Install overpasses over roads/railways

Overall effectiveness category Awaiting assessment
Number of studies: 5
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Background information and definitions

To mitigate the effects of roads on wildlife vegetated, vehicle-free overpasses may be constructed. These tend to be constructed for mammals but could also be designed for or used by reptiles.

Wildlife overpasses are constructed to provide safe road and rail crossing opportunities for wildlife. A range of different structures can be used as overpasses including purpose-built “green bridges”, on which natural vegetation is established, through to general purpose crossing structures that are accessible to both wildlife and vehicles. Overpasses are often used in combination with wildlife barriers that prevent animals accessing the road and which funnel animals toward the overpasses (see Install barriers and crossing structures along roads/railways). Studies summarized within this intervention cover both overpasses created specifically for wildlife and those that were created for other purposes but where information about use of such structures by reptiles is included. Studies mostly report on the use of such structures, such as the number of crossings made, rather than on wider population-level effects of their presence.

Study locations

Key messages

Five studies evaluated the effects of installing overpasses over roads/railways on reptile populations. Three studies were in Spain, one was a review of studies in Australia, Europe and North America and one study was in Australia.

COMMUNITY RESPONSE (1 STUDY)

Community composition (1 study): One before-and-after, site comparison study in Australia found that the composition of reptile species on a vegetated overpass was more similar to woodland on one side of the overpass than the other.
Richness/diversity (1 study): One before-and-after, site comparison study in Australia found that a vegetated overpass was colonised by two reptile species each year over five years.

POPULATION RESPONSE (1 STUDY)

Occupancy/range (1 study): One before-and-after, site comparison study in Australia found that a vegetated overpass was colonized by 14 of 23 native reptile species and one non-native reptile species.

BEHAVIOUR (4 STUDIES)

