In 2006, wildlife biologists in New York state noticed that bats were mysteriously dying in large numbers. Covered with a white fungus, the animals woke from hibernation in midwinter and starved to death. "It's like a science-fiction scenario that came out of nowhere," says bat biologist Paul Cryan of the U.S. Geological Survey (USGS) in Fort Collins, Colorado.
Now, researchers report in the 6 August issue of Science that the rate of decline is so severe it could cause one bat species to vanish from the northeastern United States within 16 years, potentially hurting agriculture and forests.
White-nose syndrome is thought to be caused by a cold-loving fungus, Geomyces destructans, which typically grows on the snouts and wings. Afflicted bats wake early from hibernation to try to remove the fungus. Once they awake, their metabolism speeds up. They lose body fat, and there are no insects to eat, so they starve. The fungus is thought to have been introduced from Europe, where it has been found on other species of bats that are unaffected. Since 2006, it has spread as far as Oklahoma in the United States.
The fungus has been detected in nine species of bat, the most common of which is the little brown bat (Myotis lucifugus). Bat ecologist Thomas Kunz of Boston University assembled a team of researchers to analyze the outlook for the species. First, they calculated the average mortality rate—73%—observed in the caves where the bats hibernate. Then postdoc Winifred Frick ran a population model that assumed that the death rate would eventually decline, as typically happens when the spread of disease depends on the density of the population. "You would expect if you have fewer individuals, the rate of transmission will lower," Frick says.
In the team's worst-case scenario, which assumes that 45% of the little brown bat population continues to die each winter, there is a 99% probability of regional extinction within 16 years. "That hit us like a brick," Kunz says. (The little brown bat lives throughout North America, so the species itself won't go extinct). If mortality declines to 10%, some little brown bats would last for 80 years, but the population would be dramatically smaller. Because brown bats eat a large amount of insects, including pests, a decline in their number could threaten farm fields and forests.
The new figures "emphasize the gravity of the situation," says microbiologist David Blehert of the USGS in Madison. "We absolutely need to look at how we can protect" the bats, Blehert says, but he cautions that any interventions shouldn't pose a risk of inadvertently harming the bats.
Right now the available options are limited. The U.S. Forest Service has been restricting access to caves to try to prevent people from accidentally spreading the fungus. However, Kunz is pessimistic about the odds of success because migrating bats also transmit the fungus between caves.
Another stop-gap approach is for people in the northeast to build simple bat houses. By offering smaller, warmer summer habitat for bats, these houses could boost the odds that bats will reproduce successfully, Kunz suggests. In the long term, the best hope is that the bats develop resistance to the fungus. Kunz thinks there may be hints that this is already happening in two colonies near Boston, which have seen fewer bat deaths this past winter.