Green and golden bell frogs were so common in Sydney, Australia that residents hid them in their mailboxes and toilets. “They would be everywhere,” says Anthony Waddle, a biologist at Macquarie University in Sydney. But in the 1960s, the population began to decline. More recently, numbers have plummeted. Surveys over the past 15 years indicate that the species has disappeared from 90% of its range. They are now listed as endangered in New South Wales, with numbers continuing to decline. Globally, the green and golden bell frog is considered “Near threatened' on the Red List of the International Union for the Conservation of Nature.
There are many factors contributing to the decline of bell frogs: Invasive fish introductions, habitat loss, cats and water pollution are some examples. One of the biggest persistent and difficult to control threats is chytridiomycosis (chytrid for short), a fungal disease caused by Batrachochytrium dendrobatidis (Decided). Chytrid was first discovered by scientists in Queensland, Australia in 1993, although the disease had been present in the country since 1993. at least 1978. Since its initial discovery, the disease has spread to every continent except Antarctica, infecting more than 500 amphibian species and causing at least 90 species to become completely extinct. It is the deadliest known pathogen for wildlife.
Amid the devastation, there are few hopeful findings. But a deceptively simple solution could help solve the shocking declines in some species. Low-tech frog saunas allow infected green and golden bell frogs to recover from chytrid and develop immunity to subsequent Decided exposures, according to A study led by Waddle and published June 26 in the magazine Nature.
The research “was all about coming up with a practical way to control this terribly nasty disease in the wild,” Waddle says Popular science. Bd is a cold-loving fungus that stops growing at about 82 degrees Fahrenheit and dies at 86 F, he explains. Amphibians that can withstand heat can clear chytrid infections if they are kept warm long enough. In Australia and other places with distinct seasons, winter is generally the worst time for chytrid-related losses.
Biologists have observed for more than a decade that some frog populations with access to warm shelters within their habitat are more resistant to chytrid. Study from 2011 noted that boulders warmed by the sun in a waterfall likely supported critically endangered species. The new study finds encouraging evidence that artificial “hotspot shelters” built of brick and plastic sheeting can do the same.
“It's a very simplistic but elegant experiment,” says Ana Longo, a disease ecologist and evolutionary biologist who studies amphibians at the University of Florida and who was not involved in the new research. “The beauty of this experiment is that it could be [widely deployed]” she says. “Many species could potentially benefit from this. It's very cheap to do and easy. This could easily be implemented by wildlife managers and conservation organizations,” Longo adds.
The first step in building an effective amphibian sauna was to determine the preferred temperature of the green and golden bell frogs. Using a piece of metal gutter with a hot and a cold side, Waddle and his colleagues determined that the species seems to gravitate toward a pleasant 84 F on average, which is perfect for fending off chytrid, if given the chance.
Next, the researchers had to confirm that basking at the desired temperature would actually help infected frogs. They found that, with access to a habitat with a thermal gradient and the ability to choose where they wanted to be, frogs infected with chytrid effectively recovered within days. They measured the intensity of infection by swabbing the frogs’ skin and testing it for fungal DNA. Six days after being moved to the thermal gradient habitat, the 20 tested frogs had an average infection intensity of zero.
The scientists further tested whether recovery from one chytrid infection helped the frogs survive subsequent exposures. In an experiment with 55 frogs, they found that a previously recovered subgroup was about 23 times more likely to survive a second exposure than a naive control group.
Finally, Waddle and co conducted their experiment outdoors, in twelve different semi-natural mesocosms enclosed by nets and with gravel ponds, artificial vegetation and various shelters. In each mesocosm they placed a pile of rocks enclosed in a small plastic greenhouse. Each mesocosm housed 19 or 20 frogs exposed to chytrid. They compared mesocosms with shaded greenhouses to those with greenhouses exposed to full sun, and found that frogs in mesocosms with unshaded, warmer shelters had significantly lower infection intensity over the duration of the 15-week experiment.
The temperature difference between the two greenhouse treatments was less than 10 degrees F. The shaded greenhouses were still warm. But the difference in infections between the two groups was at several points 100 times worse for frogs with access to the marginally cooler structures, Waddle says. In both setups, frogs with a previous chytrid infection and recovered fared better against a new infection. “As scientists we always think, 'It's not going to work,'” says Waddle, so he was surprised and excited to see how clear the results were.
Other studies have suggested interventions such as cutting down trees for sunlight ponds and even actively heating water to mitigate the effects of chytrid. “However, this is the first study to propose an easy-to-implement and minimally invasive method for providing artificial thermal refuges to amphibians to help them combat chytrid infections,” says Erin Sauerone of the study's authors and a wildlife disease ecologist at the University of Arkansas. Compared to removing vegetation or installing heaters in ponds, basic brick and tarp greenhouses are cheaper and are unlikely to have serious negative impacts on other organisms that share the ecosystem with frogs.
Now, conservationists trying to save the green and golden bell frogs have an emerging, evidence-based tool at their disposal. Waddle hopes to encourage Sydney residents to build the structures (which cost about $50 to make) in their backyards in a citizen science effort. And in a follow-up experiment, he and his colleagues have already installed 50 of the greenhouses around the Brickpit in Sydney Olympic Park—one of the few remaining sites with a population of the amphibian. Over the next two years, they will monitor the site to see if the dwindling frog populations rebound, if other species take advantage of the structures, and/or if the shelters cause problems. For example, they don’t want to “create a snake buffet,” Waddle says. While waiting for those results, he hopes to test the concept in other amphibians with similar thermal biology.
Still, there are limits to what frog saunas can do. In significantly cooler places, the greenhouses may not get warm enough to make a difference. And so far, the concept has only been tested in one species. Amphibians vary widely in their temperature preferences and tolerances. Alpine frogs and otherwise cool-adapted frogs, such as the Sierra Nevada yellow-legged frog in California, which has been decimated by chytrid, are unlikely to benefit from hotboxes. Completely aquatic salamanders would also be impossible to treat this way, Longo notes. And some of Sauer's previous research even indicates that higher temperatures apply to certain species may aggravate chytrid infections“It would be really irresponsible to extrapolate this to all other species affected by cancer,” says Waddle.
But for green and golden bell frogs, and probably others, rocks and a plastic cover can be a blessing in a sea of threats. “It takes these kinds of bold ideas” to make a difference, Longo says. Many scientists might see an infected and declining population and just give up, she says. This study offers an alternative: an active response, one that “can ensure the persistence of a species.”
