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