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Invasion of the habitat snatchers

Out of Eden: An Odyssey of Ecological Invasion
By Alan Burdick
Farrar, Straus and Giroux, 324 pp., illustrated, $25

Not far from where I live in the Champlain Valley, a lovely slough named Dead Creek meanders for a few miles. People come to its edge by the hundreds to see the snow geese arrive -- Vermont's largest wildlife spectacle, and one that's only a couple of decades old. The water in the creek is muddy, partly because the clay soils hang in suspension once they're fanned by the sweeping tails of carp, an introduced species. In the warm months, volunteers come to uproot the water chestnut, an Asian import that has spread through much of the East Coast. So you tell me: Is Dead Creek natural?

That's the question veteran journalist Alan Burdick has engaged in his new book, except on a global scale. In one of the most comprehensive and readable accounts of the phenomenon, Burdick describes the incredible movement of organisms around the planet in recent decades, and the consequences of their travels -- nothing less, in some sense, than the homogenization of the earth.

His story begins in Guam, the Pacific Island that for more than half a century has been host to a military airfield. Sometime in the 1960s Guam's tropical birds began to disappear, and no one could figure out why: DDT? Avian malaria? Habitat loss? As it turned out, the cause was an introduced animal, Boiga irregularis, the brown tree snake, which apparently arrived sometime around 1949, perhaps coiled in the dashboard of a Jeep. Guam proved to be perfect territory for the snake. It slowly spread through the jungled interior, dining on rats and mice but also chickens, pigeons, kittens, and pretty much all the native forest birds of the isolated island. ''As the forest birds have disappeared, the insects they once fed on, the gnats, butterflies, and moths, have flourished. The flutter of feathered wings has been replaced by a flurry of tiny membranous ones, in turn prompting a frenzy of spinning among the spiders. All the better for hungry, multiplying skinks and geckos."

Biologists have long since given up on eradicating the snakes from Guam. Now they aim merely to keep them from spreading to places like Hawaii where they would wreak similar devastation -- no easy goal, since the snakes routinely climb the struts of airplanes and coil themselves in the wheel wells. And it is no easy feat, in some larger measure, to keep the same sort of thing from happening elsewhere in a world where the volume of global trade has doubled every seven years for the last few decades.

Most of that trade travels by ship, and the second half of Burdick's book is a fascinating account of the novel science of ballast water ecology. He profiles James Carlton, a biologist who works from Mystic Seaport in Connecticut and who, in the 1970s, sampled the ballast water of a Woods Hole research vessel as it sailed across the Atlantic. Carlton found all kinds of creatures, sucked into the ballast tanks at the beginning of the voyage and expelled into the waters of the port where they eventually docked -- a kind of involuntary draft of zoology from one corner of the world to another. And this occurs day after day on the 35,000 ships plying the world's waters, many of them longer than a football field and capable of carrying the equivalent of a small lake in their tanks.

As a result of such traffic (and of other vectors -- scientists recently found Antarctic penguins with a poultry virus they apparently contracted from garbage tossed overboard), the world's biota become ever more intermingled. Sometimes the results are striking -- the European green crab that has decimated shellfish harvests on the East Coast, the Atlantic jellyfish that wiped out 80 percent of the Black Sea fishery, the zebra mussels that suddenly appeared by the billions in the Great Lakes. At other times the effects are more subtle.

And always they are philosophically difficult to tease apart. Should we consider such invasions unnatural? They have always happened, of course, though man-made technologies have vastly sped up the work of tide and wind. Yet, isn't man a part of nature? And now that we know what we're doing, shouldn't we take the steps to slow it, like treating ballast water? But what if that costs a lot? You get the picture -- it's a jumble.

On the shores of Dead Creek, as I've said, the volunteers pull the water chestnut before it can crowd out other plants that provide more support for wildlife. They don't, however, try to fish out the carp. And they welcome the arrival of the snow geese. For people trying to figure out how our species fits, or doesn't, into the natural order, there is no more interesting subject than the blending of the native and the exotic. And no more interesting an introduction than this fine book.

Bill McKibben is a scholar-in-residence at Middlebury College, and the author of ''Wandering Home: A Long Walk Across America's Most Hopeful Landscape, Vermont's Champlain Valley and New York's Adirondacks."

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