- The authors of a recent study observed a dramatic shift in bat mortality at the turn of the 21st century to deaths by wind turbine collisions and white-nose syndrome.
- Though human travel between North America and Europe is believed to be responsible for the introduction of white-nose syndrome to North America, the study did not attribute death by this fungal infection to human activity.
- Close and frequent contact between bats and people or other animals can lead to spillover events, in which disease is transmitted from the reservoir species to the species it comes in contact with.
For the first time since 1970, a team of scientists has reviewed global trends in bat mortality. Unprecedented in scope, the review, published in Mammal Review, reveals that over half of all mass mortality events in bats over the past 226 years resulted from human activity.
The authors observed a dramatic shift in mortality at the turn of the 21st century to deaths by wind turbine collisions and white-nose syndrome. Killing millions of bats, these factors surpassed all other historical causes of bat deaths.
Though human travel between North America and Europe is believed to be responsible for the introduction of white-nose syndrome to North America, the study did not attribute death by this fungal infection to human activity.
Study authors searched through a multitude of reports since 1790 for evidence of mass mortalities, which they define as events involving 10 or more bat deaths.
They classified the events by cause of death and identified nine major categories: intentional killing by humans; biotic factors not including disease (such as predation); abiotic factors; environmental contamination; accidents (such as entrapment); wind turbine collisions; infectious viral and bacterial disease; white-nose syndrome; and unexplained causes.
Estimates are likely conservative, since mortality report requirements vary between countries. Still, the study documented 1,180 mass mortality events in total affecting 152 of 1200 known species.
Found on every continent but Antarctica, bats promote biodiversity and ecosystem health worldwide.
“Researchers have discovered that many plants are pollinated…crop and plant pests eaten, and plant seeds spread by bats,” said David Hayman, Massey University senior lecturer and co-author.
Bats famously pollinate plants involved in tequila and cacao production, and can eat up to 1,000 insects an hour, reducing pest populations for farmers.
As bat populations fall, their roles as reservoir species – animals that host a pathogen without experiencing disease symptoms – have also drawn concern. Viruses running through bats’ veins include those causing rabies, Ebola and SARS.
Evidence suggests the incredible viral diversity found within bats results from their unique metabolism. Being the only mammal capable of flying, bats experience dramatically heightened metabolic activity and body temperatures during flight, ultimately producing conditions similar to fever in other mammals. Raina Plowright, Montana State disease ecologist and co-author, said this boosted immune response prevents bats from getting disease when infected with the same viruses that may endanger humans and other animals.
“Bat and human health are closely linked,” Plowright said, largely because of bats’ useful role eating insects, pollinating plants, and dispersing seeds. But close and frequent contact between bats and people or other animals can potentially lead to spillover events, in which disease is transmitted from the reservoir species to the species it comes in contact with.
“As bat habitats have been cleared, we have seen new viruses emerge from bats to affect domestic animals and humans,” Plowright said. “Habitat loss can send bats into human areas searching for food, leading to closer contact between bats and people.”
However, documentation of disease spillover from bats to people is very rare.
Human population growth and change in land use can also spark spillover events. Some hypothesize deforestation prompted last year’s Ebola outbreak in West Africa as fruit-eating bats were forced to seek food in nearby communities upon the destruction of their natural habitats.
The landmark review is a key step in bat conservation. Understanding the causes and magnitude of bat mortality, after all, is necessary for prevention.
“Maintaining healthy bat populations while preventing human contact is a really important thing for us to do,” said Hayman.
Since bats are long-lived and slow to breed, population stability requires high survival rates in adults. But the onset of white-nose syndrome and a growing wind energy industry threaten these rates. The authors doubt their observations are sustainable. Climate change is also expected to introduce fresh challenges to bats worldwide.
Wind turbine collisions represent the most prevalent cause of mass mortality events since 1790, with the majority of these events taking place after the year 2000. Bats depend on echolocation while navigating, and can mistake turbines for trees, especially when wind farms are located near forests where bats roost.
Efforts to make wind farms safer for bats include turning turbines off during low-wind conditions, and setting out high frequency sound-emitting devices that deter bats. Bladeless turbines are also an alternative, though some doubt their ability to convert wind energy into electricity efficiently.
“Many possible approaches are still in the research and testing phase, …but few are proven ready to use,” said Paul Cryan, USGS research biologist and study co-author.
In the mean time, Cryan said preventing turbine blades from rotating at full speed on gentle-wind nights when bats are active, could reduce bat fatalities by 50 percent or more.
First observed in New York, white-nose syndrome is the second most prevalent cause of mass mortality events, killing at least six million North American bats since its discovery a decade ago. White-nose syndrome appears to be endemic and harmless to bats in parts of Europe and China. Explanations of their resistance remain unclear, as different species may utilize different mechanisms. But attempts to combat the rapidly spreading disease in the United States and Canada have so far been unsuccessful, Hayman said.
The cold-growing fungus Pseudogymnoascus destructans is the culprit behind white-nose syndrome, thriving in chilly caves ideal for hibernation. It fatally infects the skin tissues of hibernating bats, and is apparent by a ring of white growth around the nose.
Infection disrupts hibernation, causing bats to prematurely burn through fat reserves. Though bats withstand infection by some of the world’s most dangerous viruses, this soil fungus could mark the end of some North American species.
Since no treatment yet exists for white-nose syndrome, resource managers are working to educate the public about how people can accidently spread the fungus on their clothing and gear, especially when visiting U.S. National Parks with bat caves. The U.S. Fish and Wildlife Service closes some caves in infected states, or caves with hibernating bats, and they ask that cavers and spelunkers respect their advisories. Scientists who conduct research in bat caves also use decontamination methods and wear special clothing to prevent its spread, Cryan said.
“The fungus probably came to North America with the help of humans, but since its arrival, bats have been probably spreading it through their natural movements,” Cryan said. “We humans are doing our best to not make the situation worse by spreading the fungus farther than the reach of the bats.”
|CORRECTION: A previous version of this story overstated the documented risk that bats pose to people through disease transmission. We regret the error and thank University of Texas ecologist Merlin Tuttle and the “Bat Fans” who read his blog for pointing it out.|