Bullfrogs, fire ants, zebra mussels, kudzu and English ivy are common plants and animals, harmless or even beneficial in their native habitats. But if moved into new territories, these hardy and invasive species can be devastating to both human enterprise and biological diversity. American wildlife officials as well as the general public are paying increased attention to the invasion of non-native species.
"HAVE YOU SEEN THIS FISH?" reads a wanted poster placed around a pond in Crofton, Maryland, some thirty kilometers north of Washington D.C. The warning beneath a picture of a long fish with toothy jaws and snake-like scales reads: "The northern snakehead from China is not native to Maryland waters and could cause serious problems if introduced into our ecosystem."
Only two adult snakeheads have been found in Maryland waters, but state wildlife authorities as well as the general public are concerned.
Eric Schwaab is the director of the Fisheries Service for the Maryland Department of Natural Resources, says "this fish is really just a sort of a top-level predator and in that sense isn't really different from many of the other top level predator species of fish that occur here naturally. The danger is that it will potentially displace those or otherwise upset these systems."
The northern snakehead eats frogs, aquatic birds, small mammals, and other fish, including native predatory fish. It can survive for several days without water and can move on the ground using its strong pectoral fins. Mr. Schwaab says that means that even with a small flood, the fish could get into the nearby Little Patuxent River and from there to other bodies of water.
How did the Asian predator get into the small pond in Maryland? Northern snakeheads are imported live into the United States and sold in fishmarkets as an Asian food delicacy. Two years ago, a Chinese-born Crofton resident ordered a couple of live northern snakeheads from an Asian fish market in New York. He kept them in his aquarium for a while, but when they became too large, he released them in Crofton Pond. There, they continued to grow until local fishermen found them. By that time, they were about three-quarters-of-a-meter long and posed a significant risk to the local water ecosystem.
The Maryland Fishery Services immediately began a search action to eradicate them, but it may be too late: the pair of snakeheads have had babies.
This is not the first time a non-native species has been found in Maryland waters. Eric Schwaab says state officials are already trying to curb the spread of several other species, harmful to its environment. "That runs the gamut from green crabs which are brought here for bait in our coastal bays to zebra mussels which originated in this country in the Great Lakes region and continued to spread in this direction," he says. "But unlimited there, we have species like mute swans and nutria, which is a large marsh rodent that was introduced here that significantly threaten different aspects of our functioning eco-systems."
In other parts of the country, non-native zebra mussels that came as stowaways on ships from Europe have infested waterways, clogging inlets and pushing local species of mussels and other freshwater shellfish to extinction. Climbing oriental vine kudzu was fist brought to this country as an ornamental plant, but because of its ability to grow fast and spread easily, it was planted on stream banks to prevent erosion. Soon it spread beyond control and is now threatening farms in southeastern states, as far north as Pennsylvania.
In the same way, some species native of America cause trouble elsewhere in the world.
For example, America's western corn rootworm hitched a ride on a commercial flight to Belgrade and has since been munching its way through the heart of Europe's farmland.
Yvonne Baskin, a science writer from the western state of Montana, says as global trade develops, the threat of invasive species grows. She warns about it in her new book titled "A Plague of Rats and Rubbervines." "Say [for example], the connections now between the U.S. and China which had been politically shut off for many decades," she says. "Trade has burgeoned and one of the unexpected results was the arrival of shipping crates that apparently had contained the Asian long-horned beetles, which had burrowed into them and crawled out and got into the street trees of Chicago and New York and are now causing a lot of grief for people who have had their neighborhoods stripped of very beloved shade trees."
Environmentalists fear these beetles might spread into the United States forests. Yvonne Baskin says non-native species are usually more dangerous in their new environments because there they do not find their natural enemies.
Ecological organizations throughout the world have programs to control the spread of invasive species. And new global trading laws are incorporating clauses to the same effect. But Yvonne Baskin says the general public needs more education on the dangers of introducing non-native species into new environments.
"When we fly into an airport from overseas and we see they are asking whether we've been on a farm, they are asking whether we are carrying fruit or meat products, we sort of don't tell them about the apple in our pocket and ignore quarantine laws," she says. "And perhaps, if we understood the danger that comes from bringing foot-and-mouth disease if you visited England, or bringing fruit flies with you in the exotic fruit, or the kinds of things that are traveling around the world, maybe we would all be more considerate of quarantine and of the measures to take quick action, whether it involves pesticide spraying or other solutions against things that do arrive."
Releasing non-native species anywhere in the United States is illegal, but the man who released the northern snakeheads into Crofton Pond says he did not know it. Maryland authorities are now spending their tax-payers' money in an effort to prevent potentially grave consequences of one man's mistake.