aliens or nonnatives -- that is, living things that come from elsewhere.We describe exotic species with much the same language we use in talking about human immigrants and warfare.
In news reports, "invading" species are always "voracious." Well, as are all organisms. They eat. Some species eat a bit more than others. If silver and bighead carp were natives, we could just as easily talk about "voracious" fishes like the gizzard shad and bigmouth buffalo competing for jobs -- I mean, food.
Mark Davis, chairman of the biology department at Macalester College and author of the new book "Invasion Biology," says the language that characterizes the debate about exotic species--even among scientists--reveals biases that make for poor science and policymaking. "All I've been arguing for is a more nuanced characterization of what's been happening," says Davis. He says he's finding traction for his ideas: "People are thinking more carefully about the words they are using, the assumptions they might be bringing in."Environmentalists often warn that alien species might cause a cascade of extinctions and loss of biological diversity.
Rarely happens, Davis says, except on islands, in lakes or in other insular environments. Throughout the United States, local ecosystems have perhaps 20 percent more plant species than they once did because of the addition of foreign species.
"How many species of plants in the U.S. have gone extinct because of the thousands of nonnative plants that have been introduced?" asks Davis. "Zero!"
While extinctions seem an inherently bad thing, it's not really possible to say whether or why biodiversity at a global scale is somehow more important that increased diversity at a local scale.Environmentalists often say exotic species indicate, almost by definition, a "degraded" or "unhealthy" ecosystem.
Yet there's little evidence that natives are more valuable than recently arrived species. Indeed, except by knowing the history of an organism, it can be tough to distinguish natives from nonnatives.
Case in point: For years conservationists had fretted over weeds overrunning the Galapagos Islands, a world heritage site of biodiversity. Because the weeds were out of control, conservationists assumed they were nonnatives, recently introduced by human traffic. Then -- whoops! -- they discovered the plants were native. They belonged. In fact, they had been present since prehistoric times.
Dov Sax, assistant professor of ecology and evolutionary biology at Brown University, says he began to question exotic-species orthodoxy after a professor at University of California, Berkeley, described the Bay Area's abandoned plantations of Australian eucalyptus trees as a "biological desert." Says Sax, "There was all kinds of stuff growing in there. I found there were really a similar number of species in both [native oak and eucalyptus] woodland types. Exotics weren't always doing the awful things people seemed to think they were doing."
"There isn't such a thing as a healthy ecosystem or a sick ecosystem," Davis says. "When someone is referring to a healthy ecosystem, what they are referring to is an ecosystem the way they want it to be. It's really kind of a way to manipulate the audience, because who can be opposed to ecosystem health?"If exotic species were unequivocally bad, controlling them would be easier.
But many were introduced because they are valuable, including crops (from apples to wheat), horticultural plants (hostas and Norway maples), and game species (ring-necked pheasants and brown trout). Silver carp are the most cultivated fish in the world. They have been introduced to 88 countries, primarily for food, but also to clean up algae without chemicals.
Filter feeders such as zebra mussels strain out, digest and encapsulate algae that grows as a result of polluted runoff. Says Mark Sagoff, an environmental-ethics scholar at the University of Maryland, "Were it native, the zebra mussel would be hailed as a savior, not reviled as a scourge."Like it or not, the world is increasingly made up of what scientists have called "novel ecosystems" -- biological stews of old and new.
Land managers can't control the thousands of exotics that reach us through globalized trade and travel. "A lot of conservation biology in the past has been built around the idea of preventing change," says Sax. "That old mantra is going to get thrown out, because it's going to be impossible to prevent change."
When Davis expounds on exotic species, he eventually comes to LTL, his shorthand for Learn to Love them. "When I first say that, people in the audience almost get physically sick. It's amazing how extensive the indoctrination has been -- nonnative species are bad. We've got to get rid of them. Boy, if you want nature to stop, you're going to be miserable."
Rather than treating all exotics as potential disasters, Davis suggests triage: vigorously trying to control threats to human health (such as avian flu) and the economy (gypsy moths and emerald ash borers). He wouldn't invest much effort or money in fighting species such as buckthorn or purple loosestrife that are already established and are not doing much damage.
Meanwhile, Davis and Sax advocate less rhetoric and more-rigorous research into the role of exotic species in modern-day ecosystems.
So what about silver and bighead carp? They may not be as damaging as sea lampreys in the Great Lakes or Dutch elm disease in our cities, but the carp do have the potential to wreak tremendous change. They make up most of the biomass in some river stretches.
Kevin Irons, large river ecologist for the Illinois Natural History Survey, has found evidence that the carp are competing with native gizzard shad and bigmouth buffalo for food. Says Irons, "It appears to me the Asian carp are winning."
Our best chance to prevent their spread would have been before we imported them. But now is our second-best chance -- to close off the canal before they can travel from the Mississippi drainage into the Great Lakes. That would have the added benefit of blocking the infiltration of nonnative species from the Great Lakes to the Mississippi.
Will we do it? Don't bet on people's willingness, especially in the absence of clear-cut danger.
Meanwhile, it's time to make lemons into lemonade -- or carp into fried fish. Silver and bighead carp are two of the most popular food fish in the world.
"I love walleye," says Irons. "But these guys really taste good."
Now, if only we could convince the rest of America that millions of tons of tasty, lowfat protein are there for the taking.
In other words, LTL.
Greg Breining is a St. Paul writer who reports on matters of science, nature and travel. His latest book is "A Hard-Water World: Ice Fishing and Why We Do It."
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