In the modest town of Agoura Hills, CA, plans are underway to construct the largest wildlife overpass in the world. Crossing over 10 lanes of the 101 Freeway, the Liberty Canyon overpass will be approximately 165 feet wide and 200 feet long. The project aims to connect severely isolated wildlife populations within the Santa Monica Mountains to those in the nearby Santa Susana Mountains. Without such a connection, there is a significant risk that the local mountain lion population will go extinct in the next 50 years.
The design will include tunnels to accommodate more reclusive wildlife, a corridor of riparian vegetation, and sound walls to dampen the noise and headlights of the freeway. According to Clark Stevens, the architect and habitat restorationist behind the design of the overpass, a wildlife crossing is more successful when you provide multiple ways for wildlife to utilize it. For example, deer are more likely to cross over the top of the bridge, while predators such as bobcats are more likely to cross through the tunnels. The riparian corridor will restore familiar scent-paths for animals, helping to draw them through the crossing. In order to accommodate mountain lion behavior, the crossing will be sloped on both sides to provide high vantage points with a wide view.
With a typical home range of 200 square miles, mountain lions are particularly sensitive to the habitat fragmentation caused by freeways. Young males need to disperse into new territories to avoid inbreeding and to avoid competition with dominant males. Freeways prevent this movement, leaving isolated populations more vulnerable to extinction due to their limited genetic diversity. Immigrants are necessary because they introduce variety into the gene pool and boost overall resilience. The Liberty Canyon wildlife crossing will enable mountain lion populations to disperse among each other and avoid inbreeding depression. To read more about the Liberty Canyon wildlife crossing, see Clark Stevens’ website or the Caltrans Project Study Report.
Benson, John F. et al. “Interactions Between Demography, Genetics, And Landscape Connectivity Increase Extinction Probability For A Small Population Of Large Carnivores In A Major Metropolitan Area.” Proceedings of the Royal Society B: Biological Sciences 283.1837 (2016): 20160957.
Caltrans. “US-101 Liberty Canyon Wildlife Crossing: A First for California.” 2017.
Goldman, Jason. “How To Design A Wildlife Crossing Wildlife Will Use.” KCET. 2017.
Groves, Martha. “Caltrans Proposes Wildlife Overpass On 101 Freeway.” Los Angeles Times. 2017.
National Park Service. Local Mountain Lion Population Faces Precipitous Decline In Genetic Diversity Within 50 Years, Possible Extinction. Los Angeles: National Park Service, 2016.
Stevens, Clark. “Wildlife Overpass.” Clark Stevens / Architect. 2017.
by Eddie Nolan, Student Affiliate ASLA, Ecology + Restoration PPN Officer – Co-editor for The Field