The Washington PostDemocracy Dies in Darkness

Thousands of frogs are dying in Australia. Scientists aren’t sure why.

In a handout photo, a green tree frog, a common sight across the northeast of Australia, is seen. (Jodi Rowley)
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MELBOURNE, Australia — Jodi Rowley squelched through a pond in rain boots, her headlamp piercing the blackness of the winter night. Following the sound of croaks, she and her three colleagues scanned the water for signs of life.

With swabs on hand, the team gathered samples from the 22 little frogs they found this month on that expedition near Albury, in southern New South Wales, in the hope of deciphering a phenomenon that is perplexing animal lovers and scientists.

“It is a really complicated murder mystery,” Rowley said.

Across Australia, dead frogs are turning up in the thousands — and no one knows why.

A team of scientists led by Jodi Rowley undertook fieldwork in New South Wales on July 6 to gather data about a mysterious affliction killing Australian frogs. (Video: Jodi Rowley)

It started last winter, when Rowley, a herpetologist, noticed increased social media reports of frog carcasses in backyards and local creeks. She was concerned, but knew that amphibians’ immune systems slow down in the cold — and it was a cold year.

But a call-out for citizen data brought in a flood of dead-frog sightings far beyond normal winter losses. Frogs, which usually bunker down during cooler weather — the middle of the year in Australia — were apparently wandering out into the open, sitting down, and dying en masse.

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“Property holders were saying that they’ve never seen this, but there’s dozens of dead frogs all over their house,” said Rowley, the herpetology department lead at the Australian Museum and University of New South Wales. More than 1,600 reports came in, covering more than 40 species around the country, many detailing multiple deaths.

After a summer reprieve, the phenomenon seems to be back this winter.

“I was bracing myself for the possibility it would happen again,” Rowley said. “And unfortunately, it does look like it.”

Australia is home to more than 240 native species of frog. They include such delights as the pobblebonk, named onomatopoeically, and the tiny assa wollumbin, found on one mountain, with males that carry tadpoles in kangaroo-style pouches. They come in black-and-yellow stripes, spooky ghost-white, and for the most ubiquitous species — the green tree frog — the color of the rainforest. They are everywhere, from the desert to the snowy Australian Alps, often heard but not seen.

“They’re cryptic, and they hide, but they’re out there in really huge numbers,” said Karrie Rose, manager of the Taronga Conservation Society’s Australian Registry of Wildlife Health. “If their populations change, there will be ripples throughout the food web.”

Frogs are indicators of the health of an ecosystem as a whole. They are eaten by birds, reptiles, even dingoes. And they keep the environment in balance by eating algae and insects. One study linked a decline in frogs to a rise in malaria in two countries, as fewer frogs snacked on disease-carrying mosquitoes.

In Australia, at least four frog species have gone extinct since European colonization. They include the only two species worldwide known to have the bizarre trait of laying eggs, eating them, and then vomiting up tadpoles through their mouths. Almost 1 in 5 surviving species are threatened, and Rowley said she fears the mass mortality events could drive additional species to extinction.

Rose, a veterinary pathologist, is working with Rowley to study the frogs’ demise.

The lead suspect is a killer that attacks by smothering its victim’s skin.

Chytrid fungus — batrachochytrium dendrobatidis — has ripped through amphibian populations since the latter part of last century. Scientists believe it originated on the Korean Peninsula and spread worldwide through trade. The fungus, which feeds on the keratin in frogs’ outer layer, threatens the survival of more than 500 types of amphibian, a 2019 study found. It is thought to be responsible for 90 extinctions since the 1970s, making it a more destructive invasive species than rats or cats.

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Rowley and Rose say the fungus is probably playing a role in the inexplicable die-off. But they doubt it’s the whole story. The fungus has been present in Australia for decades, Rose said. And some autopsies revealed internal lesions on the frogs’ nervous systems and hearts, which is not a usual symptom of fungal infection. Something in the environment must have changed.

“There's been good evidence of widespread chytrid fungus infection since about the mid-1980s. So why are we seeing such a high mortality now?” she said.

Of the hundreds of frozen frog carcasses her lab has analyzed, about 75 percent were infected with chytrid fungus. But that couldn’t explain the fate of the other 25 percent.

The scientists are exploring several theories. One could be eastern Australia’s rainy weather over the past two years, which is conducive to both fungi and frogs. A secondary disease, parasite, environmental toxin, or stressors from successive drought, fires and floods could also play a role.

Last winter, with Australian cities under coronavirus lockdown, Rowley and her herpetologist colleagues in Sydney were limited to studying frogs that happened to be in their neighborhoods, samples from sick frogs that Australians had taken to veterinary clinics, or frog carcasses that people had placed in freezers to be collected by experts. This year, Rowley is out in the field, racing to work out what is going on before frog populations are permanently affected.

She hopes a combination of institutional and citizen science will gather the data that will unlock the puzzle. Australians are being asked to record frog sounds and take pictures in their neighborhoods, using the Australian Museum’s FrogID app. “We do really need everyone’s help, because it’s a huge problem and it spans the entire continent,” Rowley added.

Rowley, 42, has been specializing in the study of frogs since she was 18. She recalls the moment she “personally fell in love” with amphibians — “these beautiful, amazing, precious creatures that I almost couldn’t believe were real when I first ventured out into Australia at night.”

Now, the frogs’ long-term prospects could hinge on Rowley solving the mystery of their mass deaths.

“If this keeps happening, if it does what it did last year this winter, then there could be really dire consequences for our amazing frogs,” she said.