Background information and definitions
Wildlife crossings over or under roads may be installed to reduce the impact of the road on animal mortality and on habitat fragmentation. They usually take the form of tunnels or bridges of a range of designs. These may not be suitable for use by mammals that spend most of their time higher up within trees. Rope bridges have been trialled, especially in Australia, to provide a means of reconnecting habitat and reducing road mortality for arboreal mammal species. Monitoring typically takes the form of documenting use of crossings rather than looking at population level effects or impacts on road mortality.
Supporting evidence from individual studies
A study in 2000–2002 along a road through highland rainforest in Queensland, Australia (Goosem et al. 2005) found that all three rope bridges across the road were used by arboreal marsupials. Across the three rope bridges, six species of possums, Lumholtz’s tree kangaroos Dendrolagus lumholtzi and fawn-footed melomys Melomys cervinipes were recorded, with 5–7 species/crossing recorded. The number of crossings was not documented. In 1995, a canopy bridge tunnel was erected 7 m above a 7-m-wide tree gap over a low-traffic road (4 vehicles/day). The bridge comprised a 50 × 50-cm rope tunnel, 14 m long, made of 10-mm silver rope attached to wooden poles, erected amongst trees on the roadside. In 2000, a 10-m-long, 50-cm-wide rope-bridge was erected 7 m high, spanning a 5-m gap over a forestry track. Additionally, a 25-cm-wide rope ladder was placed initially over the same track, then lengthened and moved in 2001 to span a 14-m-wide gap over a road carrying 150 vehicles/day. Mammal crossings were monitored in 2000–2002, through scat and hair analysis, remote photography and spotlighting surveys.Study and other actions tested
Referenced paperGoosem M., Weston N. & Bushnell S. (2005) Effectiveness of rope bridge arboreal overpasses and faunal underpasses in providing connectivity for rainforest fauna. Proceedings of the 2005 International Conference on Ecology and Transportation, Center for Transportation and the Environment, North Carolina State University, Raleigh NC, USA, 304-318.
A study in 2000–2010 of four roads through rainforest in Queensland, Australia (Weston et al. 2011) found that all seven rope bridges connecting trees at each side of the road were used and nine mammal species in total were recorded. Of these, five species were directly observed crossing bridges. The remaining four were detected solely by other monitoring methods. Totals of 2–7 species/rope bridge were recorded. No mammals were found dead on roads in the vicinity of rope bridges (though details of searches for casualties are not stated). Seven rope bridges in total were erected at four sites in 1995–2005. Two were rope tunnels, with a square cross-section. The remainder were rope ladders, 0.25–0.5 m wide. Mammal use of bridges was monitored by direct observation by spotlight, faeces collected in nets or funnels below bridges, motion- and heat-sensitive cameras and hair collection using sticky tape.Study and other actions tested
A site comparison study in 2010–2011 at three overpasses along a road through forest in Queensland, Australia (Taylor & Goldingay 2012) found that squirrel gliders Petaurus norfolcensis, a brushtail possum Trichosurus vulpecula and a ringtail possum Pseudocheirus perigrinus used a rope bridge that connected between glider poles across the overpass. Squirrel gliders were detected using the rope bridge on 33 occasions during 27 of 166 survey nights. Over the same period, one brushtail possum and one ringtail possum were detected. No gliders crossed two overpasses that did not have glider poles or rope bridges. The study was conducted on an overpass (36 × 15 m, constructed in 2008) with eight glider poles, 6.5 m high, connected by a single rope (40 mm diameter). Two overpasses without poles or a rope bridge (62–66 m long, 19–37 m wide) were also monitored. Mammal crossings were surveyed using camera traps between September 2010 and April 2011. A camera was placed near the top of one end pole and directed along the connecting rope. Cameras were also placed in the middle of overpasses without poles.Study and other actions tested
A replicated study in 2008–2011 of five rope bridges at four sites along a highway through woodlands in New South Wales, Australia (Goldingay et al. 2013) found that rope bridges were used by six mammal species. Bridges were used by squirrel gliders Petaurus norfolcensis (44 records at two bridges), feathertail gliders Acrobates pygmaeus (nine records at three bridges), common ringtail possums Pseudocheirus peregrinus (seven records at one bridge), common brushtail possums Trichosurus vulpecula (33 records at two bridges), sugar gliders Petaurus breviceps (15 records at two bridges) and black rats Rattus rattus (19 records at two bridges). Two rope bridges across the highway (42–75 m long) were monitored at one site. Single bridges (each approximately 50 m long), crossing creeks underneath the highway at each of two sites, were monitored. At the fourth site, a rope bridge was suspended from a series of poles along a 70-m-long land bridge over the highway. Sites were up to 270 km apart. Bridges, erected in 2004–2008, comprised rope mesh either laid flat or formed into tunnels. They were monitored by 1–3 camera traps/bridge for 42–503 nights/camera.Study and other actions tested
