Faced with a host of environmental threats, many of the world's plant species are believed to be at risk of extinction. But which species? To answer that question, scientists have developed a streamlined method for assessing the conservation status of large numbers of species. Evaluating the flora of Puerto Rico, they found that 459 species -- 23 percent of the flora -- should be classified as "At Risk." The process could help focus conservation efforts where they are most needed.