Seeing a familiar bird in Hawaii may make someone from the mainland United States feel more at home. But such species that invade new habitats also can displace native species. In the most comprehensive analysis to date, to be published in Friday's BioScience, researchers have found that nearly half of species on the brink of extinction were put there in part by exotic invaders often introduced into new habitats by people.
To compile a comprehensive list of species whose numbers are dwindling, a team led by David Wilcove at the Environmental Defense Fund combined the federal endangered species list with a catalog of imperiled species maintained by the Nature Conservancy. Next they determined the percentage of species affected by five types of threats--habitat destruction, the spread of alien species, pollution, overharvesting, and disease.
The most potent force, they found, was habitat destruction, which threatened 85% of species studied. Less well-studied was the substantial damage inflicted by alien species, which appeared to harm 49% of plants and animals listed. The finding "critically points out the need for understanding alien species," says ecologist Louis Gross of the University of Tennessee, Knoxville. Countering these threats to maintain biodiversity, he says, will be a "daunting task."