Where do camels belong? You may be surprised to learn that they evolved and lived for tens of millions of years in North America-and also that the leek, national symbol of Wales, was a Roman import to Britain, as were chickens, rabbits and pheasants. These classic examples highlight the issues of "native" and "invasive" species. We have all heard the horror stories of invasives. But do we need to fear invaders? In this controversial...