Attack Of the Aliens

Tall and tan and fat and ugly, achatina fulica is not something you'd want to behold on the sands of Ipanema. But you may not have a choice. Since gaining a beachhead in Brazil 19 years ago, this unlovely mollusk, better known as the giant African land snail, has proved unstoppable. Imported on the sly in 1988 as a cheap substitute for escargot, it has become a scourge, blistering Latin America's biggest country like a strange pox. Growing to the size of a man's fist and weighing one kilogram or more, it lays up to 2,000 eggs a year and eats a tenth

of its body weight a day, devouring everything from lettuce to mouse droppings to its own dead comrades. Worse, it can also carry rat lungworm, a nasty parasite that burrows into the human brain and causes meningitis, and another that can rupture the intestines. "It crawls into gardens, up walls, over the pavement," says Silvana Thiengo, a snail expert at Brazil's Oswaldo Cruz Foundation. "We found it in Copacabana. I mean, Copacabana!"

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 come in all guises, from trifling microbes to towering trees, from the mosquito to the mongoose. What they share is a penchant for stealth, spiriting around the planet on the wings of migratory birds, nestled in the threads of clothing or swimming in the human bloodstream. Scores of bioinvaders are deliberately set loose by farmers or eggheads trying to outsmart nature (importing snakes to chase rats), gardeners with exotic tastes (knotweed) or entrepreneurs too clever by half (seeking the perfect escargot). Bioinvaders are also ferocious competitors; free from the predators of their homelands, they prosper on virgin territory, monopolizing food supply and reproducing at a rate that would make rabbits blanch. Once lodged in a new land, the intruding species may never go away, forcing public authorities to battle them again and again with earthmoving equipment, fire and poison--a job as futile as Sisyphus'.

There is nothing new or automatically pernicious about wandering wildlife. Without the millennial scrambling of life forms, humankind would neither eat nor prosper. "More than 90 percent 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 tons of goods traverse the globe in a matter of 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.

World trade has increased twenty-fold over the last half century, with cargo ships, planes, and trucks providing a free ride for countless bugs and germs--an epic genetic upheaval that Jeffrey McNeely, chief scientist for the World Conservation Union, called "the great reshuffling." Pimentel reckons that the total numberof alien invading species known to science has climbed to 500,000--double the figure of just 60 years ago. Keeping track of invaders is vexing for even the most vigilant societies. A few years ago, the National Academy of Science reported that some 13,000 plant diseases are intercepted every year at international ports of entry in the United States. Yet customs inspectors are able to examine just two percent of incoming cargo and baggage. "This is the cost of globalization," says Charles Perrings, an environmental economist at Arizona State University.

Any pest, domestic or foreign, can be a nuisance, spoiling the flower patch or buzzing about the ears. But bioinvaders are especially dangerous. Some 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. An increasing number of 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. (Of course, global climate change may eventually trump all other dangers, but in part because invasive species are likely to run rampant in a warming world.) "Once you get a nonnative plant or animal species in the system, then 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--accounting for damage to watersheds, soil degradation and extinction of wildlife--may be a staggering $1.4 trillion a year. And if the majority of experts are right, the cost of bioinvasion is only going to get worse.

Fittingly, perhaps, invasive pests and pathogens seem perfectly suited to the rigors of a world without flags. Some alien plants and animals need little more than a toehold to lay siege to a new land. 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 billon 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 U.S. 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."

Like so much else in the global marketplace, the burden of bioinvasion falls unevenly across the world. The human toll is often devastating to the poorest nations, where a failed crop can start a famine. Implacable exotic pests like the cassava mealybug, gray leaf spot and witchweed claim up to half the harvests in the poorest countries, posing a "serious threat to life and livelihood" with "enormous economic and political ramifications," says Guy Preston of the Working for Water program, a South African government initiative dedicated to preserving fresh water.

Since exotic species tend to thrive in milder weather, global warming has expanded the frontiers for a host of heat-seeking organisms. Not surprisingly, one of the major ports of entry for bioinvaders in the U.S. is Florida, which is already home to half of the country's 50,000 known alien species, from rogue ornamental plants like hydrilla to escaped pets like Burmese pythons, which have been found in the Everglades. "Invasive species are at a definite advantage in a warming world," says Pimentel.

Bioinvaders can be sleeping giants, lying about for decades until a biological opportunity like a storm or a heat wave arouses them. Take the Hottentot fig. Known for its profusion of pink and yellow flowers, this carpetlike succulent plant spread only modestly in the decades since its arrival in the U.K. from South Africa more than a century ago, held in check by the cooler climate of the British Isles. But the run of mild winters brought on by global warming has set it loose. "It's rampaging," says Spencer, "smothering local plant communities as it goes."

