Science

Green tree frogs and other frog species are dying off along the east coast of Australia and scientists are asking the public for help to discover the cause, or causes. By Ken Eastwood.

Why are frogs dying?

An Australian green tree frog, similar to those found dead and dying in recent months.
Credit: Selfwood / Alamy

In early to mid-June, reports of dead green tree frogs started coming to Dr Jodi Rowley, reptile and amphibian curator at the Australian Museum. People were finding the dead and dying frogs around Scotts Head on the north coast of New South Wales and, because she was stuck in lockdown in Sydney, Rowley went on radio and asked for further reports.

She was inundated. Frogs up and down the east coast of Australia were found, sick and dying. And not just in ones and twos, but whole, long-standing populations were disappearing within a week or so. Frogs, generally more active at night, were reported to be wandering around dazed in the day, their skin patchy and discoloured, before succumbing.

“It’s been really affecting me,” Rowley says. “When my inbox is just hundreds of emails, most with photos attached, and tear-jerking stories for weeks, it’s been wearing me down. It’s awfully upsetting to see what’s going on.”

In a webinar on the frog crisis attended by more than 1000 people in late August, a visibly moved Rowley shared some of the emails coming in and the strategies she and other scientists are using to get to the bottom of what is happening.

“I’ve been finding a couple of dead frogs every day since winter started,” one anonymous email states. “I’ve always had around 30 of them that come in each night during summer, and now they’re all dead – not just the green tree frogs but other small species too … I’ve spoken to a lot of neighbours and friends in the area and they’re all reporting the same thing. We’ve been living here for 26 years and have never seen this before.”

Frogs are known to die off in winter after a cold snap. But Rowley says that because frogs decompose quickly, people rarely see dead frogs. The scale and spread of this mortality event is unprecedented.

“I have the very strong feeling that this has the potential to change things forever,” she says. “I don’t want to say, ‘It’s all over for frogs’, but it is urgent to get to the bottom of this.”

Dead and dying frogs have been reported from Far North Queensland to southern Victoria, with scattered reports in Tasmania, Perth and the Northern Territory. Seventy per cent of the reports have come from NSW, and 60 per cent concern green tree frogs – but about 25 other species have also been affected.

As well as documenting the more than 1000 reports that have already come in, Rowley is working with a team at Taronga Zoo’s Australian Registry of Wildlife Health to conduct necropsies on any frogs that are sent in. Because many herpetologists and ecologists are in lockdown, she is appealing to anyone in NSW who finds dead frogs to put them in plastic bags and freeze them, so they can be examined later.

Fewer than a dozen frogs have been examined so far and the jury is still out on the cause or causes of death, but the No.  1 suspect is the amphibian chytrid fungus.

First discovered in Australian frogs in 1993, this Asian fungus has mobile spores that seek out frogs in soil and water. Although frogs can live with a small amount of the fungus on their skin, the fungus has decimated frog populations worldwide and caused the extinction of some species.

Yet, following the initial wave of frog population declines, in recent years some Australian frog populations have been showing signs of recovery, suggesting some species might have adapted to living with the fungus, says Dr Ed Meyer, an ecologist and member of the Queensland Frog Society.

“So we hadn’t seen regularly die-offs like we had in the 2000s,” he says. “The recent die-off during winter shows that not all species are adapting well to the presence of the fungus, and that some species, including the green tree frog, may remain highly susceptible, especially when cooler conditions favour the growth and reproduction of this fungus. Which begs the question, are we being too optimistic about the recovery or adaption of some of the other species?”

Meyer says that the Frog Society began getting reports of dead and dying green tree frogs in Queensland around the middle of July. “We immediately suspected we were dealing with chytrid fungus,” he says. “In most cases frogs were found sitting out during the day, dead, near dead or unresponsive. They also had evidence of discolouration and their undersides were quite red. All of these symptoms are consistent with what we’d seen in the late ’90s and 2000s with chytrid fungus.”

He says the huge number of reported dead and dying frogs is likely to be the result of a few factors. First, through social media and means such as the Australian Museum’s FrogID app, people are more likely to hear about and report what they are seeing than they were 20 years ago, so there are many more reports. Second, the past few Queensland winters have been mild, leading to a recovery of many frog species and green tree frogs in particular.

“There’s more of them out there, so there’s more in people’s backyards, and there’s more chance of people coming across dead ones,” he says.

Chytrid fungus reproduces and grows best in temperatures of 17–28 degrees. Warmer climate frogs such as the green tree frog are less able to combat pathogens at those colder temperatures. “So it’s like a double whammy,” Meyer says. “This winter in Queensland we’ve seen shifting temperatures back into that zone that suits the fungus.

“I’m not pressing the panic button at this stage, but there is cause for concern and it’s appropriate that we do more pathological testing to confirm that it is chytrid. If it’s something other than chytrid, we need to know what it is.”

Jane Hall, Wildlife Health project officer for the Australian Registry of Wildlife Health, isn’t so sure it’s just chytrid. “We think there’s something else going on,” she says. “Chytrid is not necessarily the only pathogen that can affect frogs. It’s really important we try to tease out the difference between this frog died with chytrid and this frog died from chytrid … We can’t afford to make assumptions about what’s happening now based on what’s happened in the past. It could be the beginning of something far more dramatic.”

Hall says when the registry has completed previous necropsies on frogs – examining them “top to tail and inside out” – they have found a lot of new and undescribed pathogens that can affect frogs. “Our role is to figure out what’s going on with these animals.”

To aid this process, Hall says the registry would ideally like to examine half a dozen dead frogs of each species, from each of the regions being impacted.

Rowley says that green tree frogs are an obvious frog often seen in backyards, but she is concerned about what is happening in swamps and forests, away from the public eye.

“I can’t get out there in the field at the moment,” she says. “I can’t look for frogs myself. I’m worried about what we’re not seeing.”

Through the emails that keep coming into her inbox, she holds onto a small piece of optimism: “I know frogs have the ability to bounce back.”

This piece was produced in collaboration with cosmosmagazine.com.

This article was first published in the print edition of The Saturday Paper on Sep 11, 2021 as "Why are frogs dying?".

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Ken Eastwood is a journalist, editor, author and photographer based in the Blue Mountains.