Cruising westward at 300 feet, the helicopter is heading straight for the end of civilization. You can see it just ahead. It's a line across the surface of the Earth--a levee, built years ago to hold back the swamp. Now it works in reverse, restraining the developers. Beyond the levee there are no shopping malls, no houses, no roads, just a wet prairie full of alligators, lily pads and saw grass.

And there's something new, something growing, spreading--a pale-green substance that seems to be crawling all over the tree islands that speckle this portion of the Everglades. The pilot takes the chopper down for a closer look. You can see it, sure enough: lygodium. Old World climbing fern.

It has gone berserk. It's like the Blob. The islands are caving in at the center, crushed by the dense, matted blanket of vegetation. The willows, the hollies, the cabbage palms--they're being buried alive.

"You wouldn't see any of this three years ago," testifies the chopper pilot, Jim Dunn.

David Viker, deputy manager of the federally managed swamp, says, "It looks like a green bomb went off."

What exactly is this virulent organism? It's a houseplant. In the right context, it's a lovely little fern.

Lygodium is the classic invasive species: an organism that's been transported by human beings to habitats where it has no natural enemies. The counterattack against this intruder is just one isolated battle in what is becoming a major war from the Everglades to Rock Creek Park, from Hawaii to your own back yard. The scale of the conflict is planetary.

There are bombs detonating everywhere.

The Multifarious Menace There have always been invasive species, but ecologists and government officials say the situation has become riotous. One recent study estimated that exotic species, including diseases, cost the nation more than $130 billion a year. There is an emerging sentiment that this could be the next great environmental crisis, that without serious countermeasures we will find ourselves living in what the nature writer David Quammen has called the "Planet of Weeds."

Last year President Clinton signed an executive order requiring all federal agencies to address the problem of invasives. The order created a new entity called the Invasive Species Council. The council's executive director takes office tomorrow. But for all the bureaucratic sparks, there are no platoons of weed-whacking commandos taking to the hills with machetes.

For the general public the issue remains relatively obscure. People grasp the dangers posed by bulldozers and acid rain. It's not as easy to understand the menace of, say, Eurasian milfoil.

The issue also suffers from its scattered nature. The invaders range from bacteria to vines to feral pigs. Broadly defined, invasive species come from every kingdom of life. A few examples:

* Domestic honeybees are under attack from the invasive Varroa mite and from aggressive "killer" bees that have arrived from South America.

* West Nile virus, blamed for seven deaths last year, has reappeared among birds and mosquitoes in New York. Central Park was closed one night this past week to allow aerial spraying of pesticide.

* The Asian tiger mosquito arrived in the United States in the mid-1980s and now plagues the Washington area. It bites all day long.

* The fabled sagebrush of Nevada is being replaced by cheat grass, an invader from Europe that is explosively flammable.

* Miconia, a plant with razor-edged leaves, has arrived in Hawaii and formed impenetrable stands over thousands of acres.

* More than 5,000 prize maple trees in New York and Chicago have been cut down after infestations by the Asian longhorn beetle.

* The Asian swamp eel has turned up in canals in South Florida and may soon start devouring small fish in the Everglades.

The invaders are characterized not so much by their exotic origins as by their virulent behavior, the way they overrun natural defenses. They are, by nature, insidious. When they get loose, they tend to have perfect camouflage. Weeds are green.

Invasive species have been pestering America for more than a century. Starlings from Europe were released in Central Park in the late 1800s by a Shakespeare fan who wanted to introduce to America the birds mentioned in the plays. The great American chestnut tree was wiped out by an Asian fungus first detected in New York in 1904.

What's new is the scale and pace of the invasion. Global economic trade has put life in a blender. Sometimes the mode of transportation is the ballast water of a ship that has crossed the ocean and plied the St. Lawrence River to Lake Ontario. Sometimes it might be the treads of a hiker's boot, the perfect slot for an exotic seed. Living things are opportunistic. The ancient barriers--oceans, rivers, mountain ranges--have been breached.

Life is flying around everywhere.

