The U.S. is home to more than 200,000 native species of plants, animals, and other wild things, at least twice as many as previously thought, according to a new Nature Conservancy study, the most comprehensive look at biodiversity ever undertaken in the nation. As many as a third of the species are imperiled to some degree, the enviro group says. The Nature Conservancy, which plans to release the study today, will also announce that it is launching the most expensive conservation campaign ever conducted by a private group — a five-year, $1 billion effort to protect large tracts of remaining wild land in the U.S.