A Few Mountain Lions Quietly Rewired an Entire California Ecosystem

Keypoints:

• A Stanford study tracked nine years of trail-camera data at Jasper Ridge Biological Preserve near San Francisco
• Rising mountain lion visits between 2015 and 2020 triggered a cascading shift across the entire food web
• Deer activity dropped sharply, letting young oak trees and other woody plants recover
• Coyote and bobcat activity fell too, while the preserve is too small to support a resident lion population
• Findings were published in Ecology and Evolution

Jasper Ridge Biological Preserve sits just 45 miles south of San Francisco, a modest, 1,200-acre patch of oak woodland surrounded by suburbs. It's far too small to support even a single resident mountain lion, since the cats typically roam territories spanning 20 to 170 square kilometers. Yet a nine-year Stanford study found that occasional visits from these cats were enough to reshape nearly everything living there.

Researchers tracked trail-camera footage and vegetation surveys from 2015 to 2020 and found mountain lion sightings climbing steadily over that stretch. As lion activity rose, deer activity dropped in step, and woody plants deer typically eat or trample, including young oak trees, began recovering and growing back. 

Chinmay Sonawane, the study's first author and a Stanford biology doctoral student, said the finding pushes back against a long-standing assumption in conservation science. "In the past, small preserves like Jasper Ridge have often been dismissed for holding very little ecological value," he said, "but this study shows that when these small preserves are connected to large wilderness like the Santa Cruz Mountains, you can still see magnificent ecological phenomena like trophic cascades."

The ripple effects reached further than deer and oak trees. Coyote and bobcat activity dropped too as lion visits increased, likely because the smaller predators shifted their movements or activity hours to avoid a far larger competitor, while gray foxes appeared to benefit from that same retreat. 

Researchers still don't know exactly what drew the lions to Jasper Ridge more often, though a mother caught on camera with kittens during the study raises the possibility that females found it a safe, low-traffic place to raise young, since the cats generally avoid areas with heavy human scent and movement.

The finding carries weight well beyond one preserve. Roughly 82% of protected areas in the US fall under two square miles, small enough that they've traditionally been written off as too limited to sustain the kind of predator-driven ecological cascades typically associated with places like Yellowstone. 

Stanford professor Rodolfo Dirzo, a co-author on the study, said the pattern underscores something conservation planning tends to overlook: losing a top predator, even from a tiny, isolated patch of habitat, can quietly simplify an entire ecosystem long before anyone notices the change.