Novel causes of bat death increase in 21st century

Brown bats with white-nose syndrome. Image credit United States Geological Survey/photo by Nancy Heaslip of the New York Department of Environmental Conservation.

Dr David Hayman

A comprehensive review lead by the United States Geological Survey with Massey University, New Zealand, Montana State University, United States and the University of Glasgow, Scotland has revealed trends in the occurrence and causes of multiple mortality events in bats as reported globally for the past 200 years – shedding new light on the possible factors underlying population declines.

The researchers combed the scientific literature dating from 1790 to 2015 in search of mortality events involving more than 10 bats per event. They then divided these events into nine different categories, spanning a variety of both natural and human causes. In the end, they found and categorised a total of 1,180 mortality events from all over the world, representing more than 200 years of recorded history.

Collisions with wind turbines worldwide and the fungal disease white-nose syndrome in North America lead the reported causes of mass death in bats since the onset of the 21st century. These new threats now surpass all prior known causes of bat mortality – both natural or attributed to humans.

Prior to the year 2000, intentional killing by humans caused the greatest proportion of mortality events in bats globally; the reasons for killing varied by region, but most commonly bats were killed as pests, for food for people, for vampire bat control and to protect fruit crops. Although the proportion of intentional killing declined in recent times, such acts still continue in some parts of the world.

In addition, storms, floods, drought, and other environmental factors also historically caused mass mortality, and are likely to increase in the future due to climate change.

The reports authors say the findings are concerning because bats play vital roles in the ecosystem, such as acting as pollinators or seed dispersers in tropical regions and insect control in most parts of the world. Insect-eating bats are estimated to save farmers billions of dollars each year by providing natural pest control. Also of concern, is that populations of many of the 1300 species of bats on Earth are considered threatened or declining.

Lead author United States Geological Survey Scientist Emeritus Tom O’Shea says the trends in human related deaths are not sustainable. “Bats need high survival rates to ensure stable or growing populations.”

The authors did not find convincing evidence that bats regularly die in large proportion due to infectious diseases caused by viruses or bacteria. This is perhaps surprising as increasing evidence points to bats as natural reservoirs of several viruses that cause disease in humans. Despite often being more gregarious than other animals, bats may somehow avoid diseases that sweep through dense populations.

Co-author Dr David Hayman from Massey University’s Institute of Veterinary, Animal and Biomedical Sciences says bats may somehow avoid large-scale mortality in spite of infection within dense populations.

He says, “determining the most important causes of bat mortality is a first step toward trying to reduce our impact on their populations.”

The researchers conclude bats globally could benefit from policy, education and conservation actions targeting human-caused mortality.

The review was published today in the journal Mammal Review.

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