When Australia’s national threatened species list was updated in March, almost a third of the newly listed animals were frogs. With the additions of the Mount Ballow mountain frog (Philoria knowlesi), giant burrowing frog (Heleioporus australiacus) and the southern giant burrowing frog (Heleioporus australiacus flavopunctatus) to the Environment Protection and Biodiversity Conservation Act List of Threatened Fauna, Australia’s number of threatened frog species has now risen to 53. Given there are more than 250 known frog species in Australia, that means about 20 per cent of our native frog species are at risk of extinction.
It’s no secret frog populations worldwide are in decline, impacted by a cocktail of threats including habitat loss and modification, pesticides, pollution, invasive species and climate change. But a major driver of what’s becoming known as an ‘amphibian apocalypse’ is a highly infectious disease called chytridiomycosis (also known simply as ‘chytrid’).
Caused by the fungus Batrachochytrium dendrobatidis (Bd), this pathogen can kill a frog by damaging the keratin layer in its moist skin, affecting its ability to absorb water, regulate electrolytes and respire. This loss of function can lead to cardiac arrest.
Thought to be native to the Korean peninsula – where local frog populations seem to be immune to the pathogen – this chytrid fungus has spread to every amphibian-inhabited continent in the world via the global wildlife trade and is now found in more than 60 countries. So far, it’s known to have caused the extinction of at least 90 amphibian species worldwide and has been the leading cause of decline in 500 more.
In Australia, amphibian chytrid has contributed to the decline of 43 native frog species and driven at least four to extinction. Scientists and conservationists are still working to understand the full impact chytrid has had on Australia’s threatened frog species because long-term population data is rarely available. But fieldwork and population surveys, public reports of frog die-offs such as the 2021–2022 mass events on Australia’s east coast, and data accrued from citizen-science initiatives such as FrogID paint a pretty clear picture. Australia’s frogs are in trouble.
Chytrid fungus
The amphibian chytrid fungus likely arrived in Australia in the 1970s and has since spread to every state and territory except the Northern Territory. It’s now found along the Great Dividing Range on Australia’s east coast (including the Australian Capital Territory), as well as south-eastern South Australia, central and eastern Tasmania and south-western Western Australia.
A frog can contract chytrid if it’s exposed to water contaminated with Bd spores, or through direct contact with an infected frog or tadpole. (It can also cure itself of the disease by baking off the infection in a warm area; the fungus will die if it’s exposed to temperatures above 37°C for more than four hours.)
As with any disease, there are several factors that affect its severity.
“A frog that’s infected with the amphibian chytrid [might not] instantly get sick and die,” says Dr Jodi Rowley, curator of amphibian and reptile conservation biology at the Australian Museum and conjoint associate professor at the Centre for Ecosystem Science at the University of New South Wales.
“It’s got a lot to do with what species it is and their inherent immunity, but also a lot to do with what environment it’s in.”
The fungus zoospores spread through water or soil, so species that live near permanent water sources such as ponds and streams have a greater risk of contracting the disease. That also means male frogs are more susceptible to contracting the disease than females.
“Male frogs tend to hang out around water more often because they’re calling for their females,” Jodi says. “Whereas female frogs generally – not always, but generally – don’t want to go near a water body unless they want to breed, because males will just annoy them.”
The amphibian chytrid fungus thrives in cool, wet environments. It’s less virulent in warm, dry areas, and won’t grow at temperatures above 28°C. Because of this, chytrid outbreaks are often seasonal.
“I’ll survey frogs in summer, and often I won’t find any frogs – or very few frogs – that have the amphibian chytrid fungus,” Jodi says. “But if I go back in the middle of winter to the same place and the same species, often there’ll be quite high infection prevalence.”
It also means highland frog populations tend to be more severely impacted than lowland populations. The Australian lacelid (Litoria dayi), for example, was once a common fixture in the Wet Tropics of Queensland, with an altitudinal range that spanned from sea level to the slopes of Mt Bartle Frere (1622m). Now, chytrid has restricted the species’ range to rainforests below 400m, where warmer temperatures help keep the fungus at bay.
One of the hardest-hit species is the southern corroboree frog (Pseudophryne corroboree). Found only in Kosciuszko National Park, this critically endangered species dwells in subalpine woodlands and montane forests in the NSW Snowy Mountains. In 2014 it was estimated fewer than 50 individuals remained in the wild, but captive-breeding programs have salvaged this brightly coloured frog from extinction.
Thanks to Taronga Zoo, Melbourne Zoo, Healesville Sanctuary and the Amphibian Research Centre, more than 2000 frog eggs have been released at Kosciuszko National Park to boost its wild population.
There’s also a captive-breeding program for the critically endangered northern corroboree frog (Pseudophryne pengilleyi); staff at Tidbinbilla Nature Reserve, Taronga Zoo and Healesville Sanctuary have bred and released more than 2000 eggs in the Brindabella Range on the ACT–NSW border.
The fungus’s intolerance of warm weather also raises interesting questions about how the disease might respond to climate change.
“After the cyclones in North Queensland bashed down a lot of the trees and let more sunlight into the canopy, there was some preliminary evidence [that] the frogs actually did better because they could get warmer and drier because of the sun,” Jodi says.
“Bushfires could potentially have the same effect. By opening up the canopy, the frogs can then get nice and warm and get rid of the amphibian chytrid fungus. Or maybe the frogs are just hideously stressed because they’ve gone through a bushfire and their immune system goes down, so they get worse.”
Attempting to eradicate amphibian chytrid from Australian ecosystems is a fool’s errand as the fungus is so widespread, so conservationists are now focusing on managing other threats and breeding healthy insurance populations.
