Numbers of unique Australian moths that migrate in their billions to alpine areas have crashed, ecologists say, putting extra pressure on the endangered mountain pygmy possum.

Scientists believe the “astonishing” drop in bogong moth numbers is linked to climate change and recent droughts in areas where the moths breed.

At the same time checks on the endangered mountain pygmy possum, which exists only in Australia’s alpine regions, have revealed dead litters in the pouches of females. The moths are a key food source for the possums as they wake from hibernation.

In 2018, scientists revealed bogong moths were the only known insect to use the earth’s magnetic field to help them navigate from grasslands in northern New South Wales and southern Queensland – sometimes at distances of 1,000km. Around two billion moths are estimated to make the journey.

The ecologist Dr Ken Green has been monitoring bogong moths for 40 years. He said: “Last summer numbers were atrocious. It was not just really bad, it was the worst I had ever seen. Now this year it’s got even worse.”

The moths find caves and cracks in boulders to hide away in a torpor state. A cave at Mount Gingera, near Canberra, has been known to house millions of the moths but last month Green and colleagues counted just three individuals. Searches of about 50 known sites have turned up similar catastrophic absences.

“They haven’t just declined. They’ve gone,” he said. “We have done mountains from down to the Victorian border all the way to Canberra. We have checked every cave we know.”

A site at South Ramshead in the Snowy Mountains in New South Wales had just 1,000 moths last year. “That is very, very low,” says Green.

“This year we found just six moths. Last week we went back and there were none.”

Green believes the cause of the crash is drought in the moth’s breeding areas. The Bureau of Meteorology has said the drought was exceptional in those areas, but noted there had been similar dry periods in the 1960s and earlier.

Impact on the endangered mountain pygmy possums

Dean Heinze, an associate at Latrobe University, has been monitoring and researching mountain pygmy possums since the early 1990s. The possum populations are spread across Kosciuszko national park in New South Wales, and in Victoria at Mount Bogong, Mount Higginbotham and Mount Buller.

Scientists think the ‘astonishing’ drop in bogong moth numbers is linked to climate change, with extensive droughts in recent years in locations where the moths breed before migrating to alpine regions. Photograph: Auscape International/Alamy

As the possums emerge from hibernation in September and October, the moths are a key food source as they breed and raise young.

“Bogong moths are an incredible food source and very high in protein. Last year we found that some populations of possums were losing litters. It’s happening again this year. We think that the fewer moths means the possums are carrying less weight and losing the litters.”

In recent weeks Heinze has visited about 12 locations in Victoria and discovered dead litters in female pouches at “most of the sites.”

“It’s a widespread event,” he said. “I inspect the reproductive system of the animal and I look in their pouch to see how many young there are. I have been opening pouches and discovering dead and decomposing young ... For some, the young have been dead for days.

“The concern is that over time, if this happens more frequently we will see declines in the adult population as well. And this is the second year that they have lost litters. The litmus test will be next season.”

Around Mount Buller in Victoria, the possum habitat is fragmented by ski resorts. Captive breeding and release programs have been used to keep numbers up.

“We have put a huge effort into the population at Mount Buller,” says Heinze. “We have brought the population back from the brink and now we have this happening. I’m optimistic, but if we get more of this then it doesn’t look good. Here we are dealing with a species that only occurs in the alps, but what’s impacting it is happening hundreds of kilometres away.”

In 2016, a national recovery plan was agreed for the possum, outlining multiple threats including habitat degradation, predation by invasive cats and foxes and climate change.

A study published in October 2018 and led by scientists at the New South Wales Office of Environment and Heritage looked at the climate change threats to the possums and the moths. The study suggested the moths had a “reduced survival in a warmer world” and this would “likely further affect survival” of the possums.

Prof Lesley Hughes, an ecologist at Macquarie University and councillor at the Climate Council of Australia, said the potential role of climate change in the decline of the moths and possums were what ecologists and climate scientists had predicted.

“Unfortunately the general predictions of the ecological risks of climate change are now turning into observations for particular species. And it should be no surprise we are seeing these impacts in the alpine zone, long recognised as one of the most vulnerable ecosystems to climate risks.

“Sometimes these changes can appear to happen abruptly – one year there are millions of moths and the next almost none. This shows how particular extreme events, such as droughts or severe bushfires, can suddenly tip a species over the edge, with flow-on effects to others in the ecosystem.”

The environment minister, Melissa Price, said she was aware of the reports of low bogong moth numbers and the government was working with a leading possum researcher “to understand the potential toll that reduced number of moths may be having on the species”.

The removal of feral cats and foxes in Kosciusko national park was helping possum protection, she said, and two government-backed projects were tackling weed infestations, building resilience to climate change, and managing loss of genetic diversity among possums in the Victorian Alpine region.

Victoria’s Department of Environment, Land, Water and Planning spokesperson Adrian Moorrees said the reduction in moths “is likely to be due to recent droughts” in NSW and Queensland.

He said tools and processes were being developed “to better monitor and predict the impact of bogong moth populations on endangered mountain pygmy possums.”

A spokesperson for the NSW Office of Environment and Heritage said no litter deaths were reported in NSW possum populations, but the drought was believed to have impacted most species.

He said when moth numbers were low, possums turned to other food sources, including insects, fruits and nectar, adding “OEH will continue to implement the species recovery plans for the mountain pygmy possum.”