Italy had to abandon a project to exterminate the north American gray squirrel because people think of the rodent as cuddly, even though it is an agricultural pest and has driven the European red squirrel to near extinction. The ordinary house cat also ranks among the most predatory of the invaders, according to Peter Jackson, a retired expert on felines. Not only can cats quickly revert to the wild, where they become formidable hunters and killers, but domestic pets cause enormous depredations during their nocturnal ramblings. Judging from a survey by the Mammal Society in Britain, cats could be killing more than 250 million birds and other creatures in the United Kingdom every year. The society says the toll could be significantly reduced if owners kept their cats indoors at night or put a loud bell on their collar.
Many aliens eventually become naturalized, like potatoes and tomatoes.
Others are creatures that Noah might wish he had never allowed onto the ark, such as the Rosy wolf snail, the crazy ant and the walking catfish. But there is one species, Mr. McNeely says, that is more invasive than all the others put together. It's called homo sapiens.