Long and short-tailed bats were once common in New Zealand but have disappeared from many areas due to habitat loss and being eaten by rats, possums, stoats, and feral cats.
The tiny mammals roost together and rear their young in hollow trees, where they are especially vulnerable to attack.
Long-tailed bats have the highest threat ranking of "nationally critical".
Short-tailed bats belong to an ancient family only found in New Zealand and are confined to two offshore islands and remote forest areas in Fiordland, the central North Island and Northland.
There are three sub-species, all threatened, although the southern short-tailed bat is now classed as "recovering" due to their recent increase in the Eglinton Valley.
Department of Conservation (Doc) principal science adviser Colin O’Donnell said monitoring of bats over many years in the Eglinton Valley near Te Anau and Pureora Forest Park, west of Lake Taupo, had shown a significant rise in populations of short-tailed bats due to predator control.
“Short-tailed bats in the Eglinton Valley have done really well since predator control began in the late 1990s with the population increasing on average by 8% per year."
However, he said long-tailed bats appeared particularly sensitive to rat predation, requiring very low rat numbers over large areas for them to thrive.
“Ten years ago, long-tailed bat survival in the Eglinton Valley was dropping after each beech mast when predator numbers soared.
"But since 2010, predator control over larger areas, timed to suppress rat plagues, has allowed many more bats to survive."
In 25 years of monitoring the long-tailed bat population had turned around, from declining at 5% to increasing by 4% per year.
Doc controlled predators in the Eglinton Valley using combinations of sustained trapping, ground-based toxins, and periodic aerial 1080 over large areas to prevent predator plagues following beech masts.
