The Wildlife Crossing Over the 101: How Genes Will Show If It Works

Los Angeles has nearly 10 million people — and mountain lions. In the Santa Monica Mountains, just west of the city, the National Park Service has spent decades trying to keep them from being walled in by freeways. The Wallis Annenberg Wildlife Crossing, a bridge for animals over the 101, is the biggest step yet. The harder question is how anyone will know it worked.

A single adult mountain lion needs 50 to 150 square miles of home range. NPS

Why the lions are trapped

Lions born inside the park may never be able to leave. Too many compete for too little, and closely related animals end up breeding with each other generation after generation — inbreeding.

What keeps a small population healthy is newcomers bringing new genes. Today a lion from outside makes it across the freeway and into the park about once every 12 years. Scientists estimate it needs to be closer to once every 2–3 years; otherwise inbreeding will eventually make the population collapse.

The crossing

The Wallis Annenberg Wildlife Crossing is a vegetated bridge over the 101, built to let lions — and many other animals — pass safely between the Santa Monica Mountains and the open spaces beyond. When this was written it was under construction and expected to be finished by late 2025. Its purpose is less about any one crossing than about mixing: letting wildlife in the mountains breed with populations elsewhere, strengthening genetic diversity for every mountain lion in Southern California. It rests on more than 25 years of the park's wildlife research.

How we will know

Seeing a lion on the bridge is not the same as the bridge working. The real test is in the genes.

1. Before: the Southern California Research Learning Center is funding genetic analyses of the lions, and of other species, to set a baseline before the crossing opens.
2. After: the same analyses continue once it is built.
3. The sign of success: new genetic variety showing up in the offspring of the mountains' lions, and their overall health improving over time.

About the research center

The Southern California Research Learning Center is one of 18 across the national park system. They increase scientific work in the parks, share research that supports stewardship, and make science part of a visit — here for Santa Monica Mountains National Recreation Area, Channel Islands National Park and Cabrillo National Monument.

Sources

Written from "A Highway Wildlife Crossing is Under Construction in the Santa Monica Mountains. How Will We Know if it's Working?", Southern California Research Learning Center, National Park Service; a work of the United States government in the public domain.