Bats just don't seem to be able to catch a break.
The nocturnal creatures are facing all sorts of potentially lethal threats, from climate change and habitat loss to white nose syndrome to the spinning blades of wind turbines. And an alarming new study shines a light on another threat to bats--the light pollution in our cities.
The study, published online on June 5 in the journal Global Change Biology, shows that bats tend not to move freely in brightly lit areas. Why does that matter, you ask?
"The ability to freely move around is key to individual bat fitness and resilience of the broader bat population," James Hale, a research associate in the school of geography at the University of Birmingham in England and the study's lead author, said in a written statement.
The availability of cheap, energy-efficient lighting could "prove a real problem for bats as they move around a city," he added.
Why bats matter. Even if you're not a big fan of bats, that could spell trouble on a number of fronts. Bats play a key role in global ecology, pollinating plants and providing the guano that acts as a fertilizer, as well as eating bugs that decimate our crops and suck our blood.
Watching bats. For the study, Hale and a group of colleagues that included scientists from Lancaster University conducted field observations of bats in England's West Midlands region. The scientists observed bats as they left their roosts at night and "commuted" along lines of trees to reach areas where they could find plenty of bugs to eat.
The observations showed that while the bats would traverse narrow gaps between trees even in the presence of strong lighting, they would not cross bigger gaps even with low levels of light.
"Even bats that are broadly tolerant of urbanization are still disturbed by lighting," Hale told The Huffington Post in an email.
And while the scientists focused their efforts on a common species of bat known as Pipistrellus pipistrellus, Hale said in the email that other species, including some in North America, would likely have a similar response to light.
Good news? If the finding is a bit of a bummer for bat lovers and conservation-minded people, it could also bring new efforts to help urban bats.
As Gemma Davies, a Lancaster University scientist involved in the research, explained in the statement, "The logical next step... would be to feed these findings into the town and city planning process, by identifying areas where bat populations are low and strategically dimming or shielding street lamps and narrowing gaps" between trees.
"Adding shields, switching off lamps, or dimming them at certain times are all practical ways to save energy, reduce glare, and broader lighting nuisance and to create a more natural environment for local wildlife," Hale said in the email. "Special care needs to be taken near to habitat corridors that can act as highways for nocturnal wildlife."
Posted on 05 Jun 2015
Researchers at the University of Birmingham have discovered that bats living in a city are less likely to move from tree to tree in brightly lit areas, according to research published online today (5th June 2015) in the journal Global Change Biology.
To maintain high biodiversity in cities, wildlife must be able to move between patches of habitat, which are often separated by paved surfaces, buildings and roads. The bats studied in this experiment emerge in the evening from their roosts, often within residential housing areas, to feed on small insects in gardens, streams and other green spaces. To reach these feeding areas they often “commute” along lines of trees, which are thought to provide protection from predators and high winds.
The researchers, along with colleagues from Lancaster University, were studying the impact of artificial light on the bat species, Pipistrellus pipistrellus (the Common pipistrelle), that is found in many parts of UK cities. They wanted to find out whether the bats’ crossing behaviour was affected by the distance between trees and the brightness of lighting within the intervening gap. They found that the bats crossed via the darker parts of the gaps, but with sufficient lighting these gaps became barriers to movement. Importantly, this barrier effect varied with the width of the gap in the tree line - bats would tolerate strong lighting in narrow gaps, but even low levels of lighting in larger gaps was enough to stop them crossing.
James Hale, from the University of Birmingham’s School of Geography, Earth and Environmental Sciences, who led the study, said: ‘The ability to freely move around is key to individual bat fitness and resilience of the broader bat population. Intensification and expansion of lighting due to the availability of cheaper and more energy efficient lighting technologies could prove a real problem for bats as they move around a city. Understanding the factors that affect movement between habitat patches is therefore important for urban species survival and conservation. Our models predict that movement would be most restricted in the urban centre, which might explain why even this common species of bat is rarely found in intensively developed areas.’
He continued: ‘We have focussed our study on the Common pipistrelle, but the flight behaviour of several other bat species may be influenced by artificial lighting. More research is now needed to explore the potential disruption of movement for other species.’
Gemma Davies of Lancaster University’s Environment Centre, a co-author on the study, said: ‘We used Geographic Information Systems to combine data about the illumination from artificial lights, with the distance from trees, to predict the landscape resistance to bat movement. The logical next step for this research would be to feed these findings into the town and city planning process, by identifying areas where bat populations are low and strategically dimming or shielding street lamps and narrowing gaps in the local tree networks.’
This work was supported by the UK Engineering and Physical Sciences Research Council through the Urban Futures project, Mapping Artificial Lightscapes project and the Liveable Cities programme.
Abstract
As the global population urbanizes, dramatic changes are expected in city lighting and the urban form, which may threaten the functioning of urban ecosystems and the services they deliver. However, little is known about the ecological impact of lighting in different urban contexts. Movement is an important ecological process that can be disrupted by artificial lighting. We explored the impact of lighting on gap crossing for Pipistrellus pipistrellus, a species of bat (Chiroptera) common within UK cities. We aimed to determine whether the probability of crossing gaps in tree cover varied with crossing distance and lighting level, through stratified field surveys. We then used the resulting data on barrier thresholds to model the landscape resistance due to lighting across an entire city and explored the potential impact of scenarios for future changes to street lighting. The level of illumination required to create a barrier effect reduced as crossing distance increased. For those gaps where crossing was recorded, bats selected the darker parts of gaps. Heavily built parts of the case study city were associated with large and brightly lit gaps, and spatial models indicate movement would be highly restricted in these areas. Under a scenario for brighter street lighting, the area of accessible land cover was further reduced in heavily built parts of the city. We believe that this is the first study to demonstrate how lighting may create resistance to species movement throughout an entire city. That connectivity in urban areas is being disrupted for a relatively common species raises questions about the impacts on less tolerant groups and the resilience of bat communities in urban centres. However, this mechanistic approach raises the possibility that some ecological function could be restored in these areas through the strategic dimming of lighting and narrowing of gaps.
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