Salty wetlands help curb fungus killing frogs

A deadly fungus decimating the world's frog populations may yet have a weakness scientists can exploit to reduce its lethal impact in Australia.

Researchers have found the chytrid fungus, which has already resulted in the extinction of four Australian frog species, is less toxic in warmer, saltier wetlands.

Endangered in NSW: The growling grass frog.
Endangered in NSW: The growling grass frog. Photo: Geoff Heard

Their study, in Ecological Applications this month, looked at growling grass frog populations and found former bluestone and clay quarries provided an unlikely refuge.

Known as the southern bell frog in NSW, it is one of the largest frogs in Australia. Listed as endangered in NSW, it has suffered a considerable drop in numbers and distribution in NSW and is now largely limited to isolated populations in the Coleambally irrigation area, the Lowbidgee floodplain and Lake Victoria.

Research fellow at Melbourne University's school of botany Geoff Heard said the quarry sites studied in Melbourne had warmer, saltier water than other wetlands due to a lack of foliage. The pit depths meant the saline water table leaked into the surface water, with some wetlands recording salinity readings that were 10 per cent as salty as seawater. The conditions appear not to faze the frogs, while the impact of the deadly fungus appears weakened.

''That's the really interesting bit,'' he said. ''We've seen the frogs in really high density in some of these quarry sites and it was intriguing as to why that was the case.''

Dr Heard studied 10 wetlands in the Merri Creek catchment area north of Melbourne with Arthur Rylah Institute researchers Michael Scroggie, Nick Clemann and David Ramsey. The sites ranged from cool wetlands with average midnight water temperatures of 16 degrees between October and March - to warm quarry wetlands with average midnight water temperatures of 21 degrees. In summer, night water temperatures can reach 27 degrees.

Warmer conditions reduced infection rates among frogs which were better able to fight off the disease as, being cold-blooded, their immune system ramps up in warmer conditions. Dr Heard said overseas research has shown the fungus has a low tolerance of temperatures above 25 degrees and dies when exposed to temperatures above 30 degrees.