West Nile virus hits the Midwest. Northern snakehead fish, natives of China, eat their way through a Maryland pond. Foreign weeds fuel Western fires.

Sparrows raid your bird feeder.

They're all connected, scientists say: The effects of "alien species," new and old, are everywhere.

The new ones, such as snakeheads, grab headlines. The old ones -- sparrows, house mice, cows and kitty cats, for example -- Americans can hardly imagine living without.

A generation ago, the public paid scant attention to the ecological hazards of moving critters and plants from one part of the world to another. Today forest fires are blamed in part on dense growths of nonnative thistle and cheatgrass.

"The whole science of invasive species has become very hot," said Leonard Krishtalka, director of the University of Kansas's Natural History Museum and Biodiversity Research Center. "What we're seeing are wake-up calls to the dangers of introducing nonnative species to finely tuned ecosystems."

The West Nile virus is thought to have jumped the Atlantic when mosquitoes hitchhiked on a boat full of tires.

For centuries, however, many other invaders have reached the United States because some people wanted them here.

Take pigeons, for example. Native to the Mediterranean region, they were brought to the New World as food and as racing pets. Now, according to Cornell University researcher David Pimentel, Americans spend more than $1 billion a year trying to control and clean up after pigeons.

"There have been very few species invading the U.S. that we've intentionally tried to eliminate, and even fewer cases in which we were successful," Pimentel said.

Most of the 50,000 species introduced to the United States proved to be friendly. Turkey Red wheat, brought by Mennonites from southern Russia in the 1870s, made Kansas the nation's breadbasket.

Other aliens that crossed the seas, in Pimentel's view, have made trouble from the start.

In 1999 Pimentel headed a group of Cornell researchers who attempted to determine the price Americans pay to live with nonnative species -- including various weeds, rats, goats, horses, fire ants, cats and dogs.

The tab: $138 billion annually.

Alien plants strangle native plants. Cats eat native birds. Baltimore orioles get shoved around by hordes of English sparrows, brought to America by farmers trying to control insects (and, legend has it, by theater lovers who wanted to raise birds mentioned by William Shakespeare).

"Of the approximately 97 introduced bird species, only 5 percent, including chickens, are considered beneficial," the Cornell report concluded. "Most . . . are considered pests."

The recent scare in Maryland was traced to a pair of snakeheads purchased at a New York market two years ago, then placed in a pond.

To eradicate the ferocious fish -- already in California, Florida and other states -- wildlife officials recently dumped poison in the pond. Afterward, state biologists found the bodies of six adult snakeheads and more than 1,000 juveniles.

Tests have shown that the breeding population was wiped out, and Maryland wildlife officials have started rehabilitating the four-acre pond. There was no indication that any snakeheads had escaped from the pond and gotten into the Little Patuxent River -- which was just 75 yards away.

Missouri and Kansas struggle with their own aquatic invaders. Giant carp have proliferated since the 1993 floods washed them away from catfish pens along the Mississippi River.

Catfish farmers in the South import carp from Asia to control snails and unwanted plants harmful to their harvest.

Unleashed, however, the jumbo silver, grass and bighead carp can bully their way through states that restrict their import, such as Missouri, sucking up plankton and displacing smaller fish.

Carp may already be responsible for shrinking populations of smallmouth buffalo fish in Kansas, said Tom Mosher, a fisheries researcher for the state Department of Wildlife and Parks.

"The carp have a tendency to out-compete our native fish," he said. "They're big and ugly, and they go where they want, pushing aside the others."

With the world's strongest nation seemingly powerless to the kudzu vine, the challenge for conservationists is to identify an invasive species as early as possible and then to predict where and how fast it might spread.

Invaders travel in both directions. Species native to North America have established themselves elsewhere: trout in Japan; gray squirrels in England; largemouth bass in South America. American bullfrogs occupy European ponds.

About 1982, ballast waters carried Leidy's comb jelly, a forager of zooplankton off the East Coast, into the Black Sea. It caused a collapse of the region's anchovy trade.

The Western corn rootworm may have reached Belgrade, Yugoslavia, a few years ago by stowing away on U.S. military planes.

Policymakers began to take invasive species seriously in the 1980s, after zebra mussels from Europe rode into the Great Lakes in the ballast water of a visiting ship. They multiplied, affixing themselves in bunches to boat hulls and clogging underwater pipes.

The striped mussels have since spread through the Midwest.

Last year about 30 half-shells of zebra mussels were found in a water intake screen at the Quindaro Power Station in Wyandotte County.

What most frightens scientists is the unknown: How will a new species, placed where it doesn't belong, affect other plant and animal life?

While the zebra mussel, for example, can wreak havoc on water intakes, some people now credit the pest for making the Great Lakes look clearer, thanks to its natural filtering powers.

"We really have only haphazard ways of tracking these things," said Phyllis Windle, senior staff scientist on the Union of Concerned Scientists' project on invasive species. "The conservation community was for years behind the ball and is now playing catch-up."

Even so, history shows that humans do a lousy job of stopping foreign invaders.

In the 19th century they tried to eliminate European black rats and Norway rats that had sneaked onto ships to the West Indies, where the rats feasted on sugar crops.

Farmers imported the mongoose, which ate the Norway rats, but the mongoose couldn't get to the wily black rat -- it learned to hide in trees.

With fewer Norway rats competing for crops, the black rat population exploded. "All they ended up with was mongoose and a lot more rats," Pimentel said. "A real disaster."

Maybe people should stop trying so hard to control the animals, said John Grandy, senior vice president for wildlife at the Humane Society of the United States.

"Most of our invasive species have been with us a long time," Grandy said. "We would oppose someone introducing a new species into an ecosystem . . . but it's not the snakehead's fault.

"We need to reevaluate the argument, drop these artificial labels like 'alien' and 'invasive,' and learn to live and deal with these animals."

But then, who needs another species of sparrow?

"Do we throw up our hands and let globalization take its course?" Krishtalka asked. "What we don't want is a bland, gray world of wildlife, a monoculture, where everything is the same everywhere.

Be vigilant, he said, or be sorry: "We can only imagine what life in this country would be like without rats."

When snakehead fish were discovered in Maryland, this Crofton pond was sprayed with herbicide as the first step in eliminating the alien species. Biologists later found the bodies of six adult snakeheads.Of the 97 bird species introduced into the United States, only 5 percent are considered beneficial, according to a 1998 report. This zebra mussel was caught in a filtering system on Glen Lake, just south of Lake George, N.Y., in 1998.