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 in Rio de Janeiro. But you may not have a choice. Growing to the size of a man’s fist and weighing one kilogram or more, the Giant African Land snail 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 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, of 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 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.”

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