Use (4 studies): Three of four studies (including two replicated studies and one review) in Spain and Australia, Europe and North America found that overpasses not designed for wildlife were used by lizards and snakes and reptiles. The other study found that overpasses not designed for wildlife were not used by snakes or lizards. Two replicated studies in Spain found that wildlife overpasses were used by lizards and Ophidians (snakes and legless lizards), and one review in Australia, Europe and North America found that one of 10 wildlife overpasses were used by reptiles. One review of road crossing structures in Australia, Europe and North America found that a rope bridge was not used by reptiles.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A study in 1991–1992 along a high-speed railway within agricultural land in Castilla La Mancha, Spain (Rodriguez et al. 1996) found that two overpasses not designed for wildlife were used to cross the railway by reptiles. Lizards and snakes were recorded making a total of 112 crossings using two overpasses and 15 underpasses, 7 crossings/100 passage-days on average. Reptile use of overpasses was relatively lower than underpasses (results reported as model outputs, see original paper). Two overpasses (small roads) crossing a 25 km section of a high-speed railway were monitored. The railway was fenced with wire netting on both sides to limit access to the rails. To monitor animal tracks, a layer of sand (3 cm thick and 1 m wide), was put at one entrance to each overpass, and tracks were monitored for 15–22 days/month between September 1991 and July 1992.
Study and other actions tested
Referenced paper
Rodriguez A., Crema G. & Delibes M. (1996) Use of non-wildlife passages across a high speed railway by terrestrial vertebrates. Journal of Applied Ecology, 33, 1527-1540.
A replicated study in 2002 of a highway in Zamora, Spain (Mata et al. 2005, same experimental set-up as Mata et al. 2008) found that wildlife and other overpasses were used by reptiles. Lacertid lizards (Lacerta spp. and Podarcis spp.) were recorded crossing wildlife overpasses (0.5 crossings/day/structure) and lacertids and ophidians (snakes and legless lacertids) were also recorded crossing other overpasses, such as rural tracks (lacertids: 0.4 crossings/day/structure, ophidians: 0.1). Two wildlife overpasses (16 m wide, 60 m long) and 16 general overpasses (rural tracks, 7–8 m wide, 58–62 m long) were monitored along a 72 km section of the A-52 motorway. The motorway had barrier fencing along its length. Marble dust (1 m wide across) was used to record animal tracks for 10 days in June–September 2002. Camera traps were installed on some overpasses.
Study and other actions tested
Referenced paper
Mata C., Hervàs I., Herranz J., Suàrez F. & Malo J.E. (2005) Complementary use by vertebrates of crossing structures along a fenced Spanish motorway. Biological Conservation, 124, 397-405.
A replicated study in 2001 of a highway in Zamora province, Spain (Mata et al. 2008 same experimental set-up as Mata et al. 2005) found that wildlife, but not other overpasses were used by some reptiles. Ophidians (snakes and legless lacertids) were recorded crossing wildlife overpasses (0.3 crossings/day/structure) but not other overpasses, such as rural tracks. Lacertid lizards (Lacerta spp. and Podarcis spp.) were not recorded using any overpasses. Four wildlife overpasses (15–20 m wide, 60–62 m long) and six general overpasses (rural tracks, 7–8 m wide, 58–65 m long) were monitored along the A-52 motorway. The motorway had barrier fencing along its length. Marble dust (1 m wide cross) was used to record animal tracks daily for 10 days in March–June 2001.
Study and other actions tested
Referenced paper
Mata C., Hervàs I., Herranz J., Suàrez F. & Malo J.E. (2008) Are motorway passages worth building? Vertebrate use of road-crossing structures on a Spanish motorway. Journal of Environmental Management, 88, 407-415.
A review in 2010 of studies monitoring 329 road crossing structures in Australia, Europe and North America (Taylor & Goldingay 2010) found that reptiles used overpass crossing structures in three of 15 studies. Reptiles were recorded using overpasses in two of 15 studies and wildlife overpasses in one of 10 of the studies (in one study reptiles were present but did not use the structure). One study of a rope bridge did not record any reptiles. The use of overpasses, wildlife overpasses and canopy-rope bridges by wildlife was reported for 15 studies.
Study and other actions tested
Referenced paper
Taylor B.D. & Goldingay R.L. (2010) Roads and wildlife: impacts, mitigation and implications for wildlife management in Australia. Wildlife Research, 37, 320-331.
A before-and-after, site comparison study in 2005–2010 in eucalypt forest and woodland next to a highway in Queensland, Australia (McGregor et al. 2015) found that a vegetated overpass was colonised by reptile species native to the area. Fourteen of 23 native reptile species found in the area were captured on the vegetated overpass. One non-native reptile species was captured on the overpass but not in adjacent woodland. Capture data over time indicated that the overpass had been colonized at a rate of two species/year. Community composition on the overpass tended to be more similar to woodland on one side of the overpass than the other. A vegetated, fenced overpass was constructed in 2005 and planted with native vegetation sourced from local woodlands. Six woodland sites <1 km from and on both sides of the vegetated overpass were surveyed from June 2005–February 2010 and one site on the overpass was surveyed from February 2006–February 2010. Reptile data were collected from pitfall traps constructed of 15 m drift fences and three 20 L buckets, and hand searches for three days and two nights every two months. Animals were not marked and released immediately after identification.
Study and other actions tested
Referenced paper
McGregor M.E., Wilson S.K. & Jones D.N. (2015) Vegetated fauna overpass enhances habitat connectivity for forest dwelling herpetofauna. Global Ecology and Conservation, 4, 221-231.

Please cite as:

Sainsbury K.A., Morgan W.H., Watson M., Rotem G., Bouskila A., Smith R.K. & Sutherland W.J. (2021) Reptile Conservation: Global Evidence for the Effects of Interventions for reptiles. Conservation Evidence Series Synopsis. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

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This Action forms part of the Action Synopsis:

Reptile Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Retain/maintain road verges as habitat Action Link	No evidence found (no assessment)	0	Synopsis Link
Install overpasses over roads/railways Action Link	Awaiting assessment	5	Synopsis Link
Install tunnels/culverts/underpasses under roads/railways Action Link	Awaiting assessment	15	Synopsis Link