Fortunately, this disappearance is only seasonal. Bats are essential to the functioning of healthy ecosystems. They help recycle nutrients in the environment and pollinate plants. They also eat agricultural pests, which reduces the need for pesticides.
Bats bring enormous value to our ecosystems, but because they do their work under cover of darkness, we are not always aware of the help they provide.
More worrying than this seasonal disappearance is the fact that bat populations have been declining in North America for decades. Habitat loss due to forestry, urbanization and conversion of land to agriculture reduces suitable habitats for bats, while the application of pesticides kills the insects they feed on.
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These impacts are exacerbated by the fungus, Pseudogymnoascus destructans, which causes white nose syndrome. This deadly fungus is responsible for the death of more than six million bats in North America.
White-nose syndrome has been particularly devastating in eastern Canada where it has caused a decline of more than 90% in populations of little brown myotis (Myotis lucifugus) and northern myotis (Myotis septentrionalis).
The fungus is making its way further west, with the first recorded case in Saskatchewan in July. White nose syndrome has yet to be detected in British Columbia, but the deadly threat is looming.
Our research team at the British Columbia node of the Canadian Wildlife Health Cooperative has been working to support wildlife health for over a decade. To understand the threats currently facing the 15 species of bats living in British Columbia, we studied 275 bats that died between 2015 and 2020. We found that the most common causes of death were related to human activity.
This information can help us track bat populations over time and in response to urbanization and climate change. To help bats live, we need to know why they die.
A quarter of the bats in our study were killed by cats. It was not surprising – domestic cats are well-known predators of wildlife. In Australia, free-range pet cats are estimated to kill 390 million animals a year.
Free-roaming cats not only pose a risk to bats, but also to biodiversity. Some cities in Iceland have implemented a curfew for cats to save their dwindling bird populations.
One of our most surprising findings is that most of the bats killed by cats were female and in relatively good physical condition. This higher proportion of dead female bats could be due to cats entering maternity roosts where female bats give birth and raise their young.
Since bats have relatively few young each year, the death of healthy female bats has disproportionate implications for their population in the future.
The simplest solution here is to keep house cats indoors and monitor the time the cats spend outdoors. Cats only bring home about 20% of their prey, so owners are likely unaware of the extent of their feline friends’ hunting habits.
Recent research suggests that these actions are greatest near forested areas. Cats were found to be more likely to prey on wildlife within 500 meters of forests rather than further away. Focusing on managing cats that live near forested areas could be one way to minimize wildlife risk.
Keeping cats indoors also has benefits for cats: indoor cats live longer than cats that live outdoors.
Half of the bats in our study died of human-related causes. This is partly because the bats we studied were submitted to our lab by members of the public. Most of the bats in our study (90%) were synanthropic species, those that live alongside humans.
Reflecting these close contacts, an additional 25% of the bats in this study died as a result of blunt trauma, such as collisions with vehicles or garage doors.
Interestingly, bats that died this way were more likely to be males. The reason isn’t entirely clear, but research suggests that males can fly farther than females, increasing their likelihood of collisions with cars or buildings.
Understanding sex differences in mortality is useful as it can inform conservation and management. For example, identifying where bats fly and how far could determine where to build new roads. Creating wildlife crossings in areas frequented by bats could also help reduce fatalities.
An incomplete picture
The study of wildlife is not easy. Bats roost in many different places, from caves to barns to attics, and scientists can’t monitor bats everywhere and all the time.
Community reports contribute to information gathered about bats and help us understand the health of local bat populations.