Invaders from Earth.

Todd Hardwick will never forget the call. Their voices quaking with emotion, a Florida couple recounted the murder. First a piercing squeal from outside their window. Then, their rush to the scene to help. They were too late. Frozen in horror, they watched a gargantuan snake swallow a helpless raccoon, and then slither under their house.

Pest controller Hardwick arid his team spent two days back in 1989 enlarging the snake's tunnel to haul the reptile out. At 7 meters CM feet) and 112 kilograms (250 pounds), the reticulated python turned out to be the largest snake ever found loose in the United States.

No one knows how long the snake had lived under the Florida house. Reticulated pythons are native to Africa, not the United States, says biologist George Dalrymple. But in this country, they're sometimes sold as exotic pets. "Someone probably bought him as a cute baby," Dalrymple says. When he grew to be 10 or 12 feet long and became expensive to feed and hard to tame, the owners probably let him go.

Stories like this are cause for worry, say Dalrymple and other scientists. Exotic, or foreign, species could pose a threat to native U.S. wildlife - and maybe even to people, the scientists say. Pythons, for example, can eat small children. Other invading animals and plants can crowd out the native flora and fauna, wreaking havoc on entire ecosystems. Some invading plants have already choked off Florida waterways.

And the problems caused by invader species, Dalrymple says, will only get worse as people travel and ship goods around the world more often. That's because the majority of alien species arrive with help from traveler's. For example, in their hand luggage, some people tote exotic species to raise as pets, or they carry foreign shrubs and other greenery to plant in their gardens. Other species arrive as unintended stowaways in the cargo bins of planes and ships.

THE PROBLEM

Not all invaders are harmful. Scientists say only about n of the 4,500 foreign species in the United States cause eco-damage. "The danger is that we don't know what to expect from invader species," says David Strayer, an ecologist from the Institute of Ecosystem Studies in Millbrook, New York. In other words, scientists don't really know which species will turn out to be dangerous.

When landscapers brought the decorative melaleuca tree from Australia to Florida in 1906, for example, no one had any idea that the tall spongy plant would grow out of control. But with no natural predators to keep the exotic plant in check, the melaleuca has taken over more than 1.5 million acres of the sunny state's natural areas. That's an area about the size of the state of Delaware! And the plant continues to spread at a rate of 50 acres - about N football fields - a day. The tree grows in dense thickets, crowding out other plant species, drying up wetlands, and driving out native fish, tree

Some t i m e s intentionally imported species don't cause a problem until conditions in their new world, change. For example, when U.S. fur farmers began importing nutria, small beaver-like America in the 1940s, the animals were used to make furs. But when the fur market dwindled, nutria began to multiply out of control. Now, in Maryland's marshlands, the nutria are eating native muskrats out of house and home.

STOWAWAY SPECIES

Stowaway species - those that arrive by accident - are causing similar problems. One famous stowaway, the Black Sea zebra mussel, has made a new home in the Great Lakes. It probably arrived here in ballast water, the water a ship takes in to stay afloat while loading or unloading cargo. As biologist Jay Troxel of the U.S. Fish and Wildlife Service explains, "Whatever organism happens to be in the water when it's taken into the ship is [dumped out] in the ship's new port."

After being dumped in Lake Erie in the mid 1980s, zebra mussels multiplied and spread to the Hudson River in 1991. At their peak, there were 550 billion mussels in the river, more than all other aquatic animals combined, Strayer reports.

To feed, the mussels attach themselves to stationary objects near flowing water. Since their arrival, the mussels have clogged every water pipe in the Great Lakes and the Hudson River. But scientists are most combined that the mussels are wiping out phytoplankton, the tiny algae that are food for a vast number of fish and other marine life.

"Invasions really demonstrate how a single species inserted into an ecosystem can alter it entirely. [The invading species] can reach out and touch everything," explains biologist James Carlton of Williams College.

HALTING THE INVASION

Dealing with invader species is a tough job. Florida, which has the worst epidemic of invader species in the continental United States, spends $25 million a year combatting exotic plants alone. Each year, tree choppers and herbicide sprayers clear thousands of acres of melaleuca and other invaders. Still, the plants continue to spread.

Even the government has stepped in to try to halt the invasions. Laws control the transportation of wildlife into the United States. But they only bar entry to species already known to cause problems. New species aren't added to the list until they've proven harmful - in other words, often after it's too late.

And not everyone obeys the law. A law passed by Congress in 1990 requires ship captains to empty and refill ballast tanks before entering U.S. waters. But as Troxel says, who's to say if ship captains do so when no one is checking?

When the law can't help, there's always room for creativity. Nick Carter, a biologist with Maryland's Department of Natural Resources, recently hosted a barbecue, serving up nutria to 20 guests. Now the idea of eating the invader is catching on. Nutria soup, nutria salad, nutria chile.... Anyone have a recipe for 550 billion zebra mussels?