The amphibian disease chytridiomycosis, or chytrid, is one of the most destructive wildlife diseases known to recorded history. It’s caused by the fungus Batrachochytrium dendrobatidis (Bd), which first emerged in the 1970s but was discovered in Australia in the 1990s.
Chytrid (Pro: ky-trid) is an infection of amphibians’ porous skin – through which they usually absorb oxygen and electrolytes – which prevents them maintaining the correct electrolyte balance in their bloodstream.
Scientists now know that chytrid has caused the global decline of at least 501 species over the past half a century – 90 species are confirmed or presumed extinct in the wild, and 124 have experienced a more than 90% reduction in their abundance.
In Australia in particular, it’s brought about the decline of at least 43 of our 200 or so frog species and been implicated in the extinction of four. One of those was the incredible but bizarre gastric-brooding frog (Rheobatrachus vitellinus), which turned its stomach into a womb.
So, it’s no wonder the phenomenon is known as the amphibian or frog apocalypse. And it has dire consequences to the health of the ecosystems that frogs disappear from.
In fact, according to Dr Deborah Bower, a herpetologist and senior lecturer in Zoology and Ecology at the University of New England, frogs are like getting two animals for the price of one in terms of the roles they play in ecosystems.
“Frogs are really important because tadpoles are so ecologically different. They are wholly aquatic, they eat algae, they’re in the water column,” Bower says. “Then, they metamorphose and become frogs – they’re big, they’re semi-terrestrial, and they might be arboreal (living in trees).”
So, what’s being done to try to combat this deadly fungal disease?
Researchers are coming at the problem from several different angles
Bower and her colleague Dr Laura Brannelly – a Research Fellow in the Faculty of Veterinary and Agricultural Sciences at the University of Melbourne – have just submitted a research proposal to look at reservoirs of the fungus in other non-amphibian animals. Scientists have found that crayfish can carry Bd, it can be transported on duck feet, and can infect and proliferate on zebra fish tissue, so studying this is important for developing our understanding of how chytrid is spread and the mechanisms that allow it to persist in the environment
Because chytrid is ubiquitous in the environment and can’t be eradicated, scientists’ attentions are turning their attention to ways to mitigate the impacts on wild frog populations.
One method, which was covered in the 95th edition of the Cosmos print Magazine, is to study the genetics of frogs that are already somewhat resistant to chytrid infections. By identifying the genetic variations that grant resistance, scientists could potentially use selective breeding or gene editing to arm other frogs with those protective genes.
According to Dr Simon Clulow, a senior research fellow at the Centre for Conservation Ecology and Genomics at the University of Canberra, the answer might lie in two other approaches that might allow captive bred frogs to be released back into the wild where they can breed and propagate.
“If you actually expose frogs to a sub lethal amount of chytrid, so infect them but treat them so they clear that infection, they become more resistant down the track.”
And you don’t need to use chemicals to treat the infection. Chytrid’s ideal temperature range for growth is between 17-23°C (it is incredibly sensitive to temperature), so exposing infected frogs to anything 30°C proves lethal to the fungus.
Turning up the heat and salt
Clulow says another approach could be harnessing something called “environmental tolerance mismatch” – deliberately creating an environment that is stressful for the fungus (so it can’t perform at its maximum) but which is neutral or beneficial for the frogs.
This environmental stressor could be something as simple as temperature outlined above or slight changes to water salinity. Manipulating environmental salinity was the focus of Clulow’s PhD work, and the research won the British Ecological Society’s Southwood Prize.
“If you increase water salinity slightly, it actually impacts the fungus to the point where it blocks infection. So, it reduces the capacity for fungus to infect new individuals, and maybe also provides curative effects,” says Clulow.
In the future this might involve building ponds and wetlands purposely to try and re-establish amphibian populations. For instance, creating small satellite ponds around a large wetland that are slightly saltier or a bit warmer, or putting out small piles of black bricks that heat up more than the surrounding environment and in which frogs can take refuge in the wintertime.
There’s optimism for the future
So, all hope is not yet lost! And according to Bower, there are even still small pockets left in the wild where B. dendrobatidis isn’t found.
“New Guinea is an area where we’ve done some sampling and we haven’t been able to detect it. So we think it might be the largest climatically suitable landmass where chytrid still hasn’t emerged.
“And New Guinea has got 6% of the world’s frogs, so it’s a really significant biodiversity hotspot.”
There are also still some areas in Australia, places like Broughton Island off the coast of New South Wales and pockets of the wet-dry tropics around North Queensland, where populations of frogs which have otherwise declined are still holding on.
“So, you can yell “hey, frog!” into the forest, and they’ll call back at you and tell you where they are,” Bower says. So why not give it a go next time you’re out hunting for frogs?
You can contribute to scientific research for Australian frogs by participating in the citizen science project FrogID, so scientists can better understand how many species of frogs live in Australia and where they’re distributed.
Think of it like “Frog Shazam” – since every frog species makes a different sound, by recording local frog calls on the app you can both learn about which species live nearby and contribute to the pool of national data.
This is a really great way of observing frogs without disturbing them, which is important since handling them or their habitat could help transfer chytrid fungus between frogs.
And, according to Bower, some frogs will even call back to you if you talk to them first.
Instructions for downloading the app can be found here.
Imma Perfetto is a science journalist at Cosmos. She has a Bachelor of Science with Honours in Science Communication from the University of Adelaide.
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