The Next Extinction

"The blending of the natural world into one great monoculture of the most aggressive species is, I think, a blow to the spirit and beauty of the natural world."

The grim assessment came from Secretary of the Interior Bruce Babbitt as he walked one morning near the Potomac River, not far from his home in Northwest Washington. The Potomac gorge is crawling with invasive vines and weeds--stuff like porcelain berry and mile-a-minute weed. Babbitt may have incited chuckles when he warned last year of the dangers of purple loosestrife, but he's dead serious.

"There's a brand-new one coming up from Mexico called buffel grass," he said, navigating a trail along a creek near Fletcher's Boathouse. "It is now crossing the border into the Sonoran Desert. It carries fire wonderfully. They're actually pulling it up by hand in Organ Pipe Cactus National Monument."

The buffel grass invasion could doom the saguaro cactus, the great emblem of the Sonoran Desert. Over time, the distinct desert environments of North America could look more and more alike. Repeat that situation all over the planet and you have a recipe for a homogenized world.

Bill Gregg, a U.S. Geological Survey plant ecologist, likens the disappearance of native species to lost knowledge: "We're burning the library, slowly," he says.

Gregg says the issue of invasives began to heat up in the 1980s, when the population of zebra mussels exploded in the Great Lakes and clogged industrial intake pipes. Other explosions followed. Asian longhorn beetles began arriving as stowaways in wooden crates from China. The link between invasives and economic globalization became obvious. Gregg points out that China and the United States have similar climates and geography, and could easily provide each other with a tremendous supply of weeds and pests.

However bad the problem is now, everyone expects it to get worse. Harvard biologist E.O. Wilson argues that invasives will cause more extinctions than ordinary pollution. Robert F. Doren, a science administrator with the National Park Service, doesn't hesitate to sound the alarm: "Because of the breakdown of ecological barriers, we are now entering the sixth great extinction in the history of the planet."

The extinction problem is most severe on islands, such as Hawaii, home to dozens of endangered birds and plants. Hawaiian officials guard night and day against the arrival of the brown tree snake, which might sneak aboard military flights from Guam.

The snake arrived in Guam several decades ago and has wiped out almost all of the birds on the island. It routinely climbs power lines and triggers electrical blackouts. It has a history of biting children in their sleep.

Backyard Invaders

For the Weed Warriors, invasives are not an esoteric matter. The Weed Warriors are people like Carole Bergmann, Jayne Hench, Michelle Grace and Claudia Donegan, who live in Montgomery County and regularly attack the monstrous vines along Sligo Creek. They sense that the issue is finally getting traction.

"Citizens just started calling, unsolicited, saying something is taking over the forest in our parks," says Hench, a supervisor with the county parks department.

The weed patrol has found numerous tall trees completely smothered, humbled by a rampaging invader called porcelain berry. "It's like a bad horror movie," says Grace.

Most noxious weeds were once desired for a specific function. Tens of millions of kudzu seedlings were planted by the Civilian Conservation Corps in the 1930s. Kudzu fought erosion. Now it's the prototypical gone-crazy weed. Multiflora rose was planted decades ago as a kind of living barbed-wire; now farmers have to bulldoze it out of their fields.

You could find plenty of invaders in your back yard: dandelions, garlic mustard, Japanese honeysuckle, Oriental bittersweet. The economists who study the cost of invasive species don't include the hours people spend on hands and knees yanking weeds from their gardens. English ivy is another invader: It looks great on an old brick building, but turn your back and it scampers into the woods.

Jil Swearingen, a National Park Service biologist, is tracking a long list of Washington area invaders, including common mugwort, smooth bromegrass, paper mulberry, Asiatic sand sedge, spotted knapweed, sticky chickweed, celandine, field bindweed, hound's-tongue, jimson weed, Chinese yam, Indian strawberry, viper's bugloss, lesser stitchwort, and so on.

Some of these pose no serious threat. Others could be time bombs. Bill Gregg says that if you detect an invader early enough, you can remove it mechanically, by force. Wait too long and you have to attack it with chemicals, hardly the most environmentally friendly solution.

