The aliens have landed and they're killing the natives. It may sound like the plot of a bad movie, but it's real life: Alien species threaten the survival of native plants and animals across the country. In the report, America's Least Wanted, The Nature Conservancy has named the 12 most threatening invaders of our nation's ecosystems, and five of this "dirty dozen" are on the loose in the West. The five - tamarisk, purple loosestrife, leafy spurge, the marine plant hydrilla, and flathead catfish - choke out local species or prey on them. Exotic species nationwide contribute to the decline of 42 percent of all federally listed threatened or endangered species, and 79 alien species have already cost the country some $97 billion, says the conservancy. The free 30-page report, edited by Bruce A. Stein and Stephanie R. Flack, can be ordered from Laurie Bullard at The Nature Conservancy at 703/841-4828, or seen on the web at http://www.tnc.org/science/library.