July 4th, 2012 by James Ayre
Bat populations in North America are currently being decimated by a fungal disease known as white-nose syndrome. The deadliness of this fungus may greatly depend on how social the bats are during hibernation, according to new research from the University of California, Santa Cruz.
“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 the study. One gregarious species has surprised researchers, however, by changing its social behavior.”
For the research, data taken from bat surveys between 1979 and 2010 were analyzed. This period of time covered a long and gradual population growth, followed by the drastic and immediate declines seen when white-nose syndrome appeared.
White-nose syndrome first appeared in New York state in 2006, and has continued to spread throughout the United States and Canada since then. For the new study, population trends in six bat species in the northeast were analyzed. The researchers “found that some bat populations are stabilizing at lower abundances, while others appear headed for extinction.”
“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.”
During the winter, bats hibernate inside caves and abandoned mines, the populations can vary drastically 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.”
The researchers analyzed how steep the population decline was at each site after white-nose syndrome appeared there, and if the severity of the population decline was the same in populations of different sizes. The research 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.”
There is at least one major outlier though; one species that is highly social that isn’t seeing the drop associated with other highly social bats. “The little brown bat, one of the most common bat species in the northeast, appears to be changing its social behavior, going from a species that preferred to roost in dense clusters to one in which most bats now roost apart from other bats.”
“Our analysis suggests that the little brown bats are probably not going to go extinct because they are changing their social behavior in a way that will result in them persisting at smaller populations,” Kilpatrick said.
As a comparison, another highly social species, the Indiana bat, is continuing to hibernate in dense clusters, and is likely to continue moving towards extinction.
“Since the appearance of white-nose syndrome, both species have become more solitary, but the change is much more dramatic in the little brown bats,” she said. “We now see up to 75 percent of them roosting singly. For Indiana bats, only 8 to 9 percent are roosting alone, which does not appear to be enough to reduce transmission rates.”
“Even solitary roosting habits may not be enough to save some species, such as the northern long-eared bat. Although it declined less rapidly as its colonies got smaller, 14 populations of northern long-eared bats became locally extinct within two years after the detection of white-nose syndrome, and no populations remained in the study area after five years. In contrast, the populations of tri-colored bats, another solitary species, stabilized at low levels three to four years after disease detection.”
“The northern long-eared bats may be particularly susceptible to the disease, so they continue to get hit pretty hard even after transmission rates are reduced,” Langwig said.
The two species of bats that have been the least affected by white-nose syndrome are mostly solitary, big brown bats and eastern small-footed bats. Although they are mostly solitary, they sometimes roost in small clusters. The researchers say that the reasons for them being less affected aren’t completely clear yet.
It’s possible that the sites where these species roost are less conducive to the disease. The researchers studied the influence of the different microclimates within hibernation sites, and found that in the drier and cooler sites the disease caused less of a population decline. “It appears that the driest and coolest caves may serve as partial refuges from the disease,” Kilpatrick said.
The majority of the bat population data used in the study was collected in surveys conducted by state agencies over the past 40 years. The research was funded by the National Science Foundation, Bat Conservation International, and the U.S. Fish and Wildlife Service.
The study has just been published July 3 in the journal Ecology Letters.
Source: University of California – Santa Cruz
Image Credits: Al Hicks, New York State Department of Environmental Conservation; Ryan von Linden, New York State Department of Environmental Conservation
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