Install barriers along roads/railways

Supporting evidence from individual studies

1. A replicated study in 1991–1992 in an outdoor facility and along a highway in Nevada, USA (Ruby et al. 1994) found that desert tortoises Gopherus agassizii interacted less with solid than non-solid barriers and that a concrete barrier was effective in keeping tortoises from a road. In daytime, desert tortoises spent less time close to and touched or pushed solid barriers less often (4–19 minutes, 0.4–4 touches/trial, 0.1 pushes/trial) than non-solid mesh barriers (5–23 minutes, 2–12 touches/trial, 5 pushes/trial) and in shorter trials made less attempts to climb over solid than non-solid fences (30 minute trial: 1 attempts/trial vs. 2 attempt/trial, 2 h trial: 0.1 vs. 0.1, night time: 1 vs. 0.1). The authors reported tortoises frequently attempted to walk through fences they could fit their head through. In a separate maze experiment, tortoises showed no preference for solid or mesh fencing (see paper for details). In a trial by a highway, nine of 10 tortoises approached a concrete barrier and walked along it for an average of 13 m before seven walked away from the highway (the remaining two tortoises settled in place). Tortoises were placed individually in pens with solid (e.g. cabin timber, aluminium flashing, cement blocks, telephone poles) or non-solid walls (e.g. chain link, chicken wire, mesh cloth) for 30 minute (solid: 41 trials, non-solid: 22), 2 hour (160, 100 trials) or overnight trials (40, 80 trials) and behaviours monitored. Tortoises participated in one trial/wall material and three trials maximum each. In separate trials, 16 tortoises were placed in a T-shaped maze with a choice between navigating towards solid or mesh fencing (40 total trials) and 10 tortoises were placed by a concrete barrier next to a highway and observed. All trials took place in 1991–1992.

   Study and other actions tested

   Referenced paper

   Ruby D.E., Spotila J.R., Margin S.K. & Kemp S.J. (1994) Behavioral responses to barriers by desert tortoises: implications for wildlife management. Herpetological Monographs, 8, 144-160.

2. A replicated study in 2005–2006 in a Wildlife Management Area, in New York, USA (Woltz et al. 2008) found that plastic fences of at least 0.6 m high excluded all painted Chrysemys picta and snapping turtles Chelydra serpentina. Fences 0.6 m and 0.9 m high were more effective at excluding turtles (100% excluded) than 0.3 m high fences (84–100%). Opaque, corrugated plastic fences were used to construct three nested, circular enclosures of heights 0.3, 0.6 and 0.9 m high. Local painted (74 individuals) and snapping turtles (62 individuals) were placed in the centre of each arena and left for 15 minutes to attempt to scale the fences.

   Study and other actions tested

   Referenced paper

   Woltz H.W., Gibbs J.P. & Ducey P.K. (2008) Road crossing structures for amphibians and reptiles: informing design through behavioral analysis. Biological Conservation, 141, 2745-2750.

3. A replicated study in 2011–2012 along two roadside verges across salt marshes in New Jersey, USA (Reses et al. 2015) found that where barriers were installed, diamondback terrapins Malaclemys terrapin nested more often on the marsh-side of barriers than on the road-side, and that terrapins were less likely to breach barriers with smaller gaps at the bottom. After barrier fences were installed, diamondback terrapins laid more nests on the marsh-side of the fence than on the road-side (results presented as statistical model outputs, see original paper for details). In separate arena trials, diamondback terrapins were less likely to breach fences with smaller gaps at the bottom (0 cm gap: 10% breached; 3–5 cm: 37–60 %; 6–8 cm: 96–100%). In 2011 and 2012, sections of two causeways (589–623 m long) with corrugated tubing fencing (15 cm diameter) were surveyed on foot for terrapin nests. Surveys were carried out every week in June–July 2011 and twice in June–July 2012. Trials to test whether terrapins could breach the fences with different sized gaps at the bottom (0–8 cm) were carried out in a fenced enclosure with 40 individual terrapins (total of 74 trial).

   Study and other actions tested

   Referenced paper

   Reses H.E., Davis Rabosky A.R. & Wood R.C. (2015) Nesting success and barrier breaching: assessing the effectiveness of roadway fencing in diamondback terrapins (Malaclemys terrapin). Herpetological Conservation and Biology, 10, 161-179.

