In the 1950s, California wildlife authorities used to fly over remote lakes and creeks in Yosemite National Park and deliver precious cargo: hatchery-raised trout.
The policy was great for fishing enthusiasts. But for the yellow-legged frogs that shared those waters, the arrival of hungry trout was a disaster.
These days, wildlife officials are using helicopters to transport the fist-sized frogs to some of the very same waters, which are now being rid of non-native trout.
It's part of a strategy that has helped the frog population grow at a rate of 11% a year since 1991, according to a report this month in the Proceedings of the National Academy of Sciences.
"If we give frogs some room to thrive, they will," said Vance Vredenburg, a biologist at San Francisco State University who worked on the study.
Non-native trout are only part of the problem facing Rana sierrae, a reddish-brown frog with yellow undersides.
The amphibians also have been contending with the deadly chytrid fungus. It causes their skin to thicken, interfering with their ability to absorb water and regulate respiration.
The fungus first swept through the Sierra Nevada range in the 1970s, triggering rapid, massive die-offs. It is now ubiquitous in the region. In 2014, yellow-legged frogs were added to the federal endangered species list.
But now the frogs are rebounding, and scientists believe the animals have developed some type of natural defense against the fungus, Batrachochytrium dendrobatidis.
Their optimism is based in part on laboratory experiments conducted at San Francisco State University.
Researchers collected wild frogs from three populations that long had been exposed to the fungus, along with frogs from three other populations that hadn't encountered the fungus. The scientists paired frogs from exposed groups with those from unexposed groups and infected each pair with a strain of the fungus collected in the wild.
The frogs that never had encountered the fungus wound up with fungal loads that were five times higher, on average, than those from the exposed populations. This trend held up for all four strains of the fungus that were tested, according to the study.
It's not yet clear why some of the frogs were less vulnerable to the fungus than others. Perhaps they had evolved some natural defense or immune response against it, which spread through the group via natural selection, the study authors wrote.
More research is needed to solve this mystery, Vredenburg said.
Despite these unanswered questions, researchers say they have clear evidence of a frog recovery in Yosemite based on 7,678 frog population surveys conducted between 1993 and 2012 in 2,154 separate bodies of water.
"Our research in the Sierra Nevada shows that some amphibian species are remarkably resilient," said study co-author Roland A. Knapp, a biologist at the University of California's Sierra Nevada Aquatic Research Laboratory near Mammoth Lakes, Calif. "It also suggests that, in some cases, amphibian declines may be at least partially reversible on a global scale with appropriate management."
Full recovery of the yellow-legged frogs will require continued efforts to remove non-native trout from the region, Knapp said. Wildlife management officials use gill nets, machines that stun fish with electric fields and other tools to clear out the trout.
Biologists also will have to continue transporting frogs to areas where they had been wiped out. These days, they are bringing the frogs back to Dog Lake in Yosemite's Tuolumne Meadows, which for decades was stocked with trout raised in hatcheries. (After that practice was halted, the trout disappeared because they had no place to spawn.)
Members of the 483 yellow-legged frog populations known to exist in Yosemite rapidly recolonize their ancestral habitats after they have reverted to their original, trout-free state.
Conservationists hope the pattern will repeat in other parts of the state. In August, the National Park Service approved a 30-year plan to remove nonnative fish from yellow-legged frog habitats in Sequoia and Kings Canyon National Park, about 130 miles south of Yosemite.
That plan calls for gill nets, electrofishing devices and rotenone, a natural poison derived from peas that kills animals with gills, breaks down rapidly and poses no threat to water supplies. Fish removal is already underway.
"It's been a good year for Sierra Nevada yellow-legged frogs," Vredenburg said.
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