What are invasive species?
They’re the barbarian hordes of the natural world. In every ecosystem, nature tends to produce a balance in the competition for survival, with no one creature or plant totally dominating the rest. But when plants or animals are plucked from one part of the world and suddenly deposited in a new ecosystem, they can find themselves without predators or competing species to keep them in check. With few natural barriers to their proliferation, these invaders can run rampant in the new ecosystem, colonizing it to the detriment of other species, the local economy, and the human beings who live there.

Can plants really be that troublesome?
Just consider the case of the kudzu vine. It arrived in the U.S. from Japan in 1876, imported by Southern farmers to prevent soil erosion because it sets deep roots, holding the soil in place. But kudzu grows with astonishing rapidity; a single vine can lengthen by 12 inches a day and cover an entire house in a few years, and those deep roots make it difficult to eradicate. Kudzu now covers millions of acres in the South and has invaded much of eastern North America, appearing as far north as southern Ontario. Not only does it crowd out native species and rob the soil of vital nutrients, but chemicals in kudzu vines react to nitrogen in the air to produce ground-level ozone, a dangerous air pollutant that contributes to respiratory distress in children and the elderly.

How do these species affect people?
The invaders can wreak havoc with crops and fishing stocks, spoil recreational areas—even depress property values. The Asian longhorned beetle, for example, first spotted in the U.S. in 1996, has wiped out tens of thousands of trees in the Northeast, depressing the value of the surrounding real estate. The parasitic protozoan Haplosporidium nelsoni, which first arrived in the U.S. from Asia in 1957, has reduced the dollar value of oyster harvests in Long Island Sound by 96 percent in just a decade. In the Western U.S., recreational trout fishing is now threatened by a European parasite that attacks the fish’s nervous system; in the Great Lakes, a $7 billion fishery is endangered by the Asian carp, which sucks up plankton like a vacuum cleaner and is working its way northward via the Mississippi and other rivers. Scientists estimate that the damage and control costs of invasive species already amount to $138 billion per year in the U.S. alone.

How do these species get from one region to another?
Some, like kudzu, are invited guests; many more are stowaways. Quagga mussels, for example, were originally native to the rivers of Ukraine. But by the late 1980s, they had reached the Great Lakes in the ballast water of cargo ships from Europe. The mussels quickly spread by gobbling up the plankton on which many Great Lakes fish species live. Quagga mussels now cover virtually the entire floor of Lake Michigan. “It’s like a carpet,” says Gary Fahnenstiel of the National Oceanic and Atmospheric Administration. “They average about 5,000 per square meter, all the way across.” And now they’re colonizing lakes in the West, having arrived via boats hauled by trailer from the Midwest.

Is this just a problem in the U.S.?
Invasive species are a worldwide scourge—a nasty side effect of modern transportation technology and economic globalization. Since arriving in Guam aboard merchant ships from Australia and Indonesia, brown snakes have overrun the Pacific island, virtually wiping out its entire bird population. In Australia, the cane toad, introduced in the 1930s to control pests on sugar-cane plantations, has endangered bird populations by devouring the insects on which the birds feed. New invaders arrive on foreign shores daily, smuggled in suitcases, clinging to used tires shipped overseas for recycling, and curled in the wheel wells of airplanes.

Is there any solution?
Not as long as humans continue to travel and trade. But invasive species can be slowed, at least. Biologists in Texas are battling the zebra mussel, a tiny European invader that clogs many lakes and rivers in the Midwest, by sprinkling potassium chloride crystals into the streams that feed large lakes. The potassium chloride clogs the gills of the mussels, which then suffocate. Scientists in the Midwest are beating back invasive plant species with herbicides that drastically reduce their seed production. Despite such efforts, says Florida wildlife biologist Skip Snow, invasive species are certain to continue to spread around the globe, with unforeseeable consequences. “We are engaged in a giant experiment that no one can control,” he says.

Eeek! It’s the Frankenfish
Not for nothing is the Northern snakehead called the Frankenfish. Native to China and the Koreas, where rice farmers use it to control pests in their paddies, the snakehead was introduced to the U.S. by Asian immigrants who cook the fish in a soup prized for its health-giving properties. Accidentally or intentionally, snakeheads have been introduced to rivers and lakes in the U.S., to the alarm of marine biologists. This unique freshwater fish, with sharp teeth and a ravenous appetite, can survive out of water for several days. Using their gills and muscular bodies, some snakehead species can climb out of the water and wriggle along the ground for hundreds of yards a day in search of new lakes and new prey. Snakeheads are even more worrisome because they breed rapidly—a single female can release 15,000 eggs, five times a year—and so can quickly take over lakes and ponds. They’ve been spotted in at least nine states—most notably Maryland, where the discovery of dozens of snakeheads in 2002 triggered a national uproar and fears that the fish might march to the Chesapeake Bay and claim its abundant fishery for themselves. They haven’t, but the snakehead is here to stay. “Neither man nor nature can get rid of them,” says fisheries scientist Kelly Gestring. “We have to deal with that fact and develop management strategies.”