The mountain yellow-legged frog thrives in lakes that are too cold for other amphibians. While this makes them a particularly unique species of frog, it has also recently become their greatest threat. In the early 1900's, California introduced non-native trout populations for gaming in the same lakes, ponds and marshes that the mountain yellow-legged frog lived in. These fish fed on the frogs as well as their tadpoles and their food sources. As the fish are non-native, the frogs have no natural defense.
The frogs are now absent from more than 92 percent of their historic localities in the Sierra Nevada. In Southern California, they are absent from 99 percent of their historic range. No frogs have been seen in the San Bernardino Mountains or on Mount Palomar since the 1970's. The fires of 2003 appear to have destroyed the remaining populations, and they are now considered extinct in the San Bernardino Mountains. Despite concurrent findings by the U.S. Forest Service and the National Park Service of the threat to the frogs, the U.S. Fish and Wildlife Service has yet to list the mountain yellow-legged frog under the Endangered Species Act.