"I think metastatic cancer is the strongest analogy," he says.

The Weed Skeptic

Mark Sagoff is the naysayer.

"One man's weed or pest is another man's palm tree," he says.

Sagoff is a professor at the Institute for Philosophy and Public Policy at the University of Maryland, and he demands deeper thinking about the war against weeds. He points out that native species can run amok, too, like the wild grape that grows in his own back yard. Deer, too, are native, and increasingly an urban pest.

"No one has shown that exotics are more likely than natives to be harmful," Sagoff wrote recently.

He says there's no such thing as the "balance of nature." Ecosystems change. There's no single way an ecosystem "ought" to be. Sagoff argues that the fight against invasive species sometimes echoes the anti-immigrant rhetoric of America's past. Invasives are accused of "sexual robustness, excessive breeding, low parental involvement with the young, a preference for degraded conditions and so on," Sagoff wrote.

But he isn't completely complacent. He acknowledges that historically significant ecosystems are being altered, and says there may be legitimate aesthetic reasons to object to the change, in the same way that the French might protest the opening of a new McDonald's.

Ecologists say Sagoff's view of invasives doesn't take sufficient account of the rate of change. "People sometimes say this is just speeded up evolution. That's not the case," says Gordon Brown, who works on invasives for the Interior Department. "It's too accelerated."

Tim Flynn, a Hawaiian botanist, points out that Hawaii is so isolated that for most of its history a new species arrived only once every 10,000 years or so. A seabird, having miraculously survived the journey across thousands of miles of ocean, might show up with a seed in its guts. "It has to be a constipated bird," Flynn said.

'Urban Safari'

A few invaders do have fangs, which is what keeps Todd Hardwick busy in South Florida. He drives a pickup with the words "Pesky Critters" on the side. He's the guy to call when you find a reptile in your swimming pool.

"Basically I capture animals from all over the world without ever leaving South Florida. Every day is an urban safari," he says, showing off what amounts to a private zoo in his back yard. He's got rheas, emus, iguanas, an Asian muttjack, and lots of snakes. A foghorn sound fills the air. That's from one of the rheas, an ostrichlike creature. It wants to mate.

"That's an Asian water monitor," he says, pointing to a caged lizard. "Six feet long, 50 pounds, cold-blooded predatory reptile. This thing rips, shakes and tears its prey apart."

Hardwick says there are 5,000 invasive primates on the loose in South Florida. He thinks the capuchin monkeys may be forming troops in the wild.

International traffic in exotic creatures grows every year. Prices are coming down. At one pet shop in Broward County recently the boa constrictors, normally $119, were on sale for $99. A python was only $59. An iguana: $11. "Great gift idea," a sign on the cage said.

Most of these creatures arrive at Miami International Airport. On a typical day recently the arrivals included scorpions, lizards, chameleons, tree frogs, king snakes and basilisks. "We're the hotbed for venomous reptiles," said Mike Barandiaran, a wildlife inspector for the U.S. Fish and Wildlife Service.

He was looking over an Air Canada shipment, making sure it didn't contain anything illegal. You can't import a Nile crocodile, for example, though he's seen people try. "They're used to feeding on large mammals in Africa," he said, "so a human being is not a big deal."

Barandiaran's method of inspection requires rapid peeking--he'll open the top of a sack ever so slightly even as some crazed lizard tries to poke its head out. Sometimes he uses a tube to look inside. Snakes are the least of his problems. The small mammals bite.

There are 12,000 shipments of animals a year here. There are eight inspectors.

In his office Barandiaran has a box with a foot-long caiman that arrived without proper papers. A caiman is similar to an alligator, and when small is prized as a pet. "That caiman will be six feet, possibly 10 feet, long," Barandiaran said. When the creature gets large and nasty, owners get spooked, and in many cases toss their pet into a canal.

The result: A colony of caimans is breeding near the Turkey Point nuclear power plant.

Florida Showdown

One day this spring the citrus canker invasion of South Florida almost led to gunfire. The canker, a fungus, has blighted thousands of acres of lime trees. The government has ordered the destruction of every lime tree within 1,900 feet of a known infestation.

