For decades, a deadly fungal disease has ravaged amphibians around the world, wiping out frogs, toads and salamanders from the mountain lakes of the United States to the rainforests of Australia. Known as chytridiomycosis, the disease is estimated to have wiped out at least 90 amphibian species and contributed to the decline of hundreds more.
“Chytrid is an unprecedented wildlife pandemic,” says Anthony Waddle, a conservation biologist at Macquarie University in Sydney, Australia. “We're seeing species and populations disappear.”
But like many formidable foes, chytrid has a weakness: The main culprit, a fungus known as Batrachochytrium dendrobatidis (Bd), thrives in cool climates and cannot tolerate heat.
Now, a new study provides evidence that conservationists may be able to keep the fungus at bay by giving frogs a warm place to spend the winter. Researchers found that simply stacking sun-warmed bricks is enough to attract Australia's endangered mountain cricket frogs. These thermal shelters increase the frogs' body temperature, helping them fight off fungal infections and perhaps allowing them to survive for longer periods.
“If you give frogs the ability to cure an infection with heat, they will cure the infection,” said Dr. Waddle, lead author of the new paper published Wednesday in the journal Nature, “and they will likely become resistant in the future.”
Once common in southeastern Australia, the green-and-gold bell frog has now disappeared from many areas and is listed as endangered in New South Wales.
Sydney is home to some remaining fire bell frogs, but chytrid fungus occurs frequently during the winter and early spring, when daytime temperatures can reach as high as 60 degrees Fahrenheit. In the first of several experiments documented in the new paper, Dr. Waddle and his colleagues found that the fire bell frogs prefer warmer climates when they are available. When placed in habitats with a temperature gradient, the frogs gravitated toward areas that averaged 84 degrees Fahrenheit, warmer than the ideal temperature for chytrid fungus.
In a second experiment, the researchers placed infected frogs in different climates. Some frogs spent several weeks in a cooler habitat set at 66 degrees. These frogs harbored high levels of the bacteria for several weeks. Over the next few months, more than half of the frogs died, Dr. Waddle said.
But frogs kept in warmer environments, or exposed to a wide range of temperatures, recovered from infection more quickly, the researchers found.
Frogs that recovered from chytrid with the help of these “heat treatments” were less susceptible to the disease in the future: When they were re-exposed to the fungus six weeks later, 86 percent of the frogs that did not benefit from the hot habitat survived, compared with 22 percent of the frogs that had never been infected.
Finally, the researchers tested these findings in large enclosures outdoors, which more closely resembled real-world conditions: The scientists stacked bricks full of holes in each enclosure and placed a small greenhouse on top of them. The greenhouse was exposed to sunlight in half of the enclosure and shaded in the other half.
The researchers then released different species of frogs into each enclosure: some of the frogs had never been exposed to Bd, but others were infected with the fungus or had been previously infected.
Both the shaded and unshaded shelters attracted frogs, who took up residence in the brick holes. But the frogs that approached the sun-warmed bricks had body temperatures about six degrees higher than those in the shaded shelters, the scientists found. That increase in temperature was enough to reduce the amount of fungus the frogs harbored. “Just a few degrees can make a world of difference for the frogs,” Dr. Waddle said.
The researchers found that frogs that had previously encountered chytrid and survived had relatively mild infections, even when they were denied access to sun-warmed shelter.
Dr. Waddle said the results suggest that the thermal refugia act as a kind of “crude immunity,” helping frogs survive initial Bd infections and making them less susceptible to future infections, “which then seed the population with resistant frogs, reducing the chytrid population.”
While the strategy won't work for all threatened amphibians (not all amphibians chase heat, for example), it could benefit many people as a low-cost intervention, Dr Waddle said, and he hopes to test the technique with other frog species.
Meanwhile, he has set up a shelter in Sydney Olympic Park, where wild frogs live, and he is mobilising the public to encourage locals to “build a frog sauna”. “We're trying to get people to build frog saunas in their backyards.”“