For decades, a deadly fungal disease has been ravaging the world’s amphibians, wiping out frogs, toads and salamanders from the mountain lakes of the United States to the rainforests of Australia. The disease, known as chytridiomycosis or chytrid, has wiped out at least 90 species of amphibians and contributed to the decline of hundreds more species, according to one estimate.
“Chytrid is this unprecedented wildlife pandemic,” says Anthony Waddle, a conservation biologist at Macquarie University in Sydney, Australia. “We see species and populations disappearing.”
But like many other formidable enemies, Chytride has an Achilles heel. The fungus that is the main culprit – known as Batrachochytrium dendrobatidis or Bd – thrives in cold weather and cannot withstand heat.
Now, a new study provides evidence that conservationists can keep the fungus at bay by giving frogs a warm place to ride out the winter. A simple pile of sun-warmed rocks, the researchers found, attracts the green and golden bell frog, a vulnerable Australian species. These thermal shelters raise the frogs’ body temperatures, helping them fight off fungal infections and potentially survive long-term.
“If we give frogs the opportunity to cure their infections with heat, they will,” says Dr. Waddle, the first author of the new paper, which was published on Wednesday in Nature. “And they are likely to be resistant in the future.”
The green and gold bell frog, once common in southeastern Australia, has disappeared from much of the landscape and is now listed as endangered in New South Wales state.
In Sydney, where some of the remaining bell frogs reside, chytrid often flares up in winter and early spring, when daytime temperatures can reach maximum temperatures in the 60s. In the first of several experiments documented in the new paper, Dr. Waddle and his colleagues found that the frogs preferred milder climates when available. When placed in habitats with a temperature gradient, the frogs moved to areas that were, on average, 84 degrees Fahrenheit warmer than is ideal for Bd.
In a second experiment, the researchers placed frogs infected with fungi in different climates. Some frogs spent weeks in the relative cold, in habitats set at 66 degrees. Those frogs harbored high levels of mold for weeks. In the months that followed, more than half of them died, said Dr. Waddle.
But frogs housed in warmer environments or given access to a wide range of temperatures recovered quickly from their infections, the researchers found.
Frogs that recovered from chytrid using this type of ‘heat treatment’ were also less susceptible to the disease in the future. When they were exposed to Bd again six weeks later – without the benefit of a warm environment – 86 percent of them survived, compared to 22 percent of the frogs that had not been previously infected.
Finally, the researchers put these findings to the test in large outdoor enclosures that more closely resembled real-world conditions. The scientists stacked a number of hole-riddled bricks in each enclosure and covered each stack with a small greenhouse. Half of the greenhouses were in the sun and the rest were in the shade.
They then released an assortment of frogs into each enclosure. Some frogs had never been exposed to Bd before, while others were actively infected with the fungus or had survived a previous infection.
The shaded and unshaded shelters each attracted frogs, which made themselves at home in the holes in the stones. But the frogs who had access to the sun-warmed rocks maintained body temperatures about six degrees higher than frogs given shady shelters, the scientists found. That increase in temperature was enough to reduce the amount of mold the frogs harbored. “Just a few degrees difference can tip the balance of the frogs,” said Dr. Waddle.
Frogs that had survived previous encounters with chytrid also had relatively mild infections, the researchers found, even if they were denied access to the sun-warmed shelters.
The results suggest that thermal refuges could act as a kind of “crude immunization,” said Dr. Waddle, which helps frogs survive their first bout with Bd and makes them less susceptible in the future. “Then you seed the population with resistant frogs that would lower the population level of chytrid.”
The strategy won’t work for every endangered amphibian — not all of them seek heat, for example — but it could be a low-cost intervention that benefits many, said Dr. Waddle, who hopes to test the approach with other amphibians. frog species.
In the meantime, he has installed the shelters in Sydney Olympic Park, which is home to a wild population of frogs. He also calls on the public and encourages locals to “build a frog sauna,” he said. “We’re trying to get people to put them in their backyards.”