Rare Aussie creature rediscovered in 'lost world' at edge of cliff
They once lived across a wide range, but Watson's tree frogs are now confined to small clifftop peaks.
Already facing a perfect storm of attacks from disease and habitat loss, it was feared bushfires may have wiped out a rare population of endangered amphibians. But one threatened species expert refuses to give up hope despite their croaks not being detected on their former clifftop stronghold for years.
The Castle south of Sydney takes over three hours to reach from the carpark. Scrambling to the top is a further 45 minute climb, and because of the sheer drop it isn’t recommended for people afraid of heights.
It was along this 1km-long plateau, which strangely resembles a tadpole with a long tail, that threatened species expert Dr Rachel Melrose knew a population of Watson's tree frog had lived. She describes The Castle as a ‘lost world’ – one of several peaks in the Morton National Park that provided sanctuary from waves of invasive chytrid fungus which sadly wiped out other amphibian species elsewhere.
“The Castle is a multi-tiered monolith — a top tier with a cliff-line and another tier below it. They’ve been called islands in the sky by people a long time ago,” Melrose told Yahoo News.
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Why are the frogs so rare?
Before the fungus spread, the Watson's tree frog had been widespread, spanning from just above Kiama all the way to Victoria.
What was special about The Castle and the neighbouring Byangee walls was their tadpole pools dry up during the warmer months, so the disease may not have been able to establish itself like in a permanent dam or creek.
Despite The Castle burning “really hot”, Melrose began searching to rediscover the Watson's tree frog as soon as it was safe to return. Many of the trees up there are still scorched black four years on.
“I kept climbing up there after it had been raining. After one big event all the pools were full but there was just nothing in them,” Melrose said.
“Last Christmas it rained for a week and I kept saying to everyone I knew: They’re going to be here.
“But there was a lot of scepticism because we’d already been looking for a very long time.”
Threatened species expert's find at cliff edge
The Department of Environment team had set up audio equipment to listen for the male’s trilling sound, and searched for tadpoles in the pools. But it wasn’t until January this year that Melrose was proven right.
“It was pretty humbling getting up there again. You’re perched on top of the highest point for a long way,” she said.
“I'd passed heaps of ponds and they were empty. So I thought they're not here again. Then I walked out to look at the view because you can see the ocean.
“And then the tadpoles were there, just there on the edge of the east-facing cliff.”
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How did the rare frogs survive the intense heat?
It’s unthinkable the frogs could have survived the intense heat that scorched The Castle. So Melrose and her colleagues at the NSW Department of Environment think the species live elsewhere and only use its plateau opportunistically to breed.
“We think they’re moving around, but they have this drive to lay eggs at the highest point they can find,” she said.
“People are speculating at what they do during fire. It seems to me some of the cracks and gullies on the side of The Castle could have been a place they were taking refuge — it’s very cool and dark even when it’s hot outside.”
The Commonwealth and the NSW Threatened Species Scientific Committee listed the Watson’s tree frog as endangered in 2022, following the Black Summer bushfires.
Like many small species, it remains unknown how many individuals survived the disaster, or whether those that did became inbred after their populations became isolated.
But, amid ongoing concern for the species, the survival of the frogs on The Castle has given researchers hope there could be more remnant populations within the national park.
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