Steer clear: Bats 3x less active by roads

U. LEEDS (UK) — Bat activity is as much as three times lower near major roadsides, according to a new study that 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 John Altringham, professor of biological sciences at the University of Leeds, 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 reported in the Journal of Applied Ecology, 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 paper’s lead author, walked along 20 routes perpendicular to the motorway, stopping at set points along the way, up to 1.6 kilometers from the road. Using ultrasonic detectors to record echolocation calls as bats flew past, she assessed bat foraging activity levels and then analyzed the recordings to identify different species groups. The researchers took into account factors like 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.”

The road is likely 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 kilometers 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, says Altringham.

“Conservation should be evidence-based. 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.”

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