A replicated, site comparison study in 2007–2011 along a highway in Victoria, Australia (Soanes et al. 2013) found that canopy rope bridges across highways, along with glider poles, were used by squirrel gliders Petaurus norfolcensis. Three of seven squirrel gliders crossed roads when canopy bridges were present. The proportion of squirrel gliders crossing roads where canopy bridges or glider poles were installed (29%) was higher than that which crossed roads when such structures were absent (0%). However more still (70%) crossed at a narrow, single-lane road with low traffic flows and no artificial crossing structures. Camera traps recorded 1,187 crossings at canopy bridges. It took 9–13 months for gliders to habituate to and use bridges. In July 2007, two rope bridges and three glider poles were installed at five sites along a 70-km-long section of a four-lane divided highway. Canopy rope bridges were 70 m long, 0.5 m wide and 6 m high. Camera traps monitored bridge (August 2007–May 2011; 787–873 nights/bridge) and pole use (December 2009–March 2011; 22–87 nights/pole crossing). In 2010–2011, 42 gliders were radio-tracked at sites with and without crossings and at a single-lane-road site (<10 m wide).Study and other actions tested
A study in 2008–2009 of a forested and urban area in Porto Alegre, Brazil (Teixeira et al. 2013) found that rope canopy bridges over roads were used by three mammal species. Rope canopy bridges were used by brown howler monkeys Alouatta guariba clamitans (4 of 6 bridges), porcupines Sphiggurus villosus (2 of 6 bridges) and white-eared opossums Didelphis albiventris (1 of 6 bridges). Six canopy bridges were installed in 2001–2006 at sites close to a protected reserve where brown howler monkeys had been killed on roads or used power lines to cross them. Each bridge consisted of a horizontal ‘ladder’ made from rope and rubber hose (4 x 12 m parallel ropes with rubber hose ‘steps’ at 80 cm intervals and interlaced ropes forming a ‘X’ between each step). Camera traps and trained local observers monitored each of the six bridges for a total of 33–152 days during 6–15 months in 2008–2009.Study and other actions tested
A replicated study in 2012–2014 at five sites along a highway through eucalyptus forest in Victoria, Australia (Soanes et al. 2015; an expansion of Soanes et al. 2013) found that canopy rope bridges were used by four species of arboreal marsupial to cross the road. Remote cameras detected 455 crossings of canopy bridges by squirrel gliders Petaurus norfolcensis, 229 by common brushtail possums Trichosurus vulpecula, 386 by common ringtail possums Pseudocheirus peregrinus and two by brush-tailed phascogales Phascogale tapoatafa. The study was conducted along two sections of the Hume Freeway, located 200 km apart. In 2007–2009, four 60–85-m-long canopy bridges, made of 15-mm-diameter rope woven into a flat net, 50 cm wide, were erected. They were 6 m above the road. A fifth bridge, 170 m long, was erected at ≥4 m high. Wildlife crossings were monitored between June 2012 and February 2013, using motion-triggered cameras.Study and other actions tested
A study in 2012–2016 in a forest site within a university campus in New South Wales, Australia (Goldingay & Taylor 2017) found that northern mountain brushtail possums Trichosurus caninus and common ringtail possums Pseudocheirus peregrinus used canopy bridges but koalas Phascolarctos cinereus and squirrel gliders Petaurus norfolcensis did not. Twenty-two passes of northern mountain brushtail possums and two of common ringtail possums were detected on rope bridges. Koalas were detected 75 times and squirrel gliders three times in two nearby trees but were not detected on rope bridges. The trial was conducted in a 30 × 100 m eucalyptus-dominated forest patch. Rope-bridges of four designs extended 8–11 m between different pairs of trees. One rope bridge had 8-cm gaps between rope strands, one was made of woven-mesh with 1-cm gaps between strands, one was a ladder wrapped around internal wires to produce a sausage shape and one consisted of a woven mesh bridge with rope-ladder sides. One or two camera traps were used to monitor each rope-bridge and single cameras were used on two nearby reference trees, for 2.8–3.1 years/tree, between December 2012 and February 2016.Study and other actions tested
A study in 2012–2013 at a forest site in the Lower Urubamba region, Peru (Gregory et al. 2017) found that canopy bridges over a pipeline route were used by 25 arboreal mammal species with use increasing over 10 months, and crossing rates were higher over the bridges than at ground level. Twenty-five arboreal mammal species were recorded crossing over 13 canopy bridges (see original paper for details). Overall, use of the bridges increased over 10 months (total 40–55 crossings/100 nights). Crossing rates were higher over the bridges (total 45 crossings/100 nights) than below them at ground level (total 0.3 crossings/100 nights), although the difference was not tested for statistical significance. A gas pipeline route (10–25 m wide) was cleared through an area of native forest in June–August 2012. Thirteen canopy bridges (with branches from one or more trees connecting across the clearing) were preserved along a 5.2 km stretch of the route. Ten bridges remained functional by the end of the study in August 2013. Three failed due to exposure/tree damage. From September 2012, camera traps recorded crossing activity over the bridges (1–4 cameras/bridge) and at ground level below (2–3 cameras/bridge) for 11–12 months.Study and other actions tested
A study in 2007–2015 at five points where a highway bisected woodland in Victoria, Australia (Soanes et al. 2010) found that arboreal marsupials using rope bridges did not suffer high predation rates when doing so. Among 13,488 detections of arboreal marsupials (from rope bridges and glider pole crossings combined – separate figures not given in paper), there was one recorded predation attempt. This was an unsuccessful night-time predation attempt on a squirrel glider Petaurus norfolcensis using a rope bridge, by an unidentified bird. In July 2007, five crossing structures were installed along 70 km of highway. Two were rope mesh canopy bridges (70 m long, 5 m wide) and three were poles for gliders (one or two poles/crossing, 12–14 m tall). Crossings were monitored with motion and heat activated cameras, from July 2007 to February 2015. Cameras recorded 5–10 images, 3 s apart (2007–2011) or a 10–20 s video (2011–2015). Predation attempts were detectable when animals were ≤5 m from each end of a canopy bridge, and ≤1 m from the top of each glider pole.Study and other actions tested