The planet's most reckless species has given bioinvaders some of their biggest breaks. The African snail, originally from East Africa, found its way to steamy Asia a century ago, perhaps tucked away in the cargo of some trading vessel, and never stopped. In 1936 a gardener from Formosa (today's Taiwan) took a couple of specimens to Hawaii to bejewel her rock garden and touched off an environmental emergency that still hasn't ended. Achatina fulica later landed in Guam and then Saipan, apparently stowing away in the gear of U.S. soldiers, sparking one of the less celebrated occupations of World War II. So thickly did living and dead snails line the island highways "dat de jeeps er slipten," one Dutch scientist reported. Twenty years later, a boy returning from vacation in Hawaii landed in Florida with a pair of snails in his pocket, touching off an invasion that took seven years to eradicate.

By then, you'd think the word on African snails would have oozed out, but apparently not to Brazil. In 1988 an ambitious merchant showed off a box of them at an agricultural fair in Curitiba, waxing about the fortunes to be made off the mother of all escargot. A cottage industry was born, as snail farmers began to export their harvest for a tidy sum. But along came Brasília's economic-stability plan, which caused the country's currency to spike against the dollar, pricing tropical escargot out of the world market. Scores of breeders ended up dumping their glut into the wilds. The authorities eventually banned the snail, but too late. At last count, the giant African snail had spread to 23 of Brazil's 27 states. Now civil-defense teams scramble around the map, shoveling up yesterday's delicacy by the unsavory ton.

Importing nature can be a blessing. A parasitic wasp from South America has helped millions of African farmers control the cassava mealybug, which ravages that continent's staple food, while Australia has successfully turned a killer virus from the Czech Republic against the ubiquitous European rabbit. Often, though, nature bites back. The Indian mongoose was shipped to the West Indies to hunt rats, but ended up feasting on almost everything that crawled or croaked; a handful of ground-nesting birds and up to a dozen amphibians and reptiles were driven to extinction.

Indeed, bioinvasion's worst legacy may be the havoc it wreaks on other forms of wildlife. In the United States, as much as 40 percent of all extinctions can be blamed on invasive weeds, predators or pathogens, says Pimentel. In South Africa, many alien weeds come hard-wired for combustion, worsening the hazard in a nation where wildfires cause $461 million in damage a year. So thick are the palisades of thorny mimosa--an aggressive weed akin to the touch-me-not--that India's endangered one-horned rhino can no longer move about freely in Kaziranga National Park. Chinese authorities have spent some $800,000 battling the American white moth, which now destroys more than 1.3 mllion hectares of woodlands each year.

And yet getting rid of bioinvaders can often prove equally problematic. Brought to China from the Philippines to eat mosquitoes, the mosquitofish has lately become a tyrant, spreading throughout the marshlands in southern China and driving several native aquatic species to the brink. The only way to kill the mosquitofish is by dousing the water with rotenone--a poison so potent it also kills almost everything else that swims. Still, doing nothing may "threaten China's most important species--the Chinese people," says Wang Fanghao, who lectures at China's Agricultural Science Academy.

Perhaps the only sure way to curb bioinvasion is to plug the gaping holes on international borders. If customs inspection in the United States is lax, in much of the rest of the world it's almost laughable. Only in 2005 did India get around to asking arriving passengers whether they were carrying any fruits, vegetables or plants--all major pathways for disease. But customs controls have their limits in the global economy. Thanks to tough import laws, Australia has developed one of the best bioinvasion defenses of any nation. Yet in the late nineties Canadian salmon farmers cried foul, charging Australia with unfair trade barriers. The World Trade Organization agreed, forcing Australia to open its market--a ruling that scientists fear could undermine other quarantine regulations.

And yet in a time when germs ride the wind and tide, even the most zealous border guards may be of little use. In most countries, exotic species are simply too entrenched to eradicate. Now a few scientists, mindful of Ralph Waldo Emerson's dictum that "a weed is a plant whose virtues have not yet been discovered," are trying to put bioinvaders to work. In India, one team of researchers is helping rural families turn lantana camara--a scraggly weed that overruns native woodlands--to good use as a substitute for bamboo. Not all pests and weeds may prove so compliant, but that doesn't mean scientists should give up and grab the flamethrower. "The issue isn't stopping bioinvasion, but understanding it," says Perrings. In the end, that means learning to live with the enemy.