This raises serious issues for how road construction projects mitigate their impact on these protected species.
The findings - published today in the Journal of Applied Ecology - show that the negative impact of a major road stretches a considerable distance, with bat activity three times lower at the roadside than 1.6km away. As bats are protected under UK and EU legislation, the research could have legal consequences for road builders.
"UK and European law protects all species, so construction work must not have a negative impact on bat populations," says Professor John Altringham from Leeds' Faculty of Biological Sciences, who led the research. "This study shows that the impact of roads on bats is far-reaching, and road construction projects must take this into account or they are potentially breaking the law."
The study measured bat activity and diversity along unlit sections of the M6 motorway in Cumbria, in the North of England, which carry between 30,000 and 40,000 vehicles a day.
PhD student Anna Berthinussen, the lead author of the paper, walked along 20 routes perpendicular to the motorway, stopping at set points along the way, up to 1.6km from the road. Using ultrasonic detectors to record echolocation calls as bats flew past, she assessed bat foraging activity levels and then analysed the recordings to identify different species groups. The researchers took into account other factors such as the time after sunset, habitat and weather.
Just under 3,500 'bat passes' were recorded and three main groups of bat species identified - Pipistrellus, Myotis and Nyctalus. Activity gradually increased as the researchers got further from the road, with three times as much activity measured at 1.6km relative to at the roadside. Although their numbers declined, Pipistrellus bats were recorded at all locations, but Myotis were mainly seen at further distances from the road.
"The results were really clear cut when all other factors were taken into account, showing a very strong correlation between bat activity and diversity and distance from the road," says Anna. "Bat activity showed no sign of levelling off before the last recorded point, so it's likely that activity would continue to increase beyond the distance set for this study."
Professor Altringham believes the road is acting as a physical barrier to the bats, cutting off colonies from established foraging sites thereby reducing the area and quality of the available habitat. "Most species of bat fly relatively close to the ground, or close to trees and hedges, so they are reluctant to cross a wide open space such as a major road, particularly when it is occupied by heavy, fast moving traffic. If they do attempt to cross, it is typically at traffic height, with a high risk of collision. Loss of habitat and increased mortality will both lead to population decline."
"Most bat species forage up to about three kilometres from their roost. If a road cuts across their home range, reducing access to part of it, they will struggle to find sufficient food unless the colony relocates away from the road, putting them in competition with other colonies," he says. "If they stay, reduced food supplies will mean less successful breeding. Either way, it will be some time before the impact on population size is seen, since bats can live for 10-15 years or more and reproduce slowly."
"New road schemes often incorporate mitigation for their impact on wildlife, such as the 'bat bridges' or 'bat gantries' recently proposed for the A11 in Suffolk*, which are supposed to make roads safe and more 'permeable'. Sadly, we have little or no evidence for the effectiveness of these measures. Monitoring standards are poor and mitigation methods are essentially unproven. If we want to have any confidence in the effectiveness of mitigation, such as bridges, underpasses and tree-planting, we need to see major improvements in the quality and application of pre- and post- construction monitoring."
The results are relevant to small insectivorous bats worldwide and highlight the impact of roads on wildlife in general. They also highlight a widespread concern among conservation scientists. "Conservation should be evidence-based," says Professor Altringham. "We need to look more objectively at the impact we have on the natural world, and at the effectiveness of conservation efforts, if we are to make best use of the limited resources conservationists have at their disposal."
Graham Askew, Simon Walker, BBSRC (Jan 2018), £699,781
Jennifer Tomlinson, Royal Society (Jan 2018), £512,801
Jennifer Tomlinson, Royal Society-Research Fellows Enhancement Award (Dec 2017), £94,681
Helen Miller, AB AGri Grant (Dec 2017), £73,600
Simon Walker, Royal Society Enhancement Award (Dec 2017), £10,000
Carrie Ferguson, Bryan Taylor, Harry Rossiter, The Physiological Society (Dec 2017), £7,392
Ralf Richter, Royal Society (Dec 2017), £6,000
Christine Foyer, British Council Newton Fund (Dec 2017), £49,840
Adrian Whitehouse and colleagues in School of Chemistry and University of Liverpool, MRC (Nov 2017), £622,319
Michelle Peckham, Neil Ransom, MRC (Nov 2017), £495,159
Dave Lewis, British Council India (Nov 2017), £22,540
Elton Zeqiraj, Royal Society (Nov 2017), £15,000
Hannah Dugdale, Royal Society (Nov 2017), £15,000
Shaunna Burke, Cancer Research UK Innovation Grant (Nov 2017), £20,000
Alex O'Neill and colleagues in Chemistry, BBSRC (Nov 2017), £431,865
Jessica Kwok, Wings for Life (Nov 2017), £87,365
Tom Bennett, BBSRC (Oct 2017), £523,679
Neil Ranson, Darren Tomlinson, BBSRC (Oct 2017), £494,318
Nikita Gamper, BBSRC (Oct 2017), £490,426
Amanda Bretman and colleagues from UEA, NERC (Oct 2017), £430,886
Juan Fontana, Rosetrees Trust consumables grant (Oct 2017), £22,500
Helen Miller, DSM Nutritional Products AG (Sep 2017), £69,988
Neil Ranson, Juan Fontana, Mark Harris, Michelle Peckham, Ralf Richter, Peter Stockley, Patricija Van Oosten-Hawle and colleagues in Engineering, FMH and MAPS, Wellcome Trust Equipment Call (Sep 2017), £418,000
Jamie Johnston, Physiological Society (Sep 2017), £10,000
Frank Sobott, Adrian Goldman, Mark Harris, Andrew Macdonald, Stephen Muench, Sheena Radford and colleagues in FMH and MAPS, Wellcome Trust Equipment Call (Aug 2017), £415,000
Ralf Richter, David Brockwell, Eric Hewitt, Jessica Kwok, Emanuele Paci and MAPS/FMH, BBSRC (Jun 2017), £600,000
Eric Blair, Adrian Whitehouse, Nicola Stonehouse, Alison Baker, Richard Bayliss, Joan Boyes, Ryan Seipke, Sally Boxall and MAPS/FMH, BBSRC (Jun 2017), £376,000
Stefan Kepinski, Yoselin Benitez-Alfonso, Tom Bennett, Michelle Peckham, BBSRC (Jun 2017), £331,000
Roman Tuma, Lars Jeuken, Paul Millner, Sheena Radford, Peter Stockley and MAPS/FMH, BBSRC (Jun 2017), £222,000
Vas Ponnambalam, Darren Tomlinson, Stephen Wheatcroft, BHF (May 2017), £107,878
Graham Askew in collaboration with Bangor University, BBSRC (Mar 2017), £477,383
Stephen Muench, BBSRC (Mar 2017), £132,945
Nic Stonehouse, MRC (Mar 2017), £906,341
Bill Kunin, Steve Sait, BBSRC (Mar 2017), £602,831
Adrian Goldman, EU (Mar 2017), £546,576
Sheena Radford, Wellcome Trust (Mar 2017), £1,836,482
Beatrice Filippi, Royal Society (Mar 2017), £15,000
Tom Bennett, Royal Society (Mar 2017), £15,000
Jamie Johnston, Royal Society (Mar 2017), £15,000
Ryan Seipke, BBSRC (Feb 2017), £52,116
Mary O'Connell, BBSRC (Feb 2017), £46,986
Hannah Dugdale, NERC (Feb 2017), £504,138
Anastasia Zhuravleva, EPSRC (Jan 2017), £100,792