MELBOURNE, Australia — Jodi Rowley crossed a pond in rain boots, her headlamp piercing the blackness of the winter night. After the sound of croaking, she and her three colleagues scanned the water for signs of life.
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With swabs on hand, the team collected samples of the 22 tiny frogs found on this expedition near Albury in southern New South Wales this month in hopes of deciphering a phenomenon that puzzles animal lovers and scientists alike.
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“It’s a really complicated murder mystery,” Rowley said.
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Across Australia, dead frogs are turning up in the thousands – and no one knows why.
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video-caption">A team of scientists led by Jodi Rowley undertook fieldwork in New South Wales on July 6 to collect data on a mysterious disease killing Australian frogs. (Video: Jodi Rowley)
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It started last winter when Rowley, a herpetologist, noticed an increase in reports on social media of frog carcasses in local backyards and streams. She was worried, but knew that amphibian immune systems slowed down in the cold – and it was a cold year.
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But a call for citizen data has resulted in a flood of sightings of dead frogs far beyond normal winter losses. The frogs, which usually hide in cooler weather – mid-year in Australia – apparently wandered out into the open, sat down and died en masse.
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“The owners were saying they had never seen this, but there are dozens of dead frogs all over their house,” said Rowley, head of the department of herpetology at the Australian Museum and the University of Nova Scotia. South Wales.More than 1,600 reports have arrived covering more than 40 species across the country, many detailing multiple fatalities.
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After a summer respite, the phenomenon seems to be back this winter.
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“I was preparing for the possibility of it happening again,” Rowley said. “And sadly, it looks like it.”
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Australia is home to over 240 native species of frogs. They include delicacies such as the onomatopoeically named pobblebonk and the tiny assa wollumbin, found on a 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.
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“They’re enigmatic and hiding, but there are huge numbers of them,” said Taronga Conservation Society Australian Wildlife Health Registry manager Karrie Rose. “If their populations change, there will be ripples throughout the food web.”
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Frogs are indicators of the health of an ecosystem as a whole. They are eaten by birds, reptiles and even dingoes. And they maintain the balance of the environment by eating algae and insects. A study linked a decline in frogs to an increase in malaria in two countries because fewer frogs nibbled on disease-carrying mosquitoes.
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In Australia, at least four species of frogs have disappeared since European colonization. They include the only two known species in the world to have the bizarre trait of laying eggs, eating them, and then vomiting tadpoles out of their mouths. Nearly one in five surviving species is threatened, and Rowley said she fears mass mortality events could drive other species to extinction.
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Rose, a veterinary pathologist, works with Rowley to study frog deaths.
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The prime suspect is a killer who attacks by choking his victim’s skin.
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Champignon chytride — Batrachochytrium dendrobatidis – has torn amphibian populations since the turn of the last century. Scientists believe it originated on the Korean peninsula and spread around the world through trade. The fungus, which feeds on the keratin in the outer layer of frogs, threatens the survival of more than 500 types of amphibians, according to a 2019 study. It is thought to be responsible for 90 extinctions since the 1970s, which which makes it a more destructive invasive species than rats or cats.
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Rowley and Rose think the fungus likely plays a role in the unexplainable death. But they doubt that’s the whole story. The fungus has been present in Australia for decades, Rose said. And some autopsies revealed internal damage to the frogs’ nervous system and heart, which is not a typical symptom of fungal infection. Something in the environment must have changed.
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“There has been good evidence of widespread chytrid fungus infection since about the mid-1980s. So why are we seeing such high mortality now? ” she says.
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Of the hundreds of frozen frog carcasses his lab analyzed, about 75 percent were infected with the chytrid fungus. But that could not explain the fate of the remaining 25%.
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Scientists are exploring several theories. One could be the rainy weather in eastern Australia over the past couple of years, which is good for both mushrooms and frogs. A secondary disease, parasite, environmental toxin or stressors resulting from successive droughts, fires and floods could also play a role.
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Last winter, with Australian towns under coronavirus lockdown, Rowley and her fellow herpetologists in Sydney were limited to studying frogs that were in their neighborhoods, samples of sick frogs that Australians had taken to veterinary clinics or frog carcasses that people had placed in freezers to be recovered by experts. This year, Rowley is in the field, racing to figure out what’s going on before frog populations are permanently affected.
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She hopes a combination of institutional and citizen science will gather the data that will unlock the puzzle. Australians are encouraged to record frog sounds and take photos in their neighborhood, using the Australian Museum’s FrogID app. “We really need everyone’s help because this is a huge problem and it spans across the continent,” Rowley added.
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Rowley, 42, has been specializing in the study of frogs since he was 18. She recalls the time she “personally fell in love” with amphibians – “those beautiful, amazing and precious creatures that I almost couldn’t believe were real when I first ventured into Australia at night.
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Now, the frogs’ long-term prospects may hinge on Rowley solving the mystery of their mass death.
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“If this continues, if he does what he did last year this winter, then there could be really dire consequences for our amazing frogs,” she said.
