Karen Oberhauser was scrambling up a mountain about 100 kilometers northwest of Mexico City when she began to fear for the future of the monarch butterfly. It was the winter of 1996–1997, and Oberhauser, an ecologist then working at the University of Minnesota and more accustomed to the flat, low-lying U.S. Midwest, huffed and puffed during the steep, high-altitude hike. Her head ached in the thin air. But when she stopped to look around, she saw millions of monarchs draped like living jewels on fir trees that hugged the slopes.