Scientists Identify Best Wildlife Crossings

Mountain Lion F92 and her two cubs, F126 and 127, feast on a deer carcass in the Santa Ana Mountains, one of the study areas. Photo courtesy of UC Davis Wildlife Health Center

A new report offers recommendations on the location and type of wildlife crossing structures proposed for two of the busiest freeways in the world. Interstate 15 in western Riverside County and U.S. 101 in Los Angeles and Ventura Counties both bisect large areas of natural habitat and serve as major barriers to wildlife movement and gene flow, especially for mountain lions.

The report summarizes input from some of the world’s foremost experts on wildlife connectivity and crossing structures and combines it with landscape characteristics and wildlife data to prioritize locations for wildlife crossings. The experts evaluated locations that would improve connectivity for all kinds of wildlife, including for the imperiled mountain lion populations in the Santa Ana and Santa Monica Mountains.

“The loss of mountain lions in this area could have potentially harmful impacts on this ecosystem,” said Seth Riley, report co-author and wildlife ecologist at the Santa Monica Mountains National Recreational Area. “Wildlife in the Santa Monica Mountains is essentially trapped on an island of habitat, and a crossing structure would allow two-way migration to increase gene flow generally, and specifically increase genetic diversity for mountain lions.”

Genetic diversity levels for mountain lions in the Santa Ana and Santa Monica Mountains are lower than have been measured anywhere else in the west.

Wildlife crossing structures would allow juvenile mountain lions, among other wildlife, to move out of larger populations from east of the Santa Ana Mountains and north of the Santa Monica Mountains into the isolated coastal mountain ranges.

The report evaluated 11 potential locations in the Santa Ana Mountains and seven in the Santa Monica Mountains. Each site was rated for evidence of wildlife use, including mountain lions, landscape connectivity, and the extent to which land was protected or at risk of development. Specific recommendations include:

  • Construction of a new wildlife overpass over I-15 south of Temecula Creek Bridge, as well as enhancements to the area underneath the existing bridge.
  • Construction of a new wildlife overpass over the 101 Freeway at the West Liberty Canyon site. A tunnel option was deemed inferior because constructing a tunnel large enough to serve a broad range of species would be prohibitively expensive and complicated from an engineering perspective.
  • Accompanying measures such as effective wildlife fencing to funnel animals to crossing points and appropriate vegetative cover on or near structures are critically important.
  • In both mountain ranges, additional locations offer the potential for new construction or enhancements that could provide secondary crossing sites.

Data from the study is already being used to optimize and inform planning.

Caltrans is working with other partners on a plan to build a wildlife crossing over the 101 Freeway in the Liberty Canyon area and released the draft initial study and environmental assessment for the project last fall.

The Nature Conservancy recently acquired approximately 73 acres in the Rainbow Canyon area along I-15 near Temecula, a critical property needed for the placement of one of the highest priority wildlife crossings identified in the report.

UC Davis researchers are teaming with civil engineering faculty and students at Cal Poly Pomona and Caltrans, with the support of the NCCP Local Assistance Grant program and the San Diego Association of Governments, to better define the most feasible locations and types of crossing structure modifications that could improve the ability of mountain lions and other wildlife to cross I-15 in the future, including a future Rainbow Canyon crossing in the area where The Conservancy purchased land.


The Nature Conservancy is a global conservation organization dedicated to conserving the lands and waters on which all life depends. Working collaboratively in 72 countries, the Conservancy engages local communities, governments, the private sector, and other partners. To learn more, visit or follow @nature_press on Twitter.”

Santa Monica Mountains National Recreation Area (SMMNRA) is the largest urban national park in the country, encompassing more than 150,000 acres of mountains and coastline in Ventura and Los Angeles counties. A unit of the National Park Service, it comprises a seamless network of local, state, and federal parks interwoven with private lands and communities. As one of only five Mediterranean ecosystems in the world, SMMNRA preserves the rich biological diversity of more than 450 animal species and 26 distinct plant communities. For more information, visit

The UC Davis Karen C. Drayer Wildlife Health Center seeks to advance the health of wildlife in balance with people and the environment. Through projects and programs, the center focuses on critical issues involving free-ranging and captive terrestrial and aquatic wild animals. As key components of the UC Davis One Health Institute and School of Veterinary Medicine, the center provides broad-based service, training and research activities as they relate to wildlife health and conservation.


Suzanne Guldimann

Suzanne Guldimann is an author, artist, and musician who lives in Malibu and loves the Santa Monica Mountains. She has worked as a journalist reporting on local news and issues for more than a decade, and is the author of nine books of music for the harp. Suzanne's newest book, "Life in Malibu", explores local history and nature. She can be reached at

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