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Bat numbers continue to fall

But some evidence bats may be seeking areas less conducive to deadly fungus

Marshall Helmberger
Posted 4/4/18

SOUDAN MINE— Bat numbers here have declined yet again according to the latest winter survey, although there is reason to hope that the worst of the impact from white-nose syndrome could be over. …

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Bat numbers continue to fall

But some evidence bats may be seeking areas less conducive to deadly fungus

Posted

SOUDAN MINE— Bat numbers here have declined yet again according to the latest winter survey, although there is reason to hope that the worst of the impact from white-nose syndrome could be over.

The note of optimism is based on a sharp decline in the number of bats that have emerged from the mine this winter. Last winter, thousands of bats emerged from the mine mid-winter, virtually all of which died due to the cold. But according to James Pointer, interpretive supervisor at the Lake Vermilion Soudan Underground Mine State Park, fewer than ten bats have emerged from the mine this winter.

Whether that’s because so few bats remain, or because bats are finding better places to hibernate in the mine, is unclear. The labyrinth created by the mine offers differing conditions for bats, and some places are less conducive to the growth of the fungus that causes white-nose syndrome.

There’s some evidence that bats in the mine are figuring that out and are gravitating to areas where there’s more fresh air and cooler temperatures, which appears to limit the growth of the fungus.

There’s also evidence in the latest bat survey that bat mortality in the mine remains significant. According to Pointer, the latest survey found just 116 bats in the three levels (the 12th, 17th, and 27th) that are surveyed every year. Portions of those levels are part of a standard survey route that park staff cover every year. Every five years, the park conducts a more thorough survey across the bulk of the mine.

Last year, DNR staff counted 440 bats in the three-level survey route, which was a significant drop from the 600-800 bats that had been typical before the introduction of white-nose syndrome. “What we don’t know is if they died, or are simply moving elsewhere,” said Pointer.

The survey, which involved just two park staff on March 7, was limited this year intentionally, said Pointer. “We’re trying to be conscious of what we’re doing,” he said, in hopes of minimizing disturbance in the mine during the hibernation season. “We’re even trying to limit maintenance work during that time,” he said.

Bat researchers first documented the presence of the deadly fungus in the Soudan Mine five years ago, but researchers didn’t find evidence of a significant number of bat deaths until last winter. That would make the current winter the second year of the die-off phase. The Soudan Mine has been known as the state’s largest bat hibernaculum, which was previously home to an estimated 10,000-15,000 bats.

So far, the effects of the fungus have played out similarly at Soudan as they have elsewhere in the U.S., where 90-95 percent of infected bats have died. It typically takes two or three years from the time the fungus is first detected in a hibernaculum before it begins to impact large numbers of bats. In the case of the Soudan Mine, the first sign of significant mortality appeared during the winter of 2015-16, and bat deaths spiked significantly from there last winter. It remains unclear whether the remaining bats have figured out ways to resist the fungus or have a greater tolerance. Pointer noted that all of the bats they recorded during their most recent survey were little brown myotis. Another species, the northern long-eared bat, has also been found in significant numbers in the mine in the past, but that species is known to be highly susceptible to white-nose syndrome, so it’s possible that mortality of that species has been significantly higher.

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