Action: Install overpasses as road crossing structures for bats
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- Three studies evaluated the effects of installing overpasses as road crossing structures for bats. Two studies were in Europe and one in Australia.
COMMUNITY RESPONSE (1 STUDY)
- Richness/diversity (1 study): One site comparison study in Australia found that the same number of bat species were recorded at an overpass and in nearby forest and bushland.
POPULATION RESPONSE (0 STUDIES)
USAGE (2 STUDIES)
- Use (2 studies): One replicated, site comparison study in Ireland found that three bat species used overpasses but up to three-quarters of bats crossed the road below at traffic height. One study in the UK found that an overpass with planters was used by two-thirds of crossing bats, and an unvegetated overpass with a paved road over it was not used by crossing bats.
Overpasses (solid structures such as bridges built for pedestrians or vehicles) may help to guide bats safely over roads. This would both reduce the number of bats killed on roads and increase the permeability of roads for bats to maintain connectivity across the landscape. Studies have been summarised below if they provide data that can be used to assess effectiveness, such as a control or the proportion of bats that are or are not using overpasses.
For evidence relating to bat gantries/bridges (wire or mesh structures), see ‘Install bat gantries or bat bridges as road crossing structures for bats’.
Supporting evidence from individual studies
A replicated, site comparison study in 2008 at six overpasses over a road in agricultural and woodland habitat in southern Ireland (Abbott et al 2012) found that three bat species or species groups flew over overpasses but 39–75% of activity was recorded over the road below them, and lower activity was recorded on overpasses than in adjacent habitats. Overpasses were used by three bat species or species groups (common pipistrelle Pipistrellus pipistrellus, soprano pipistrelle Pipistrellus pygmaeus, Myotis spp.), but 39% of common pipistrelle passes, 49% of soprano pipistrelle passes, and 75% of Myotis spp. passes were recorded over the road below overpasses. Bat activity was lower (by >10%) on overpasses than in adjacent habitats (data reported as statistical measures). The overpasses (8–11 m wide x 6–11 m high x 58–76 m long) had minor roads over them. The motorway (65–70 m wide) had four lanes carrying an average of 20,000 vehicles/day. Bat detectors recorded bat activity above and below each of the six overpasses and simultaneously at two adjacent linear features on two nights in May–September 2008.
A study in 2013 at two overpasses over two roads in the UK (Berthinussen & Altringham 2015) found that an ‘environmental’ bridge was used by almost two-thirds of crossing bats, but an unvegetated overpass carrying a paved road was not used by crossing bats and only three bats crossed the road nearby. Significantly more bats used an ‘environmental’ bridge (62%, 118 of 192 bats) than crossed the road below at traffic height (19%, 36 of 192 bats). An unvegetated overpass was not used by crossing bats and only three bats were observed crossing the road nearby (12–20 m away) at heights of 2–20 m. Both overpasses were designed as crossing structures for bats, alongside other purposes. The ‘environmental’ bridge (30 m long x 5 m wide x 6 m high) had solid vertical sides (2 m high) and was covered with deadwood and planters of hawthorn Crataegus monogyna. The overpass (40 m long x 15 m wide x 8 m high) had a paved road over it and no vegetation. It was designed to carry traffic and provide a crossing structure for bats. Observations of crossing bats and recordings of bat calls were made during 6 x 60 minute surveys at dusk or dawn at each overpass and the road below in June–August 2013.
A site comparison study in 2014–2015 of one vegetated overpass over a road within a forest reserve in Brisbane, Australia (McGregor et al. 2017) found that the overpass had higher bat activity but the same number of bat species as adjoining forest and bushland. More bat passes were recorded on the overpass (average 52 bat passes) than in the adjoining forest (27 bat passes) or bushland (29 bat passes), although no statistical tests were carried out. Nine bat species were recorded on the overpass and in the adjoining forest and bushland. The overpass was hourglass shaped (70 m long x 15 m wide at the midpoint and 20 m wide at the ends) and was planted with natural vegetation and mature saplings (70 shrubs and six trees/100 m2). It was built over two dual lanes of a major urban road bisecting forest and bushland. Bat activity was recorded using bat detectors over two consecutive nights/month between December 2014 and July 2015 at two stationary points on the overpass and along eight 75 m transects perpendicular to the road.
- Abbott I.M., Butler F. & Harrison S. (2012) When flyways meet highways - The relative permeability of different motorway crossing sites to functionally diverse bat species. Landscape and Urban Planning, 106, 293-302
- Berthinussen A. & Altringham J. (2015) WC1060: Development of a cost-effective method for monitoring the effectiveness of mitigation for bats crossing linear transport infrastructure. Department for Environment, Food and Rural Affairs (Defra), UK report.
- McGregor M., Matthews K. & Jones D. (2017) Vegetated fauna overpass disguises road presence and facilitates permeability for forest microbats in Brisbane, Australia. Frontiers in Ecology and Evolution, 5, 153