The yellow-spotted bell frog (Litoria castanea), last observed in 1970s, has long been thought to be extinct in the wild. Scientists believed it was probably a victim of the deadly chytrid fungus that has devastated amphibian populations around the world.

But last year, Luke Pearce, a fisheries conservation officer in the Australian state of New South Wales (NSW), thought he saw a yellow-spotted bell frog in an isolated stream where he was looking for another endangered species. He returned a year later with herpetologist David Hunter of the NSW Department of Environment, Climate Change and Water. Together, they found a population of around 100 adult frogs.

 They also found tadpoles, six of which were collected and raised to maturity. The six frogs have now been placed in a captive breeding program at Taronga Zoo in the Sydney suburb of Mosman.

"This was definitely the most exciting moment of my career and I will be surprised if I repeat it," Hunter told the AFP news service.

So why did this one isolated population survive when all other yellow-spotted bell frogs have disappeared? Hunter thinks the population could have some sort of resistance to the chytrid fungus, although he says it is too early to speculate if that is true, or why.

Interestingly, despite the amount of time since the frog had been seen in the wild, the Australian government never gave up on it. A formal recovery plan for the species has been in place since 2001.

The location where the pair discovered the frogs has not been disclosed to protect the remaining habitat.

Image: Yellow-spotted bell frog, © Gordon Grigg, via the New South Wales Department of Environment, Climate Change and Water