Yellow-legged frogs were once common in high-altitude lakes in the Sierra Nevada mountains, but they have become increasingly rare. Trout feed on tadpoles and young frogs. Few lakes in the Sierra Nevada had any trout in them until a hundred years ago. At that time, many lakes were stocked with trout for recreational fishing, and now trout are common in virtually all bodies of water in the Sierra Nevada. Researchers removed the trout from one lake, and the frog population soon quadrupled. Since frogs are capable of moving several miles over land, removing trout from just a few lakes is clearly the way to restore the frog population to its former levels.