Brazil’s unwelcome snail is just one of a burgeoning breed of pests and pathogens that have broken free of their native habitats around the world. Biologists somewhat quaintly call them exotic species. The rest of the world knows them for what they are: bioinvaders. They are competitors free from the predators of their homelands, prospering on virgin territory, monopolising food supply and reproducing at an astronomical rate.

There is nothing new or automatically pernicious about wandering wildlife. “More than 90 per cent of food crops like wheat, corn and rice, and almost as many strains of livestock, are exotic species,” says Cornell University’s David Pimentel, a leading scholar on the subject. But bioinvasion has taken a quantum leap in a borderless world where billions of people and tonnes of goods traverse the globe in hours, making a mockery of customs inspectors and quarantines. Indeed, the very forces that make the international economy flourish — trade, travel, transport and tourism — also make it vulnerable to invasive species. “This is the cost of globalisation,” says Charles Perrings, an environmental economist at Arizona State University.

Some bioinvaders wipe out harvests, choke waterways and desiccate the landscape, inviting wildfires. A deadly few microbes cause pandemics, like mad-cow disease and AIDS. Even when they aren’t an outright menace, exotic plants, animals and pathogens impoverish nature by crowding out a whole suite of homegrown species or creating mongrel hybrids through interbreeding. More and more scientists now agree that bioinvasion is the most immediate — and surely the fastest-growing — threat to plant and animal life on the planet after deforestation and breakneck development. “Once you get a non-native plant or animal species in the system, it’s very difficult to get the habitat back to where it was,” says Mark Spencer, an expert on invasive species at the Natural History Museum in London. “We are at an ecological tipping point.”

In the United States alone some 50,000 bioinvaders cause an estimated $120 billion in damages to harvests, trees and fisheries every year, according to Pimentel. Throw in India, the United Kingdom, Australia, South Africa and Brazil, and the cost nearly doubles, to $228 billion. Globally, says Pimentel, bioinvasion’s toll on the economy and the environment may be a staggering $1.4 trillion a year.

Barely a decade after washing out of the ballast tanks of European ships into the inland waterways of the United States, the Baltic zebra mussel has spread from the Great Lakes to the Mississippi Delta, choking water pipes, clogging hydroelectric-power plants and driving native water plants and mussels to the brink by monopolizing food and oxygen. Officials spend $1 billion a year combating zebra and quagga mussels — just two of the 88 exotic species loose in the United States.

Often in the natural world, what goes around comes around. So while Old World mollusks choke US waterways, Yankee imperialists like the American mink and the signal crayfish are “taking over British waterways, outmuscling native competition and spreading disease”, warns Britain’s environmental authority. Then there is Japanese knotweed, a peripatetic ornamental plant so aggressive it can crack roads, fissure buildings and simply overwhelm native plants. London Development Agency director Gareth Blacker, who is excavating a vast East End brownfield site to build a sports complex for the 2012 Olympics, says unexploded World War II bombs will be “less difficult to deal with than knotweed”. So thick are the palisades of thorny mimosa — an aggressive weed akin to the touch-me-not — that the endangered one-horned rhino can no longer move about freely in Kaziranga National Park.

But, that doesn’t mean scientists give up and grab the flamethrower. “The issue isn’t stopping bioinvasion, but understanding it,” says an expert. In the end, that means learning to live with the enemy.

—Mac Margolis / Newsweek