Making a comeback Joel Sartore, National Geographic Photo Ark/Getty

For decades a deadly fungus has been slaughtering amphibians around the world, driving many to the brink of extinction or even beyond.

But now one frog’s recovery shows that, with a little luck and habitat preservation, some amphibians may be able to evolve resistance to the fungus.

The Sierra Nevada yellow-legged frog (Rana sierrae) lives in the mountains of California, but its population has been in decline for more than 100 years.


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It started when non-native predatory trout were introduced into the lakes and rivers it inhabits in the late 19th century for recreational fishing, devouring its tadpoles, and really hit high gear when the chytrid fungus arrived in the 1970s.

“By the early 2000s, it had disappeared from 93 per cent of its historical localities,” says Roland Knapp at the University of California’s Sierra Nevada Aquatic Research Laboratory. In 2014 the frog was listed as endangered.

But there is hope. Knapp and his colleagues looked at more than 7000 population surveys of the frog conducted over the past 20 years in Yosemite National Park, and found that frog numbers have been recovering over that time, by an average of 11 per cent per year.

Bouncing back

“The question is: why are they recovering now?” says Knapp. “The fish and fungus are still there.”

Part of the answer is that there are fewer non-native fish, he says, since the National Park Service put an end to fish stocking in 1991.

But more importantly, the frogs seem to have developed some resistance to the fungus. In lab tests Knapp found that frogs from populations that had been dealing with the fungus for years were more likely to survive infection than those from populations that had so far escaped it.

“This shows there is hope that at least some species can recover, given the time and the habitat in which to do it,” he says.

Kerry Kriger, founder of the conservation group Save the Frogs, agrees that it is good news for the yellow-legged frog. It builds on similar findings from research on stony creek frogs he did in Australia, he says, which showed evidence that the frogs can survive and even clear their chytrid fungus infections.

But he cautions that not every species may have the opportunity to develop resistance, if their surviving populations are too small or they are wiped out entirely.

“None of this suggests that chytrid fungus is not a threat,” he says. “Chytrid can wipe out entire populations and governments have significant work ahead if chytrid-induced extinctions are to stop.”

Journal reference: PNAS, DOI: 10.1073/pnas.1600983113