An obstinate Broward County man, pushing 90, didn't want to go along. He had several favorite lime trees in his yard. When the feds came he got his shotgun and prepared to fire. That brought a swarm of cops. At gunpoint the old man dropped his weapon and gave up the battle. His trees were burned.

South Florida, lacking a killer freeze, is particularly susceptible to biological pollution. The Brazilian pepper has completely covered two large areas in the middle of Everglades National Park. The only way to get rid of it is to bulldoze everything, scrape away the limestone bedrock and cart it all away in trucks.

Melaleuca is probably the most hated invader, an ornamental tree that long ago escaped into the Everglades. It forms impenetrably dense stands that can only be knocked back with heavy doses of herbicide. The worst thing you can do is try to burn it. The leaves contain a highly flammable oil, and when a stand burns, the intense heat wipes out every other form of life nearby. The tree, meanwhile, emits millions of seeds, which take root all through the fire zone. The ultimate irony is that, in its native Australia, the melaleuca is an endangered species.

Lygodium, originally sold in nurseries, may turn out to be more diabolical than melaleuca. Its tiny spores can fly for miles in a windstorm. The oldest patches are in southern Martin County, where it pillars up from the forest floor, riding cypress trees to the sky. Dead, whitened tree trunks poke through the flourishing lygodium like skeletal fingers. The weed gradually drifted west and hopped the levee into the Loxahatchee National Wildlife Refuge, where it has proliferated only in the last decade. Aerial surveys revealed 39,000 acres in South Florida infested with lygodium in 1997. By 1999, the figure had risen to 100,000 acres.

The antidote may be yet another exotic species, an Australian moth that feeds on the fern. Biologists are still studying the moth to make sure it won't go berserk itself.

In the meantime, refuge managers attack lygodium with herbicide and machetes. At one point the refuge officials hired some college students to hack away at the fern, but their thrill at having an outdoorsy summer job vanished quickly in the steam of the swamp.

"They lasted about a week," said Mark Museus, the refuge manager. "They didn't want to work in 90-degree weather up to their chests in water with snakes and alligators all around."

Slippery Eels

Heavy rains flooded a plant nursery near the boundary of Everglades National Park last fall. When the waters receded, workers sloshed their way through the nursery, trying to clean up. Then they felt something around their ankles and boots. Something slithering. The creatures were tubular and moved like serpents.

Asian swamp eels.

Bill Loftus, a government biologist, says they were probably dumped in canals by Asian immigrants who wanted to create a food source. But the eels don't stay in one place. They don't even have to stick to water--they can wriggle across a moist road.

"This guy can burrow right into the muck and survive there," Loftus said.

He was standing by an eel-infested canal that runs westward toward the park. Loftus worries that if the eels get into the Everglades, they'll eat shrimp and small fish, and disrupt the food supply of migratory birds.

But maybe they won't.

"Ecology is sort of an inexact science," he said.

The Great Experiment

The world is a laboratory, and this experiment has little scientific supervision. No one can keep track of all the variables, all the new inputs, the stuff dropping unexpectedly through the skylight and into the bubbling vat of life.

"It's an irreversible experiment. That's the problem. With no control," says Loftus.

Tim Flynn, the Hawaiian biologist, finds it hard to be optimistic: "Sometimes it almost feels like a lost cause."

Among the newest invaders in America is an aquatic fern called giant salvina. It has been found in nine states from Florida to the far West. It grows on the surface of lakes and ponds and can form mats three feet thick.

"It will kill everything beneath it," says biologist Randy Westbrooks. "It's bad news. Bad news."

And in early July the federal government said it had found an invasive algae, Caulerpa taxifolia, among eel grass in waters off the coast of Southern California. The algae is toxic to sea life. It ruined thousands of acres of underwater habitat in the Mediterranean Sea in the 1980s.

Where did it come from? Aquariums. The algae looks good in a fish tank. Officials suspect that it mutated after exposure to ultraviolet aquarium lights.