“Unfortunately, I don’t think we’re going to be able to get rid of it,” Jodi says. “You can do some small things to tip the favour towards the frogs and away from the disease, but it’s really about managing other threats so that the frogs can hopefully live with it and maybe even bounce back.”
Insurance populations are being bred in conservation breeding programs across Australia. In addition to the southern and northern corroboree frogs, targeted species include the Kroombit tinker frog (Taudactylus pleione), bred by Currumbin Wildlife Sanctuary in Queensland; spotted tree frog (Litoria spenceri), by Zoos Victoria; and Booroolong frog (Litoria booroolongensis), by Taronga Zoo in Sydney.
Other threats to manage include climate change – leading to more frequent extreme weather events such as bushfires, droughts, storms, floods – as well as habitat loss and modification, pollution and pesticides, predation or competition with introduced species, and other diseases such as ranaviruses.
“Trying to understand how the amphibian chytrid fungus interacts with all these other threats is really important, because increasingly with climate change, we’re not just going to be dealing with one threat – we’re going to be dealing with all these kind of synergistic effects, one on top of each other,” Jodi says.
Genetically modified frogs?
In a laboratory at The University of Melbourne, geneticist Dr Stephen Frankenberg is researching ways to genetically engineer frogs’ immune systems against chytrid. It’s an ambitious project that sounds like science fiction, but recent breakthroughs in gene editing and immunology have placed it in the realm of possibility.
Stephen and his team are working with transgenes, which are genes taken from one organism, engineered, then inserted into another.
“The idea is to have a transgene that you can just knock into the genome that will provide some sort of degree of immunity as a first line of defence [against chytrid] – just enough to tip the scales in favour of the frog species,” Stephen says.
Surprisingly, the transgene in question isn’t derived from amphibians but from camels and alpacas. Stephen says members of the Camelidae family produce nanobodies – tiny, specialised antibodies – that are particularly suited to the job.
Supported by a $3 million grant from the Colossal Foundation, the Texas-based biotechnology company best-known for its ongoing efforts to ‘de-extinct’ the woolly mammoth, Stephen and his team are now screening those nanobodies to identify which ones have an affinity with chytrid and can neutralise the pathogen.
The project is still in its early days, so Stephen can expect some unexpected hurdles ahead. One major focus is ensuring the transgene won’t have any inadvertent side effects.
“It’s important to make sure that you’re putting it into a part of the genome where it’s not going to affect the expression of any other genes and have any negative impact,” Stephen says.
“You don’t want that frog to be slightly less fit than it would have been otherwise…even if the benefit from the immunity is greater than the disadvantage from that other effect.”
Stephen and his team are currently using cane toads (Rhinella marina) as their lab model. Testing the transgene in frogs won’t happen for another two or three years, but Stephen already has a few species in mind that he’d like to target, including the green and golden bell frog and corroboree frog.
Then, it’s a matter of testing the fitness of those frogs and breeding them up for release.
In the face of the amphibian apocalypse, genetically modified frogs might be just the ticket. The concept of genetically modified wildlife is still new enough to make the public feel squeamish, but Stephen believes people will come around to the idea when they realise it can be used as a force for good.
“Because of the progress of technology, there are so many potential applications now for genetic modifications for the purpose of conservation,” he says. “It’s a bit of a no-brainer in terms of the benefits.”
“We’ve seen it happen in the past with technologies that seem, at face value, scary to many people, but then people get used to it,” he says, citing IVF and genetically modified foods as an example. “They’re far less demonised now than they were a couple of decades ago. People just get used to the idea.”
A fighting chance
There’s no silver-bullet solution to managing amphibian chytrid, but there are small things we can do to give frogs a fighting chance. Dr Anthony Waddle is a conservation biologist at Macquarie University who developed a simple yet effective solution to mitigating outbreaks in urban areas: frog saunas. These purpose-built shelters provide frogs with a warm place to raise their body temperatures and stave off the disease.
Aware that many frogs are well-adapted to urban environments and sheltering in artificial habitats such as letterboxes, toilets and masonry bricks, Anthony devised a way to repurpose these materials to create a humid, muggy environment where a sick frog can keep warm and bake off its infection.
These ‘frog saunas’ are essentially piles of masonry bricks painted black to trap in the heat and covered by a greenhouse to keep them warm and humid. The DIY thermal shelter costs about $40 to make and uses materials readily available at Bunnings: a small drop-over greenhouse, 10-hole clay masonry bricks, black paint and cable ties. “It’s nothing super high-tech, but it works really well,” Anthony says.
Frog saunas have already been rolled out at Sydney Olympic Park to support its green and golden bell frog population.
“There are these areas in Greater Sydney and within the city where there are frogs [and] frog ponds. And these [habitats] have been built, but there’s chytrid there. When the winter comes around, there’s a lot of mortality,” Anthony says.
He plans to build about 100 more frog saunas throughout Greater Sydney this coming year, but hopes to one day roll out saunas in urban areas across the species’ entire range. He has also done workshops in the ACT – where the green and golden bell frogs are being reintroduced – and freely emails out his DIY sauna construction guide to anyone looking to build one in their backyard.
Building frog saunas is just one way to support local frog populations. Anthony says that just making your garden more frog-friendly – planting natives, reducing chemical pesticides and keeping predatory pets indoors – will also make a difference.
“The urban environment doesn’t need to be just for cars and concrete,” he says.
“You can create natural landscapes and you can futureproof them using [plants] that are native, that are resilient to climate change, and build frog ponds. And this does make a massive difference. If you start planting native trees you’ll get more birds, and if you create little water ecosystems, like a little pond, frogs will come to it.”
