VREDENBURG, V T: Exotic species and the decline of amphibians: unintended consequences on a global scale.

A puzzling aspect of amphibian declines is that amphibians, particularly frogs, have been declining even in large protected areas. These habitats are generally thought to be relatively unaltered and undisturbed. However, the introduction of nonnative fish is a common practice throughout the world, and may be an important cause of declines. In the Sierra Nevada, California, the mountain yellow-legged frog (Rana muscosa) has disappeared from >80% of its historic habitat during the past century. Obvious habitat destruction is not to blame because nearly its entire habitat is on public land that is protected from timber harvests and development. Surveys of >2000 historically fishless lakes and ponds showed that the distribution of nonnative trout and mountain yellow-legged frogs were largely non-overlapping. However, some land management agencies and scientists have rejected the idea that trout introductions could be a major cause of decline because 1) introductions began as early as the 1890s, 2) major frog declines did not occur until the late 1970’s, and 3) several reports existed of apparently co-existing populations. To test the hypothesis that introduced trout could be mechanism driving this pattern, a fish-removal experiment was conducted over a five-year period. Frogs quickly recolonized the fish-removal lakes (n=3) from nearby source populations, while numbers of frogs in fish containing control lakes (n=8) remained low and constant. These data suggest that in some areas, the removal of exotic species may be sufficient to reverse the decline of some amphibians.