Mountain lions left this suburb a century ago; one returned to Stanford's preserve in 2015, and woody plants grew 64-fold in just 11 years

Large predators are supposed to need large spaces. This assumption has shaped decades of US conservation planning, with small wildlife refuges often dismissed as too small to matter ecologically. A new study from Stanford University just threw a wrench in that thinking.

According to the study, ‘Mammal Community Responses to Increasing Puma Activity in a Suburban Preserve,’ published in Ecology and Evolution, researchers spent nine years tracking what happened after a mountain lion began regularly passing through Jasper Ridge Biological Preserve, a roughly 1,200-acre stretch of land about 45 miles south of San Francisco. The preserve itself is too small to be able to support a resident population of mountain lions. In the nearby Santa Cruz Mountains, cats generally range over territories from 8 to 66 square miles. But this one visiting cat rearranged the entire local food web.

The paper reports that puma activity rose after 2009 in a nine-year camera-trap record spanning 17 cameras and more than 61,000 trap days, with over 38,000 independent detections overall. It also notes that mammals inside the preserve shifted their activity patterns more than those outside it, suggesting the predator’s presence was altering the community even without a resident population.


Deer disappeared, and the plants noticed
Mountain lions are increasingly showing up in trail camera images taken at Jasper Ridge between 2015 and 2020. Deer sightings on those same cameras also fell relative to previous years during that same stretch. Less deer meant less browsing pressure on the landscape, and vegetation surveys showed young oak trees and other woody plants beginning to return.

Ecologists refer to this as a trophic cascade, where the impact of one predator ripples through several levels of an ecosystem. It’s the same fundamental idea behind what happened when wolves came back to Yellowstone. According to the long-term research, ‘Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction,’ published in Biological Conservation, the 1995 to 1996 reintroduction of wolves to Yellowstone was followed by a measurable recovery of aspen and willow along parts of the park's northern range as elk browsing pressure eased. Cascades like these have been studied mostly in vast wilderness areas, and that's what made the Jasper Ridge findings stand out. Clearly, a powerful predator like this doesn't need a national park to leave a mark.

The researchers revisited 97 aspen stands along four streams in Yellowstone’s Lamar River catchment in September 2010 and compared them with surveys from 1998. By then, browsing on the five tallest young aspens had fallen from 100% of measured leaders to under 25% in uplands and under 20% in riparian areas, while aspen recruitment and cottonwood recruitment both increased as browsing pressure eased.

It's not just about the deer
The Jasper Ridge team also found a second pattern, this one involving medium-sized predators. With the mountain lion becoming more of a regular sight, coyotes and bobcats became less active in the area, perhaps changing their territory or schedule to avoid a much larger cat. As those two species pulled back, grey foxes appeared to fill in the gap, showing up on camera more frequently. And with more foxes around, the brush rabbits in the preserve seemed to get more cautious as well.

The researchers are upfront that this second, lower part of the cascade isn’t fully proven. Other factors, like changes in fog and temperature at the site, could not be ruled out as contributors. But the top of the chain, the decline in deer sightings along with the disappearance of coyotes and bobcats, was a clear, direct effect that could be tied to the mountain lion.

Scientists have dubbed this dynamic: the ecology of fear. Animals do not need to actually be hunted to change their behavior. Even just sensing the presence of a larger predator nearby often changes where and when they move, and that ripple effect can cascade all the way down the food chain. This broad trend, where predators influence ecosystems partly through fear and not just direct kills, manifests itself in landscapes far beyond the Santa Cruz Mountains.

A large citizen-science mammal survey across the eastern US published in eLife found that many mammal species actually held steady or even higher numbers in moderately developed landscapes compared with untouched wild areas, challenging the old assumption that development automatically pushes wildlife out. It’s a reminder that land on the edge of suburbia can still be a real wildlife habitat.

The authors also say the work is still provisional on the lower trophic levels, because changes in fog and temperature could not be ruled out as alternative explanations for the vegetation response.

Why this matters for the rest of the country
It’s not just the story of one wandering cat. It has a real message for how the US manages land. Across the country, 82 percent of protected areas are smaller than 5 square kilometers, or roughly 2 square miles. These pocket-sized preserves have, in the past, been largely ignored by conservationists, who have thought them too small to matter. The Jasper Ridge results show that a small preserve can still host the kind of ecological drama often associated with places like Yellowstone, if it is connected to a larger wild corridor, in this case, the Santa Cruz Mountains.

Why this particular mountain lion began to frequent Jasper Ridge is unknown. Researchers have one theory: that the preserve is perceived as a safe place for female mountain lions to raise young. A mother with kittens was caught on camera during the study period.

What is clear is that these cats do everything within their power to avoid people. They are primarily nocturnal and rely on scent and sound to steer clear of humans. Ironically, humans are still the number one cause of mountain lions’ deaths in the US, mostly from hunting and vehicle collisions. As the Stanford researchers point out, humans exert their own version of the ecology of fear on the landscape, too, whether we realize it or not.