Light pollution hurts urban bats. Trees can help.

You may think that Halloween would be the most contested time of the year, but these winged mammals deserve a second annual celebration.

In honor of the National Bats Recognition Day, we are looking at urban bats and their way of life among us. (Get the truth behind six bat myths.)

Excessive artificial lighting, also known as light pollution, can have a negative effect on many nocturnal animals, for example by disorienting them or disrupting their reproduction.

But that did not stop bats from settling in the cities. For example, 18 of the 25 German bat species live in Berlin, which also includes 20% of forests.

"Trees bring many benefits to bats," including roosts, shelter from wind and predators, and better foraging opportunities, says Tanja M. Straka, researcher at the Leibniz Institute for research in zoo and wildlife.

Straka led a recent study that suggests trees have another benefit: Reduce the impact of street lights on urban bats.

The relationship is complex, but overall, forest cover reduces impacts, such as reduced foraging. or fewer opportunities for rest, on these creatures of the night.

For their research, Straka and his colleagues recorded the activities of five species and groups of bats. flying over 22 green spaces in Berlin in summer 2017.

The team observed how bats responded to street lights powered by ultraviolet (UV) light or light-emitting diodes (LEDs). These lamps were located either in an area with a high tree cover, or in a zone without trees. (Related: "The civic cemetery is alive with shocking numbers of bats and spiders.")

The activity of bats was generally higher around the lit areas and tree cover. But some species reacted differently to both types of lighting with regard to forest cover, according to the study published in March in the newspaper Borders in ecology and evolution.

For example, the common pipistrelle and Nathusius pipistrelle have increased their activity around ultraviolet lamps, "especially when the tree cover is dense," says Straka. This is not surprising: ultraviolet light attracts tasty insects and forest cover protects bats.

All studied bats were less active around LEDs light sources, which attract less insects than ultraviolet rays. But higher flying bat species were more active around LED lamps in wooded areas. (Read the myths about LED lighting.)

The team's decision to focus on lighting at different wavelengths in relation to forest cover was a wise decision, says McCracken, who did not participate in the research. (Learn more about efforts to improve the reputation of bats.)

"It's an interesting study," he says.

As for bats negatively affected by artificial light, they "could possibly leave these regions where the rate of colony reproduction could suffer," adds the co-author Christian Voigt, also a researcher at the Leibniz Institute, by email.

In addition to planting more trees, the most important remedy for bats is to keep their habitat as dark as possible, says Voight.

So why should we support cities in conflict? On the one hand, pest control. Bats eat a lot of stinging insects, such as mosquitoes and midges that thrive around urban wetlands, notes Straka via email. (The small brown bats from the United States, for example, eat up to 1,000 mosquitoes per hour.)

In addition, bats pollinate plants and disperse seeds in community gardens and backyards. They are also "good bioindicators" of the situation of other animals in the area, she says.

You still do not understand urban bats? You can always raise your glass today in front of the long-billed bat of Mexico, the main pollinator of tequila.

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