Over or under the road – Effectiveness of some bat road crossing mitigation measures

Roads have been shown to impact bats negatively. To explore the effectiveness of different road mitigation measures we studied bat activity at two existing underpasses for watercourses at a highway, and at four experiential hop-over sites by the uses of screens, and at a control site in Denmark. The study used synchronized ultrasound recordings, in combination with night vision video and manual observation to describe the flight behaviour of the bats. More than 2500 bat passes were analysed at seven sites.

The study mainly focused on small low flying bats, Myotis daubentonii and Pipistrellus pygmeus. Low-flying bats have the highest risk of fatal road crossing and are most vulnerable to road construction near forest edges, hedges and river crossing are high-risk sites.

Underpasses were only found to be efficient for M. daubentonii and were not used by other bat species. Hop-overs and screens were only efficient in cases where alternatives did not exist. The bats’ behaviour at the hop-over sites was very variable and they may adapt to the new obstacle by change their flight routes. Screens and hop-overs may be used to reduce the risk of fatal road crossing in cases where underpasses are not possible to construct. For both M. daubentonii and P. pygmaeus correct positioning of the screens is of crucial importance and screens placed in the wrong position can be without any effect or even, in worst cases cause negative impact because they forces the bats to cross the road at more risky places or catch the bats between the screens. The studies emphasises the importance of proper planning of mitigation measures and the need for detailed understanding of the bat commuting routes and behaviour.

Road, mitigation, biodiversity, bats, fauna passages