Mating season (June 2004) in Sixty Lake Basin. The large lake in the foreground is a frog population source and the three lakes in the background are trout removal lakes now colonized by large frog populations. Credit: Rob Bingham, University of California, Berkeley

Ecology of infectious diseases data gathered over seven years has played a key role in convincing the National Park Service and the California Department of Fish and Game to remove trout from high-altitude lakes in California's Sierra Nevada.  The trout are causing the disappearance of the mountain yellow-legged frog.

Funded through the EID program, biologist Vance Vredenburg of the University of California at Berkeley showed that introduced-trout have devastated native frog populations over the past 50 years in formerly fish-free, high-Sierra lakes, but that removing the fish can allow the frogs to flourish once more.

"The mountain yellow-legged frog used to be the most common inhabitant of the high Sierra, but frog populations have declined dramatically enough to put it on the endangered species list," said Vredenburg.

"The worldwide decline in frog and salamander populations is a harbinger of more serious threats posed by the current rapid environmental changes our planet is undergoing," said Sam Scheiner, EID program director at NSF. "Possible culprits include the spread of disease, increased UV radiation and predation by introduced species.  This study helps to tease apart those complex causes and shows that, in these frogs, the decline is due to increased predation. For these populations, removing the trout will save the frogs. Such studies provide hope that we can reverse the large environmental changes we’re causing."

As part of the research, Vredenburg removed trout from five lakes and documented a rebound in the frog population in all of them. Three years after trout removal, the frog populations in all five lakes were indistinguishable from populations at lakes that had never seen a trout.

"The response was incredibly dramatic and rapid," Vredenburg said. "Every time you plant hundreds of thousands of fish, you're hammering a nail in the frogs' coffins."

Vredenburg has also teamed up with other researchers to determine the effect of a chytrid fungus, Batrachochytrium dendrobatidis, on the mountain yellow-legged frog. The fungus, which is threatening frog populations around the world, attacks tadpoles as well as adults, and can kill adult frogs. It was discovered in the Sierra Nevada in 2001.

Loss of wetland habitat has also reduced populations of frogs and toads and endangered several species of amphibians with restricted ranges, say scientists like James Collins, NSF Assistant Director for Biological Sciences (on leave from Arizona State University).  Collins and others believe that alarming new events have added to these trends. For example, frog and toad populations have declined dramatically in the past several years, many in high-altitude places in the United States, Puerto Rico, Costa Rica, Panama, Colombia and Australia. Studies suggest that these population declines may be caused by infections, perhaps promoted by environmental stressors.

For more information on studies of the chytrid fungus, please see Outbreak: Rapid Appearance of Fungus Devastates Frogs, Salamanders in Panama.