IT'S MIDNIGHT ON Guam, and an eight-foot-long brown tree snake has just emerged from a toilet bowl. After hours of slithering through sewage pipes, she's hungry. She slides across the bathroom floor into the room where a baby lies sleeping. The snake slithers up into the crib, and then under the boy's blanket. Her jaws clamp down on his fingers. The boy screams. His father bolts into the room, grabs the slinky monster and throws it out an open window.
There are so many snakes on Guam they're literally crawling out of the woodwork. In some areas there are as many as 13,000 per square mile, and they eat everything they can sink their teeth into. Eventually, many biologists think, the infestation is going to spread to Hawaii, which is just a short flight away. "It's only a matter of time," says Tom Fritts, a research biologist with the U.S. Geological Survey. "Hawaii could be devastated," agrees Julie Savidge of the University of Nebraska. In this century much has been made of vanishing species; in the next millennium we may be even more alarmed about rapidly expanding ones.
The ecological equation is simple. When there are too many predators, not enough resources and a great deal of competition, an animal population dies out. But when the opposite occurs, the population explodes. This can be every bit as bad. When the brown tree snake arrived on Guam in the cargo of a military plane 50 years ago, the ecosystem wasn't ready for the reptilian assault. The snake had no natural competitors or enemies. Virtually every forest songbird on Guam is snake droppings now.
The situation will get worse as trade, shipping and tourism bring stowaway animals - a snake in a shipping crate, a mosquito in a suitcase - to new habitats. And native species run rampant can be as much of a problem as alien intruders. ""What humans do is make the environment worse for a number of species and make it much better for some," notes ecologist James Brown of the University of New Mexico. The white-tailed deer, for example, has been thriving in second-growth forests and is now a suburban pest.
Opportunistic animals aren't pests just to us; they can be their own worst enemy. The lesser snow geese of Canada are so overpopulated they're destroying their own habitat. Over the past three decades, the population has risen from 800,000 to almost 3 million. Advances in agriculture have enabled the geese to arrive at their breeding grounds in better condition, leading to higher rates of reproduction. As these huge flocks of birds nest on the west coast of Hudson Bay, they grub intensively, upturning soil and pulling out plants by their roots. The land has been ravaged. ""The Arctic is a fragile ecosystem," says Bob Trost of the U.S. Fish and Wildlife Service. "It may take upwards of a century to come back to a productive state." There are two options: the geese will find another nesting ground and destroy it, or they will run out of food and land and their population will crash.
Managing these booms is difficult. Encouraging recreational hunting is one option, though it's unlikely to make a large dent. More efficient methods, like killing animals en masse, tend to prompt public outcry. Nobody much cares for tree snakes, but keeping them from leaving Guam is ultimately more important than eradicating them. The USDA is using snake-sniffing dogs to check outgoing cargoes.
The most natural method of controlling outbreaks is biological control - bringing in a predator or enemy. Recently the USDA introduced the phlorid fly to help cull the pesky fire ant, which has long been stinging Southerners and sucking the sap out of soybeans. The fly implants its larvae in the ant, eventually killing it. But this method has its dangers, too. When Australian authorities released a virus targeting wild rabbits, they ended up killing many domesticated bunnies as well. And some scientists say the virus could spread to humans. Maybe that's what you get for playing God.