Good News: One Endangered Frog Species Is Bouncing Back

It’s always nice to be able to deliver good amphibian news. The latest: after decades of decline, numbers of Sierra Nevada yellow-legged frogs (Rana sierrae) are on the rise in Yosemite National Park. A report on the frogs’ return was published today, October 3, in the Proceedings of the National Academy of Sciences.

One hundred years ago, California’s Sierra Nevada mountains croaked with yellow-legged frogs. R. sierrae was one of the most abundant species around. Then came the habitat destruction. People began stocking naturally fish-less local lakes with non-native fish like trout, which feed on amphibian eggs, tadpoles, and even full-grown frogs.

After that came the fungus. Batrachochytrium dendrobatidis, or Bd, is a parasitic chytrid fungus that gets into frog skin and causes it to thicken. This might sound like a good thing, but frogs are amphibians. They take in air, water, and nutrients through their porous skin. When that skin ceases to be porous, the resulting lack of oxygen and chemical imbalances can stop their hearts from beating.

Mention Bd to any herpetologist if you want to see them shudder. The teeny-tiny fungus has wreaked havoc on amphibian species the world over and is believed to be responsible for the decline or extinction of hundreds of species to date. And it only continues to spread.

But the power of Bd’s scythe depends on the species it infects. Some frogs, including the American bullfrog and the lowland leopard frog have evolved a resistance to the pathogen, and it seems R. sierrae may have done the same. In the early waves of chytrid infestation, many frogs fell to the fungus, but others were naturally resistant. In the aftermath, the survivors mated, creating a small but relatively hardy new community in the middle of Bd territory.

Still, even with this toughness, by 1996 more than 93 percent of yellow-legged frogs had vanished from their habitat [PDF]. Things were not looking so good for R. sierrae. But how bad were they, exactly?

To find out, researchers analyzed data from more than 7000 frog population surveys conducted across Yosemite National Park between 1993 and 2012. Yosemite comprises 13 percent of the frog’s total range.

To their delight, the scientists found that the frogs were, if not completely rebounding, at least hopping back from the brink a bit. Within the 20-year study period, R. sierrae populations had increased sevenfold.

What happened? The study authors aren’t completely sure, but the frogs’ resistance to Bd certainly didn’t hurt. Another big change: people finally stopped putting frog-gobbling non-native fish in the dang waterways.

In recent years, habitat loss and chytrid fungus have made the decimation of Earth’s amphibians seem inevitable. But if R. sierrae is any indication, the authors write, "some amphibians may be more resilient than is assumed, and with appropriate management, declines of such species may be reversible."

Woo!

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