4. A before-and-after study in 1999–2008 on a roadside verge along a river bank in Pennsylvania, USA (Nagle & Congdon 2016) found that after adding a chain-link fence to a highway, creating artificial nest mounds on the non-road side of the fence, and actively moving turtles off the road, fewer female northern map turtles Graptemys geographica were killed on the road. Results were not statistically tested. In the first year after a fence was installed on a new major highway, 10 northern map turtles were killed on the road, compared to 50 turtles the year before (total for previous year included a small number of wood turtles Glyptemus insculpta and snapping turtles Chelydra serpentina).  In the subsequent 8 years, map turtle deaths reduced to 0–5 individuals/year (no data for other species). The authors reported that most deaths were gravid female turtles. In 2000, a fence (1 m high and 1,150 m long) was installed on the river side of the highway to prevent turtles from crossing the road to access nesting habitat. Mounds of sand aimed at providing alternative nesting habitat were added to the river side of the fence in 2000–2001. After the first year, the fence was extended by 300 m to prevent turtles from going around it and crushed shale was added to the sand mounds and turtles were actively moved off the road. Turtle deaths on the road were counted from 1999 (the first year after a new highway opened) to 2008 (excluding 2004).

   Study and other actions tested

   Referenced paper

   Nagle R.D. & Congdon J.D. (2016) Reproductive ecology of Graptemys geographica of the Juniata River in central Pennsylvania, with recommendations for conservation. Herpetological Conservation and Biology, 11, 232-243.

5. A controlled, before-and-after study in 2009–2014 on a causeway over a saltmarsh in Georgia, USA (Crawford et al. 2017, same experimental set-up as Crawford et al. 2018) found that installing a roadside barrier with nest boxes reduced diamond-backed terrapin Malaclemys terrapin road crossings. Numbers of crossings by terrapins were lower at the site after the barrier was introduced (2–7 crossings) compared to before (10–17 crossings) and compared to sites without the barrier (12–109 crossings). Three monitoring locations along the causeway were selected based on high terrapin road mortality levels in 2009–2010. Two sites (331 m and 310 m long stretches of road) without barriers were monitored, and at a third site (162 m long, between the other sites) a barrier was installed in 2011. The barrier was 22.9 m long, positioned 5 m from the roadside and comprised short mesh fencing and six nest boxes. Terrapins were monitored on the road at all three sites every 20–90 minutes, between 08:00 and 20:00 from May–July in 2009–2014. Two years of pre-barrier and four years of post-barrier data were collected.

   Study and other actions tested

   Referenced paper

   Crawford B.A., Moore C.T., Norton T.M. & Maerz J.C. (2017) Mitigating road mortality of diamond-backed terrapins (Malaclemys terrapin) with hybrid barriers at crossing hot spots. Herpetological Conservation and Biology, 12, 202-211.

6. A before-and-after study in 2009–2015 on a causeway over a saltmarsh in Georgia, USA (Crawford et al. 2018, same experimental set-up as Crawford et al. 2017) found that installing a roadside barrier with nest boxes and adding a flashing terrapin-warning sign to alert motorists reduced likelihood of mortality for diamondback terrapin Malaclemys terrapin crossing the road. When the hybrid nestbox-fence barrier and flashing signs were added to a road, survival of crossing female diamondback terrapins increased from 24% to 53% (data reported as statistical model outputs). In 2011, a 22 m long hybrid nestbox-fence barrier was built along an 8.7 km long causeway. In 2013, two terrapin crossing signs with flashing warning beacons were added to warn motorists entering a 6 km stretch of causeway. The signs were activated for 2 hours/day during peak terrapin crossing times. Terrapins were surveyed on the causeway and in adjacent creeks during the nesting season (May–July) in 2009–2015.

   Study and other actions tested

   Referenced paper

   Crawford B.A., Moore C.T., Norton T.M. & Maerz J.C. (2018) Integrated analysis for population estimation, management impact evaluation, and decision-making for a declining species. Biological Conservation, 222, 33-43.

7. A study in 2006–2017 in shrub-steppe desert in British Columbia, Canada (Eye et al. 2018) found that building an exclusion fence to prevent snakes entering high human areas activity and associated roads did not prevent road related snake mortality. In the first year after the exclusion fence was installed, seven snake mortalities were observed near the fence. Ten to 11 years after the fence was installed, 22 of 45 (49%) snake deaths were road kill, and a further 15 (33%) dead snakes were found next to the fence. Six of the 15 dead snakes near the fence were found next to a section that had been rerouted the previous year. Western yellow-bellied racer Coluber constrictor mormon were disproportionately represented among dead snakes. In total 341 live snakes (northern pacific rattlesnake Crotalus oreganus oreganus, great basin gopher snake Pituophis catenifer deserticola, and western yellow-bellied racer) were captured around the fence in the tenth and eleventh year after it was installed. In 2006, approximately 4 km of exclusion fencing (60 cm high with 0.6 cm mesh) was built. The fence was upgraded and 200 m rerouted in 2015. Snake mortality was monitored in May–October 2016–2017 by walking the fence line 2–3 times/week and live snakes were monitored using mark-recapture methods 5–6 days/week.

   Study and other actions tested

   Referenced paper

   Eye D.M., Maida J.R., McKibbin O.M., Larsen K.W. & Bishop C.A. (2018) Snake mortality and cover board effectiveness along exclusion fencing in British Columbia, Canada. Canadian Field-Naturalist, 132, 30-35.