The Sierra Nevada yellow-legged frog has been declining for over a century. Edging ever closer to extinction, things have not looked good for the little amphibian, yet its fortunes might just be changing. Surveys over the last two decades have found that populations in some regions are inexplicably increasing, providing hope that the species may not yet be resigned to the history books.

The native amphibians have actually been facing two fronts of attack. First, they had to contend with the non-native trout that were introduced in the late 19th century for fishing. These voracious predators started to devour the eggs and tadpoles of the frogs, sending the population into a tailspin. But things really hit the proverbial fan in the 1970s, as the deadly chytrid fungus reached the Sierras, wiping out the frogs in over 90 percent of their habitat.

What was once the most abundant amphibian in the mountain range became a rarity getting rarer. But over the last two decades, the frog has experienced an unprecedented comeback. Growing in population by around 11 percent a year, there are now thought to be seven times as many frogs than when researchers first started surveying them in Yosemite in the 1990s. Yet, curiously, the invasive fish still swim in the rivers and the fungus has not gone away, making the amphibians' comeback unexpected.

It could be possible the frogs have evolved a resistance to the fungus that has been wiping out other species around the globe, according to the researchers in their paper published in the Proceedings of the National Academy of Sciences. If so, this raises the interesting possibility that other populations of amphibians, if given sufficient protection and help, may be able to develop immunity to the fungus by themselves. So far, though, only the populations in Yosemite have been found to be growing.

Further tests found that those frogs that had been up against the disease for the longest period of time were also those showing the greatest resistance, backing up the notion that the amphibians can indeed evolve to fight the fungus. Many have suggested that this could happen, but others are cautious about such results.

The Sierra Nevada yellow-legged frog may well be able to develop resistance, but this doesn’t mean that all species of amphibians will have the same ability, especially considering the speed at which the fungus can decimate populations.