Researchers studying a small suburban preserve about 45 miles south of San Francisco discovered that as mountain lion activity increased, the behavior of many other animals changed as well. The effects extended beyond wildlife, influencing plant growth and the overall health of the landscape.

Mountain lions (Puma concolor) appeared more frequently on trail cameras at Stanford's Jasper Ridge Biological Preserve ('Ootchamin 'Ooyakma) between 2015 and 2020. During that same period, researchers observed a decline in deer activity compared with earlier years when pumas were rarely seen or absent altogether.

Vegetation surveys revealed another notable change. Woody plants that are commonly eaten or damaged by deer, including young oak trees, showed signs of recovery and growth.

The findings, published in Ecology and Evolution, point to a phenomenon known as a trophic cascade, in which changes at the top of the food chain ripple through multiple levels of an ecosystem. These effects are most often studied in large wilderness regions, particularly in well-known examples involving wolves in Yellowstone National Park. The new research suggests that similar ecological processes can occur in much smaller protected areas.

"In the past, small preserves like Jasper Ridge have often been dismissed for holding very little ecological value, 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," said Chinmay Sonawane, the study's first author and doctoral student in biology in Stanford's School of Humanities and Sciences (H&S). "They are not just things that happen in places like Yellowstone far away from the city and people. They can happen in these places that are quite small and more urban as well."

The Ecology of Fear and Predator Effects

To investigate the preserve's ecological changes, researchers analyzed information collected from motion-activated cameras and vegetation surveys.

They identified two types of trophic cascades. One involved mountain lions, deer, and plant life, a relationship known as a tri-trophic cascade. The other involved smaller predators that share the landscape with pumas.

As mountain lion activity increased, coyotes and bobcats were observed less frequently. Researchers suggest these animals may have been avoiding the area or shifting their activity patterns to reduce encounters with the much larger predators.

With fewer coyotes and bobcats present, foxes appeared to benefit. Fox activity increased, which may in turn have reduced activity among rabbits, one of their primary prey species.

Scientists refer to these kinds of predator-driven behavioral shifts as the "ecology of fear." The concept describes how the presence of a top predator can influence other animals, even without direct predation. Simply knowing a predator is nearby can alter where animals travel, when they are active, and how they forage, creating effects that spread throughout an ecosystem.

Some of the lower-level impacts identified in the study remain tentative. The researchers note that the apparent effects on vegetation, foxes, and rabbits could also have been influenced by environmental factors such as changes in fog patterns or temperature.

However, the evidence linking mountain lion activity to changes in deer, coyote, and bobcat behavior was much stronger.

Those findings highlight the ecological importance of both apex predators and small protected areas. According to the researchers, 82% of protected areas in the United States are smaller than 5 square kilometers (about 2 square miles). As urban development continues to expand, these smaller preserves may play an increasingly important role in supporting wildlife and plant communities.

"Maintaining sites where there is an entire community of animals, from predators to prey to the prey's resource base, is very important," said Rodolfo Dirzo, study co-author and Stanford professor of biology in H&S. "When one piece is missing - and it's typically the top predators that require larger areas and are more sensitive to human impact - we will no longer have fully functioning ecosystems."

Why Are Mountain Lions Visiting?

Researchers do not yet know why mountain lions began using Jasper Ridge more often.

One possibility is that female mountain lions view the preserve as a relatively safe place to raise their young. During the study period, cameras captured images of a mother mountain lion accompanied by kittens.

Despite their growing presence, the animals are not permanent residents. Mountain lions in the Santa Cruz Mountains typically occupy territories ranging from 20 to 170 square kilometers (about 8 to 66 square miles). Jasper Ridge is far too small to support its own resident puma population.

Mountain Lions and Humans

Although mountain lion sightings occasionally make headlines in San Francisco and surrounding suburbs, the animals generally avoid people whenever possible, according to Elizabeth Hadly, the study's senior author and Stanford professor emerita of biology in H&S.

Mountain lions are primarily nocturnal, which means they are usually active when people are not.

"Pumas are afraid of our smell and our sounds; they don't like to see us moving," said Hadly, who is also the former faculty director at Jasper Ridge. "Pumas use all of their senses to avoid humans."

Humans remain the leading cause of mountain lion deaths, whether through hunting or vehicle collisions.

"Clearly, we exert our own ecology of fear," she said. "Humans are the ultimate predator on almost every landscape."

Dirzo is also the Bing Professor in Environmental Science at Stanford's Department of Biology in H&S, a professor of Earth system science in the Stanford Doerr School of Sustainability, and a senior fellow at the Stanford Woods Institute for the Environment.

Hadly is also the Paul S. and Billie Achilles Professor in Environmental Biology, Emerita, in H&S; professor emerita of Earth system science in the Doerr School of Sustainability; member of Stanford Bio-X; and senior fellow at the Woods Institute.

Additional Stanford co-authors include Trevor Hébert, academic technology specialist at Jasper Ridge; Kevin Leempoel, a former postdoctoral scholar in biology; Nicole Nova and Jordana Meyer, both former biology doctoral students; and Amelia Zuckerwise, a former undergraduate student in biology.

This research received support from the National Science Foundation.