This is of great concern. There are still some Mountain yellow-legged frogs in Sequoia and Kings Canyon, but they are now gone from half the sites where they were found three decades ago. For the mountain populations, there is no question what part of their problem is. The lakes in the Sierra Nevada were dug out by glaciers about 13,000 years ago. They were fishless until the arrival of Europeans, who planted trout for sport.
Trout eat the tadpoles of the yellow-legged frog.
In most lakes today, where there are fish there are no frogs. But now frogs are even disappearing from locations where there are no fish. Part of the problem may be that frogs have been isolated into smaller populations which are more susceptible to extinction, and lakes that have lost frog populations are less likely to be recolonized. While these factors may affect the Mountain yellow-legged frog, they do not explain the disappearance of the Foothills variety or the disappearance of amphibians worldwide. Scientists are looking at many possible causes.