A sudden decline in critically endangered bats is troubling experts, who recorded about 5,000 fewer southern bent-wing bats than usual at Naracoorte’s “bat cave” this summer.

These tiny russet-toned microbats roost in caves and forage for insects over long distances across south-west Victoria and south-east South Australia.

Through summer, the Naracoorte cave in South Australia – one of three maternity sites for the species – becomes home to the largest population when the bats return to give birth and raise their young.

Over the past five years, the peak population at Naracoorte has averaged 28,600 adults, but last summer, that fell by about 14-16%.

“We’ve lost quite a lot of the adults,” said bat ecologist Nicola Bail, who coordinates the species recovery team at Zoos Victoria.

“We’re really not sure what the cause of this decline is, but it is really concerning, especially with so many threats operating in the range of the southern bent-wing bat.”

Population counts at the World Heritage listed site date back to the 1960s, when as many as 200,000 bats were recorded. Severe declines since then have seen southern bent-wing bats listed as one of Australia’s eight critically endangered mammals.

“They’re still hanging on. They’re a real little fighter,” Bail said.

But the recent drop, which includes some of the lowest male survival rates in a decade, would have long-term ramifications. Southern bent-wing bats depend heavily on adult survival, she said. The bats are known to live up to 22 years, with females producing only one young a year.

Associate prof Jasmin Hufschmid, from the University of Melbourne, said the bats were “gorgeous little creatures” that played an “invaluable” role controlling agricultural pests and insects that spread disease.

“They’re very important animals for us, but we just don’t really see it because they’re really small, they’re out at night,” said Hufschmid, who researches bat health but was not involved with the counts at Naracoorte.

Possible explanations for the decline include habitat clearing, drought, fewer flying insects that the bats eat, or human disturbance at non-breeding caves, many of which are located on private land.

A spokesperson for South Australia’s environment department said collisions with wind turbines and the potential introduction of disease like white-nose syndrome – which has not been detected in Australia – also posed risks to the species.

Cassie Hlava at the Limestone Coast Landscape Board, is coordinating a federally funded project that is working with landowners, restoring caves and replanting corridors of native grasses and flowering natives. This includes planting wattles, eucalypts, hakeas and banksias – places for moths to gather and breed, boosting the food supply for hungry bats.

Southern bent-wing bats are unique to the area, she said. “We know they’ve been here around 500,000 years. We’d like to keep them in this landscape, and we would like more people to get involved in habitat restoration on their properties to support the bats.”

“The recent decline is a huge concern,” she said. “The numbers are already quite low. So when you have a drop of 5,000 bats within a year, it’s really concerning that the species is being pushed towards extinction.”