Researchers from University of California, Davis have published a study that outlines how artificial light pollution from things like city lights and highway traffic is threatening the habitats of mountain lions in Southern California.

“Well-lit streets, neighborhoods and commercial areas will reduce and fragment the areas available to mountain lions to move around,” Fraser Shilling, senior author of the study and director of the UC Davis Road Ecology Center in the Institute of Transportation Studies, said in a statement. “It’s not just the outsized human footprint that is squashing lion habitat, but the extended glow from that footprint, too.”

The study, published in the journal Philosophical Transactions of the Royal Society B, tracked 102 mountain lions (Puma concolor) via radio collars in coastal mountain regions, including the sprawling metropolises of Los Angeles and San Diego. The area is home to more than 20 million people.

Researchers tracked the mountain lions from 2001 to 2022 and analyzed the impact of disturbances like nearby lights, sky glow, and natural light from the moon.

They found that while mountain lion presence was largely unaffected by sky glow (or the diffused light in the sky from both artificial and natural light) and moonlight, the big cats seemed to stay away from areas with more upward radiance, or light from artificial sources, like city lights.

“Overall, we found that mountain lions try to avoid zones lit artificially, possibly to avoid interactions with humans,” Rafael Barrientos, lead author of the study, an ecologist with the Universidad Complutense de Madrid in Spain and a visiting scientist at the Road Ecology Center, shared in a statement. “This can have cascading effects on the redistribution of the species in the region, as well as the benefits wildlife provide in this ecosystem.”

Artificial light can contribute to habitat fragmentation, as mountain lions attempt to avoid highways and cities that emit artificial light. The study found that even in the daytime, mountain lions tended to avoid areas with more upward radiance.

Human and mountain lion conflict, particularly traffic-related incidents, has long threatened the animals. The famous mountain lion P-22, known for crossing major freeways in Los Angeles, died in late 2022 after a suspected vehicle collision, and about 70 mountain lions die in California because of vehicle collisions each year, the San Francisco Chronicle reported.

Further, major freeways can keep mountain lions from finding mates, which limits their genetic diversity and threatens the species.

The new research highlights that the animals’ avoidance of artificial light needs to be considered in the design and construction of wildlife crossings over or under highways, such as the Wallis Annenberg crossing over the U.S. Highway 101 and the proposed wildlife crossing at Interstate 15, near Temecula, a city south of Los Angeles.

“Our research has shown that even when structures exist to allow mountain lion passage under freeways, the light and noise can deter mountain lions from use of these safe crossing structures,” said Winston Vickers, study co-author and wildlife veterinarian with the UC Davis School of Veterinary Medicine. “The findings in this analysis should increase the incentive for highway agencies to block light that spreads into the habitat from roads at crossing structures.”

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