Non-native fish introductions and the decline of the mountain yellow-legged frog from within protected areas

One of the most puzzling aspects of the worldwide decline of amphibians is their disappearance from within protected areas. Because these areas are os tensibly undisturbed, habitat alterations are generally perceived as unlike ly causes. The introduction of non-native fishes into protected areas, howe ver, is a common practice throughout the world and may exert an important i nfluence on amphibian distributions. We quantified the role of introduced f ishes (several species of trout) in the decline of the mountain yellow-legg ed frog (Rana muscosa) in California's Sierra Nevada through surveys of >17 00 sites in two adjacent and historically fishless protected areas that dif fered primarily in the distribution of introduced fish. Negative effects of fishes on the distribution of frogs were evident at three spatial scales. At the landscape scale, comparisons between the two protected areas indicat ed that fish distribution was strongly negatively correlated with the distr ibution of frogs. At the watershed scale, the percentage of total water-bod y surface area occupied by fishes was a highly significant predictor of the percentage of total water-body surface area occupied by frogs. At the scal e of individual water bodies, frogs were three times more likely to be foun d and six times more abundant in fishless than in fish-containing waterbodi es, after habitat effects were accounted for. The strong effect of introduc ed fishes on mountain yellow-legged frogs appears to result from the unique life history of this amphibian which frequently restricts larvae to deeper water bodies, the same habitats into which fishes have most frequently bee n introduced. Because fish populations in at least some Sierra Nevada lakes can be removed with minimal effort, our results suggest that the decline o f the mountain yellow-legged frog might be relatively easy to reverse.