How do urban bats cope with city lights?

The common pipistrelle, Pipistrellus pipistrellus, is a wide-ranging bat, making its home in city and country alike across most of Europe, Northern Africa, and into Southwestern Asia, possibly all the way to Korea. But even though the tiny bat – its mass ranges from just 3.5 to 8.5 grams – is widespread and isn’t a species of conservation concern, it might be a good indicator of how well a species can get along in the city.

The truth is that although many species have found ways of surviving in urbanized landscapes, they’re not all necessarily thriving. For some species, cities are population sinks: critters wander into cities to find food and shelter, but don’t do a very good job of reproducing. Cities may host quite a few of those individuals, but they can’t sustain their own populations. As animals in the city die, they’re replaced by new ones dispersing into the city from more natural areas. Meanwhile for other species, cities are population sources: these are animals who not only can find sufficient food, water, and shelter in cities, but can actually reproduce and raise their young there. Their young then set off and establish new territories within the city as well.

The introduction of artificial lighting in cities has improved human cultures in certain ways, but it can also be detrimental, especially to ecological processes that for millions of years have operated on a day-night circadian cycle. That’s especially true for nocturnal critters like bats.

Something that bats, like the common pipistrelle, need is consistent cover along their movement corridors. The bats roost in their home trees, but then set off each night in search of food. In order to remain safe, the bats have evolved to fly in spaces where there’s a reasonable amount of tree cover. Under the safety of a forest’s canopy, the bats can fly about in relative darkness, free from the prying eyes of their predators. In cities, there’s often a fair amount of tree cover. One might suspect, especially since trees have a way of being planted in rows, that bats ought to be able to take advantage of these leafy highways as they flap and flutter from spot to spot.

But even the greenest of urban parks contains lots of open spaces and lots of artificial light. Previous studies have shown that bats go out of their way to avoid brightly lit areas, preferring the darker shadows. Sometimes, bats come up to a brightly lit gap in tree cover and it’s such a barrier to their travels that they simply turn around. Artificial lighting can disrupt foraging or hunting behaviors for nocturnal species, and it can even impede reproduction.

While some researchers have found that bats avoid the light, the solution isn’t simply to turn off all the lights. For one thing, it’s impractical and disadvantageous in many ways for human safety. For another, artificial lights create artificial buffets for insectivores like bats, offering important hunting opportunities. All of this is to say that when it comes to planning for the welfare of a population of urban animals, there’s more going on than first meets the eye.

So maybe we ought to think of artificial lighting not in binary terms (bright or dark) but in more continuous terms. Maybe bats can handle some level of brightness, with a threshold at which they become uncomfortable. Rather than turning off all the lights, maybe we could simply dim them in critical sections of bat habitat?

Those are the questions that University of Birmingham School of Geography, Earth and Environmental Sciences research associate James D. Hale wanted to answer. To find out, he and his colleagues focused on the West Midlands of the United Kingdom, a highly urbanized metropolitan county with a population of some 2.3 million people in an area of roughly 902 square kilometers. Since common pipistrelles move each night along the edges of tree lines, the researchers surveyed gaps within those tree lines to see if they could figure out under what conditions the bats were willing to cross. They published their findings recently in the journal Global Change Biology.

Of 27 gaps surveyed by the researchers, bats were only willing to cross 19 of them. Even for gaps the bats would risk crossing, nearly all of them (95.6%) did so by sticking to the darkest areas. That is, even if it required taking a slightly longer route across the gap, the bats found it more important to stick to dimmer rather than brighter spots. Using that data, Hale and his colleagues calculated that brighter lighting reduced the landscape accessible to urban bats by as much as 7% compared with dimmer lighting.

Hale argues that although “networks of tree cover along with broader elements of ‘green infrastructure’ are commonly recognized in urban planning policy as ‘wildlife corridors,'” whether wildlife is actually capable of using those corridors is far more complex. The ability of bats to fly from their roosts to their feeding areas is critical to their survival, and commutes longer than one kilometer are routine for them. That means that even if a single bat can roost in a relatively protected urban park, it needs a home range of more than three square kilometers to feed adequately. Whilst bat roosts within the European Union are legally protected under the EU Habitats Directive, the level of protection afforded to commuting routes is less clear,” write the researchers.

The researchers established that networks of urban trees can indeed support the movements of common pipistrelles, even when tree lines are punctuated by gaps up to 80 meters long – but only if those gaps remain fairly dark. The shorter the gap, the more light the bats are apparently willing to tolerate, but only to a point. An 80-meter gap was crossable at an average illumination of 10 lux (the amount of light thrown off by a candle at a distance of one foot), while the bats refused to cross a 20-meter gap that was illuminated at 40 lux (the dimmest light at which you could comfortably read a book). In other words, the brightest acceptable threshold for crossing a fairly short gap was still fairly dim.

And even for gaps that were already poorly lit, bats still chose to stick to the darkest parts, underscoring the idea that even minimal lighting is likely to be detrimental to their well-being in some way, at least along movement corridors. “The strategic dimming of lights in the vicinity of gaps, combined with the narrowing of gaps through tree planting, might therefore be reasonable conservation measures for this species in urban areas,” concludes Hale.

To that end, conservation has historically taken a reactive approach, but now that urban ecological research is becoming more refined, we may be in a position to become more proactive. To do so, the researchers argue, conservation efforts must “move on from detecting broad patterns in urban biodiversity to a more mechanistic understanding of the processes that drive them,” such as by understanding just what it is about the light, and where, and when, that bothers bats. – Jason G. Goldman | 12 June 2015

Source: Hale, J. D., Fairbrass, A. J., Matthews, T. J., Davies, G., & Sadler, J. P. (2015). The ecological impact of city lighting scenarios: Exploring gap crossing thresholds for urban bats. Global Change Biology. DOI: 10.1111/gcb.12884.

Header image: Common pipistrelle in flight via Barracuda1983/Wikimedia Commons.

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