by Aaron Searcy
This year’s Bat Week (Oct. 24–31) comes at a time when bats in the Great Smoky Mountains are particularly busy. Hibernating species will continue to feed as much as possible in anticipation of the long winter ahead, and on cool fall evenings, bats may swarm in groups to find a mate or locate safe places to hibernate.
This year, however, there will be fewer bats taking to the Smokies’ night skies than there were only a decade ago. Since the arrival of the deadly fungal disease known as white-nose syndrome (WNS), three of the park’s 13 known bat species—the Indiana bat, the little brown and the tri-color—have lost 91% to 95% of their populations. Though once relatively common, the Indiana bat is now listed as endangered.
White-nose syndrome takes its name from the appearance of a white fungal growth on the muzzle and wings of bats infected with the disease. The fungus thrives in dark, cool environments and spreads among cave-dwelling bats when they reduce their metabolic rate and lower their body temperature to enter winter hibernation. It can be transmitted through physical contact with bats infected with the disease or the surfaces of caves or mines where they hibernate. While humans cannot contract WNS themselves given the cooler body temperatures it requires to thrive, they can spread the disease from one cave to another by unintentionally carrying it on clothing, shoes, or gear.
Although not necessarily lethal by itself, the fungus induces far more frequent arousals from torpor throughout bats’ five- to six-month hibernation, causing them to expend their valuable stores of fat and energy before they can replenish them in the spring. Researchers also believe that, in addition to forcing bats to burn much more energy than usual, WNS may disrupt other critical physiological functions such as water balance by causing dehydration through physical damage to wing skin membranes.
The first evidence of WNS in the Smokies was discovered in a park cave in 2010 when a little brown bat tested positive for Pseudogymnoascus destructans—the cold-loving fungus responsible for the disease. Since then, WNS has spread far beyond its original boundaries in the Northeast to as far west as the state of Washington, leaving in its wake more than six million dead bats across North America and one of the fastest declines of wild mammal populations to date.
While emerging research may offer possible short-term treatments for bats affected by the disease, park staff and researchers continue to study the ecology, habitat requirements, and behavior of bats in the presence of WNS. The park’s proactive steps have included closing cave areas, monitoring hibernating bats, tracking bats on the wing using acoustic devices, and collaborating with researchers at the University of Tennessee and Indiana State University. The hope is to better support survivors of the disease and speed the recovery of the species found in the Smokies.
This monitoring has shown that not all of the park’s bat species have been equally affected by WNS. Of the 13 bat species found in the park, only ten stay over the winter to hibernate in caves and mines, while the others—red, hoary, and Seminole bats—usually migrate to warmer areas.
A 2016 winter survey in the park found decreases in the overall number of bats and a 94.4 percent decrease in the Indiana bat population in a cave that historically housed more than 2,400 bats. Counts from that year showed that Indiana bat numbers had suffered an overall loss of 97 percent, tri-colored bats a loss of 87 percent, and Rafinesque’s big-eared bats a loss of 52 percent. On the national level to date, little brown bat populations have decreased by about 90 percent, while tri-colored and northern long-eared bats have suffered losses of roughly 97 percent.
Not all news was bad news. Capture rates actually appeared to have increased since the previous pre-WNS survey for some other bats such as eastern small-footed and big brown bats according a 2016 survey. This squares with the findings of some researchers that suggest big brown bats have fared better than other species and either have natural resistances to WNS or have adapted a means of evading its most deadly side effects.
These numbers offer at least a faint glimmer of hope for bats that play a key role of healthy ecosystems across North America. Bats perform ecologically significant roles as the primary predators of nocturnal insects. One little brown bat can eat as many as 1,000 mosquito-sized insects in an hour, and a nursing female can consume her body weight in insects in a single night. Bats’ suppression of forest and agricultural pests accounts for nearly $3.7 billion worth of insect control for US farmers every year. They also pollinate night-flowering plants, provide valuable nutrients to cave ecosystems through their guano, and serve as prey for other animals including owls, hawks, falcons, and weasels.
Despite the arrival of the devastating white-nose syndrome, Great Smoky Mountains National Park continues to provide valuable habitat for regional bat species with its mature forests, water sources, protected areas suitable for hibernation, and freedom from excess light and noise pollution. For now, NPS bat monitoring efforts and maintenance of roosting sites in the Smokies is one of the best ways to ensure the survival of endangered and threatened bats
Anyone can contribute to healthy bat populations near their home by reducing outdoor lighting wherever possible, minimizing tree clearing, and protecting local waterways and wetlands. More resources can be found at www.batweek.org and www.whitenosesyndrome.org.
From top: Bat with white-nose syndrome courtesy of US Fish and Wildlife Services; Bat at Cades Cove Visitor Center courtesy of Bruce Day