Wildlife corridors: why did the sloth cross the road?

Have you ever run over an animal while driving, or had to suddenly hit the brakes so an animal could cross the road? Engineers and ecologists have come up with genius solutions, collectively called wildlife corridors, so that this happens less often. This article is about two such solutions - green bridges, which are big vegetated overpasses, and rope bridges between trees.

More than just roadkill

Roads threaten animals because of a concept called habitat fragmentation. This is when big animal populations are split into two smaller populations with less resources and less genetic diversity than the original populations. Animals may try to move between habitat fragments in search of new food or mates, but die trying to cross the road. Either they walk directly onto the road and collide with cars, or they cross by walking over power lines and get electrocuted. Wildlife corridors allow animals to safely walk over roads, un-doing the habitat fragmentation and reducing their chance of extinction.

Wolves in Germany

A 2021 study analysed the activity of animals crossing a green bridge in Germany. This bridge, one of seven in the state of Brandenburg, was built in 2012 over the important A12 highway (Figure 1). Using camera footage over a year, researchers found that grey wolves were more likely to use the bridge at dusk, at night, and in the winter. The deer and wild boars eaten by wolves were also more likely to use the bridge at dusk and at night, so the presence of wolves on the bridge did not deter their prey. Since 76% of wolves in Germany die in road-related incidents, bridges like this one are crucial for effective wolf conservation. 

Rope bridges in Costa Rica

While Germany’s wolves and deer walk straight onto roads, Costa Rica’s tree-dwelling animals cross the road using power lines. This means the tree dwellers, including monkeys and sloths, are at risk of electrocution - in fact, nearly 1000 animals died of electrocution in Costa Rica in 2018-19. To reduce this risk, Costa Ricans have built rope bridges across the country as a safer alternative for wildlife to cross roads. Most bridges consist of a single blue nylon rope (Figure 2a), while researchers at the University of Costa Rica built rope bridges specially designed for howler monkeys (Figure 2b). Howler monkeys were targeted because of their endangered status and ecological role as seed and pollen dispersers. While the specialised bridges doubled howler monkey populations between 2015 and 2021, both them and classic rope bridges were used by squirrels, opossums, and kinkajous. However, a 2021 study found that animals use telephone lines to cross roads as frequently as they use rope bridges, and telephone lines are dangerously close to power lines. Some species still are not crossing using rope bridges, many years after their construction. Although the rope bridges are helping to reduce electrocution, they are not perfect.

Heathland in the UK

Closer to home, a brand-new green bridge called Cockrow Bridge will soon open in Surrey (Figure 3). Surrey has lost 85% of its lowland heath in the last two centuries, but Ockham and Wisley Commons continue to support rare heathland species like the nightjar and sand lizard. These two commons, on either side of the A3/M25 junction, will be connected by the Cockrow Bridge into a 3 km-long stretch. Although existing heathland needs to be destroyed for construction, tree stumps and soil from the destroyed habitat will be repurposed on the bridge. Tree stumps will provide shelter to small animals, while the soil contains native roots and seeds to kickstart the bridge ecosystem. Since the public will be allowed on this bridge, it will improve our access to green spaces and bring revenue to local organisations. Therefore, Cockrow Bridge is expected to benefit wildlife and the public.

Conclusion

Wildlife corridors could be an important conservation tool by undoing habitat fragmentation, reducing roadkill, and preventing electrocution on power lines. Examples in Germany and Costa Rica look promising, and a unique heathland bridge is under construction here in the UK. Green bridges and rope bridges prove that modern infrastructure does not need to harm biodiversity, and humans can coexist with nature.

Written by Simran Patel

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REFERENCES

The Sloth Conservation Foundation. Connected Gardens: facilitating the peaceful co-existence of sloths and people. [Internet]. [cited 2025 Apr 13]. Available from: https://slothconservation.org/what-we-do/habitat-connectivity/

Tobias N. Swinging to safety: How canopy bridges may save Costa Rica’s howlers. Mongabay Environmental News [Internet]. 2023 Feb 15 [cited 2025 Apr 13]; Available from: https://news.mongabay.com/2023/02/swinging-to-safety-how-canopy-bridges-may-save-costa-ricas-howlers/

Gilbey V, Petty R. UK’s first heathland green bridge. Proceedings of the Institution of Civil Engineers - Civil Engineering. 2024 Nov 1;177(6):99–110.

Laidlaw K, Broadbent E, Eby S. Effectiveness of aerial wildlife crossings: Do wildlife use rope bridges more than hazardous structures to cross roads? Rev Biol Trop. 2021 Oct 1;69(3):1138–48.

Plaschke M, Bhardwaj M, König HJ, Wenz E, Dobiáš K, Ford AT. Green bridges in a re‐colonizing landscape: Wolves ( Canis lupus ) in Brandenburg, Germany. Conservat Sci and Prac. 2021 Mar;3(3):e364.