Reversing introduced species effects: Experimental removal of introduced fish leads to rapid recovery of a declining frog.

Amphibian population declines and extinctions are occurring even in the world's least impacted areas. The introduction and spread of nonnative predators is one of many proposed causes of amphibian declines. Correlational studies have shown a negative relationship between introduced fishes and declining amphibians, but little direct experimental evidence is available. This study experimentally manipulated the presence and absence of widely introduced salmonids rainbow trout (Oncorhynchus mykiss) and brook trout (Salvelinus fontinalis) to test the hypothesis that their introduction has contributed to the decline of the mountain yellow-legged frog (Rana muscosa). From 1996 to 2003, the introduced trout were removed from 5 lakes in a remote protected area of the Sierra Nevada, and 16 nearby lakes were used as controls, 8 with introduced trout and 8 without. To determine the vulnerable life stage, rainbow trout were placed in cages in three lakes containing amphibians. Removal of introduced trout resulted in rapid recovery of frog populations, and, in the caging experiment, tadpoles were found to be vulnerable to trout predation. Together, these experiments illustrate that introduced trout are effective predators on R. muscosa tadpoles and suggest (i) that the introduction of trout is the most likely mechanism responsible for the decline of this mountain frog and (ii) that these negative effects can be reversed.