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Roads

Bats Crossing sign

Roads, Bat Bridges and Gantries: A position statement from BCT

The construction of roads has the potential to negatively impact bat populations, through loss of roosts, foraging habitats and by severing landscape elements used as commuting routes by bats. Roads create an open space, which most bat species are reluctant to cross. Traffic further increases the barrier effect due to sudden movement, noise, headlamps, street lighting and the risk of collision. Most species of bat fly relatively close to the ground or close to trees and hedges for protection against the weather and potential predators. Those that do cross roads typically do so at traffic height, with a high risk of collision. Recent research shows that roads also have a major negative impact on bat foraging activity and diversity1.

Roads can impact bats in the following ways:

  • Fragmenting habitat through barrier effects
  • Severing commuting routes
  • Collision from traffic
  • Disturbance from lights2
  • Sudden movement and noise
  • Decreasing bat foraging activity - feeding areas can be lost or reduced in quality along the path of the road and surrounding area
  • Decreasing bat diversity

Bats are afforded protection by European and UK law in an effort to help bat populations recover from the devastating losses sustained in the last century. Mitigation for the impacts of roads is therefore an essential part of helping to ensure the survival of our bat species.

Mitigation

Eco-passages in the form of different types of under-passes (tunnels and culverts) and overpasses (hop-overs, elevated verges and green bridges) are important for providing safe crossing points for all types of wildlife, including bats. The effectiveness of such schemes in helping biodiversity should be robustly monitored, pre and post-construction, to enhance the design of future mitigation.

Wire or mesh structures placed at height over roads, known as bat gantries or bat bridges, have been proposed as artificial road crossing structures for bats and have been erected as mitigation over many roads in the UK and Europe. However, a lack of appropriate monitoring has meant there is little evidence for their effectiveness.

A new study3 demonstrates that bat gantries in northern England have been ineffective for this purpose (even nine years after construction for one gantry) as the presence of gantries did not effectively increase the height at which bats crossed the road. This study was completed on 4 gantries located directly on or up to 90m away from the original commuting route. The authors conclude that wire gantries should not be used.

The Bat Conservation Trust advises caution in the use and siting of bat gantries. We are keen to see further pre- and post-construction monitoring and research across the UK and Europe to consolidate knowledge on what constitutes effective mitigation for the fragmentation of commuting routes by roads. We would encourage more investigation of natural crossing points such as hop-overs, elevated verges and green bridges (also known as land or habitat bridges) that could deliver a benefit for the bat species involved, and also for other species impacted by such developments. This is an approach often used in Europe where the use of ‘green bridges’ is more widely adopted. The effectiveness of such schemes in helping biodiversity should be properly monitored to enhance the design of future mitigation in the UK.

ENDS

[1] Berthinussen, A. and Altringham, J. 2011. The effect of a major road on bat activity and diversity. Journal of Applied Ecology 49 (1): 82-89

[2] Stone, E.L., Jones, G., & Harris, S. 2009. Street lighting disturbs commuting bats. Current Biology 19:1-5

[3] Berthinussen, A. and Altringham, J. 2012. Do bat gantries and underpasses help bats cross roads safely? PLoS ONE 7(6): e38775. Doi:10.1371/journal.pone.0038775

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