Australian researchers are testing an unusual way to save frog species from extinction: spa and sauna treatment.
Frog species in Australia and globally are under intensifying threat of extinction. Major factors include habitat drying due to human-induced climate change, pollution and the introduction of invasive species.
More than 40% of amphibian species are under threat of extinction according to a report which analysed the International Union for Conservation of Nature Red List of Threatened Species.
A fungal killer
Another reason why frogs and other amphibians are being pushed to extinction is a virulent disease caused by the fungus Batrachochytrium dendrobatidis, also known as chytrid fungus. More than 500 amphibian species globally are under threat from the deadly chytridiomycosis disease the fungus causes.
Chytrid fungus is believed to have originated on the Korean Peninsula. The fungus has been present in Australia since the 1970s and has spread worldwide, largely due to the pet trade.
Jarrod Sopniewski. Supplied.
“Some species seem to be fine with it, but some species are heavily affected,” Dr Jarrod Sopniewski explains in an interview with Cosmos. “Around the globe in the last 50 years or so, at least 90 species have gone extinct mainly due to this fungus.”
At least 7 frog species in Australia have been lost to chytrid fungus.
Among these are the southern and northern gastric-brooding frogs (Rheobatrachus silus and vitellinus respectively) native to the northern state of Queensland. These frogs had a unique way of raising young. Instead of laying eggs in water where their tadpoles grow, the gastric-brooding frogs eat their eggs and grow the tadpoles inside their bodies.
“Eventually they would kind of vomit out little froglets,” Sopniewski laughs. “We don’t know any other frogs that do that. Unfortunately, that frog has disappeared now.”
Sopniewski is a conservation biologist at the University of Canberra. He is part of a team which is trying to prevent other frog species – like the southern and northern corroboree frogs (Pseudophryne corroboree and pengilleyi respectively) and green and golden bell frog (Litoria aurea) – from succumbing to the same fate as the gastric-brooding frogs.
Adult bell frog in the wild. Credit: Ewen Lawler.
“Green and golden bell frogs used to be one of the most common frogs in eastern New South Wales, the Australian Capital Territory (ACT) and northern Victoria,” Sopniewski says. “But since about the ‘80s, when this fungus spread, their distribution has dropped by about 90%.”
Green and golden bell frogs are also under threat from habitat destruction due to human land use, and the introduction of the invasive eastern mosquito fish which eats the frog’s eggs and injures tadpoles.
“They’ve got lots of things stacked against them, but chytrid fungus is still the main thing that is preventing this frog from reestablishing in places it was lost,” Sopniewski says.
“The research I’m involved in is working out, knowing that chytrid is here to stay at least for the medium term, how can we help frogs survive with this fungus.”
Jarrod Sopniewski in the field. Credit: Simon Clulow.
Sopniewski says that analysing the environments where green and golden bell frogs persist might indicate certain conditions that restrict the impact of chytrid fungus on the amphibians.
A spa and sauna treatment can save lives
Chytrid fungus prefers cool, wet environments.
“In places like Canberra, where it is cold, if we can add hot spots to the environment, hypothetically, frogs can go there when they’re infected with chytrid fungus [to] kill the fungus off them,” Sopniewski says.
He points to a proof-of-concept paper published in Nature last year which showed in controlled outdoor trials that hotspot shelters could offer protection from chytrid fungus to green and golden bell frogs.
“They came up with a design for these little frog saunas,” Sopniewski says. “It’s a pile of black bricks and you build a little greenhouse around them. You could put those out in the wild and, without electricity, the sun can make them a little sauna.
“They get above 28°C which the fungus doesn’t like. Hypothetically, if a frog is in that environment, the frog will be fine but the fungus will die. They showed that if you put infected frogs in these frog saunas, that does happen.”
Sopniewski adds that frogs which have cleared chytrid fungus have a short-term vaccine-like effect and are less likely to be re-infected.
Frogs that are beating the fungus also tend to live around the coast, suggesting that salt and mineral-rich estuary waters may also restrict the fungus.
“The fungus can’t handle much salt at all. Once you get to about 3 parts per 1,000 salt, less than a 10th that of sea water, the fungus dies. Green and golden bell frogs can handle 8 or more parts per 1,000,” Sopniewski says.
“We can’t just go dump kilos of salt in natural wetlands, because we don’t want to affect other things. But something that we’re trying in Canberra is ‘satellite ponds’ – little bodies of water around the edges of major bodies of water. They’re controlled, closed environments that we can add salt to. The idea is that frogs in a landscape can go have a salt bath when they’re sick.”
Control reintroduction with satellite pond and hostpot structure. Credit: Angela Kirkegaard.
Sopniewski is part of a team which has been awarded an ARC Discovery Grant to field test the frog spas in 15 wetlands around Canberra. Tests begin in November 2025 and will run for at least 2 summers.
He is hopeful that the combined effect of salty spas and hot saunas will help the frogs survive chytrid fungus infection.
“We’ll be tracking the frogs to see if they use these environmental manipulations that we’re putting in, whether they’re achieving what we want them to – so whether the hot spot structures are getting to sufficient temperatures for sufficient periods of time – we’ll be regularly swabbing frogs to see if they’re getting chytrid, and then if they’re able to clear it by using the salt bath and the heated structures.”
A 12-month pilot in a handful of sites assessed how the structures were used by native frogs.
“We were pretty confident that a low level of salt is not going to be harmful for the frogs,” Sopniewski says. “We actually found that every single pond we put out had a pair of frogs reproduce in it. That was a welcome finding because we can also exclude mosquito fish from these little ponds.”
Sopniewski also notes new research into a population of green and golden bell frogs which are hanging on in Captains Flat near Canberra. The waters are polluted with heavy metals due to a closed-down mine.
“We showed in a lab setting that heavy metal pollution could help support frogs. The takeaway from that is not to go and dump heavy metals in all our waterways. But we do, unfortunately, have a lot of polluted waterways from mining industries and there’s a chance that we could take advantage of these when we’re looking at places to reintroduce frogs.”
Researcher Milad Esmaeilbeigi swabbing a frog for chytrid in heavy metal lab tests. Credit: Milad Esmaeilbeigi.
Don’t frog-et about us
“Frogs may be overlooked when we consider endangered species,” Sopniewski says. “Across the world, among land animals, frogs are the most endangered group. Nearly half are threatened with extinction.
“They have an important role in ecosystems. They’re good at eating things and being eaten by other things. Frogs are good at keeping pest numbers down. They love eating insects.
“Tadpoles are important nutrient cyclers in water bodies. If you have 2 bodies of water – one with tadpoles in it and one without tadpoles in it – the water quality in the body with tadpoles will be much higher, the plant growth will be much higher, nutrient cycling is much faster.”
Sopniewski and others from the research team will be joined by a cast of frog friends in a DIY frog sauna workshop on Thursday, 14 August in Canberra as part of National Science Week (9–17 August. The event is funded through a seed fund from Inspire Australia to FrogWatch ACT and Region which is financed by the ACT Government.