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.

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