The impact on bat populations of a deadly fungal disease known as white-nose syndrome may depend on how gregarious the bats are during hibernation. Species that hibernate in dense clusters even as their populations get smaller will continue to transmit the disease at a high rate, dooming them to continued decline, according to a new study led by researchers at the University of California, Santa Cruz. One gregarious species has surprised researchers, however, by changing its social behavior.

White-nose syndrome has decimated bat colonies throughout the northeast since it first appeared in New York state in 2006, and it continues to spread in the United States and Canada. In the new study, researchers analyzed population trends in six bat species in the northeast. They found that some bat populations are stabilizing at lower abundances, while others appear headed for extinction. The study, published July 3 in Ecology Letters, examined data from bat surveys between 1979 and 2010, covering a long period of population growth followed by dramatic declines caused by white-nose syndrome.

"All six species were impacted by white-nose syndrome, but we have evidence that the populations of some species are beginning to stabilize, which is really good news," said Kate Langwig, a graduate student at UC Santa Cruz and first author of the paper. "This study gives us an indication of which species face the highest likelihood of extinction, so we can focus management efforts and resources on protecting those species."

The bats hibernate during the winter in caves and abandoned mines, and the number of bats can vary tremendously from one site to another. The fungus that causes white-nose syndrome grows on the exposed skin of hibernating bats, disrupting their hibernation and causing unusual behavior, loss of fat reserves and death.

Langwig and her coauthors looked at how steeply the bat populations at each site declined after they were hit by white-nose syndrome, and whether the severity of the decline was the same in large and small populations. They found that for species that hibernate alone, the declines were less severe in smaller colonies. For gregarious species, however, even small colonies declined steeply.

"We found that in the highly social species that prefer to hibernate in large, tightly-packed groups, the declines were equally severe in colonies that varied from 50 bats to 200,000 bats, which suggests that colonies of those species will continue to decline even when they reach small population sizes," said coauthor A. Marm Kilpatrick, assistant professor of ecology and evolutionary biology at UC Santa Cruz.

Trends in the declines of different bat species since the emergence of white-nose syndrome support these predictions. As populations get smaller, the declines tend to level off for species that roost singly, but not for socially gregarious species.