If global-warming predictions are right, as many as a quarter of mammals now alive could be extinct in our lifetime; in other groups of plants and animals, casualties could be as high as 40 percent. Considering that humankind doesn't have the money or know-how to save them all, some scientists are calling for ecological triage—choosing which critters to preserve and which to abandon. It's a concept that came to Stanford University biologist Terry Root, who has worked on the Intergovernmental Panel on Climate Change, after witnessing her husband's cancer treatment. "I realized I'm an oncologist for the world. I realized that for some species, it's already too late," she told a conference last year. "And then there are species who, like my husband, we can work to save. And part of what we have to do is this horribly, horribly difficult process of figuring out what we can save." Researchers are already devising intelligent ways to make Terry's choice. Marc Cadotte and a group at the University of California at Santa Barbara recently published a paper assessing which flowering plants in grasslands make it over the species-saving bar. Their general conclusion is that it behooves us to save the most genetically unique species and the ones that preserve functional ecosystems, which are often one and the same. Eventually, every species will have to be judged. Pity the poor scientists who have to spend their days crossing cute, fuzzy things off the list.