Long-term research has found that even occasional visits from mountain lions can rebuild entire ecosystems.
Researchers studying a small suburban preserve about 45 miles south of San Francisco found that as mountain lion activity increased, the behavior of many other animals changed as well. The impact extended beyond wildlife, affecting plant growth and the overall health of the landscape.
mountain lion (puma conqueror) began to appear frequently on trail cameras at Stamford’s Jasper Ridge Biological Preserve (‘Auchamin’ Auyakuma) from 2015 to 2020. Researchers observed a decline in deer activity during the same period compared to earlier periods when mountain lions were rarely seen or disappeared altogether.
The vegetation survey revealed another notable change. Woody plants commonly eaten or damaged by deer, including young oak trees, showed signs of recovery and growth.
The survey results are ecology and evolutionrefers to a phenomenon known as a trophic cascade, in which changes at the top of the food chain ripple through multiple levels of the ecosystem. These effects are best studied in large natural areas, particularly the well-known example of wolves in Yellowstone National Park. New research suggests similar ecological processes can occur in much smaller protected areas.
“In the past, small preserves like Jasper Ridge were often dismissed as having little ecological value. But this study shows that even when these small preserves are connected to larger wilderness areas like the Santa Cruz Mountains, grand ecological phenomena like trophic cascades can occur,” said Chinmay Sonawane, first author of the study and a biology doctoral student in Stanford’s School of Humanities and Sciences (H&S). “They don’t just happen in places like Yellowstone, which are far away from cities and people. They can happen in these very small, urban places as well.”
The ecology of fear and the influence of predators
To investigate ecological changes in the reserve, researchers analyzed information collected from motion-activated cameras and vegetation surveys.
They identified two types of trophic cascades. One involves mountain lions, deer, and plants, a relationship known as the tritrophic cascade. The other involved small predators that share the same landscape as mountain lions.
As mountain lion activity increased, coyotes and bobcats were observed less frequently. Researchers suggest these animals may be avoiding the area or changing their activity patterns to reduce encounters with larger predators.
Foxes appear to be benefiting as coyote and bobcat numbers have declined. Fox activity may have increased and, as a result, rabbit activity, one of their main prey species, may have decreased.
Scientists refer to this type of predator-driven behavioral change as an “ecology of fear.” This concept describes how the presence of top predators affects other animals even in the absence of direct predators. Simply knowing that a predator is nearby can change where animals move, when they are active, and how they forage, creating impacts that extend throughout the ecosystem.
Some of the low-level effects identified in the study are preliminary. The researchers note that the apparent effects on vegetation, foxes and rabbits may also have been influenced by environmental factors such as fog patterns and changes in temperature.
But the evidence linking mountain lion activity to behavioral changes in deer, coyotes, and bobcats was much stronger.
These findings highlight the ecological importance of both apex predators and small-scale protected areas. According to researchers, 82% of protected areas in the United States are less than 5 square kilometers (about 2 square miles). As urban development continues to expand, these small reserves are likely to play an increasingly important role in supporting wildlife and plant communities.
“Maintaining habitat for the entire animal community, from predators to prey to prey resource bases, is critical,” said study co-author Rodolfo Dilzo, professor of biology at Stanford University H&S. “When something is missing, which is usually a top predator that requires more area and is more sensitive to human influence, we no longer have a fully functioning ecosystem.”
Why do mountain lions visit?
Researchers still don’t know why mountain lions started using Jasper Ridge more frequently.
One possibility is that female mountain lions consider the reserve a relatively safe place to raise their young. During the study, cameras captured images of a mother lion with her cubs.
Despite its growing presence, this animal is not a permanent resident. Mountain lions in the Santa Cruz Mountains typically occupy territories ranging from 20 to 170 square kilometers (approximately 8 to 66 square miles). Jasper Ridge is too small to support the mountain lion population that lives there.
puma and human
Elizabeth Hadley, the study’s lead author and professor emeritus of biology at Stanford University’s H&S University, said mountain lion sightings occasionally make the news in and around San Francisco, but mountain lions generally avoid people as much as possible.
Mountain lions are primarily nocturnal, so they are usually active when no one is around.
“Pumas are afraid of our smells and sounds. They don’t like to see us move,” said Hadley, who is also a former dean at Jasper Ridge. “Pumas use all five of their senses to avoid humans.”
Humans remain the leading cause of mountain lion deaths, whether from hunting or vehicle collisions.
“Obviously, we have a unique ecology of fear,” she says. “Humans are the ultimate predator in almost every landscape.”
Dirzo is also the Bing Professor of Environmental Science in the Stanford Biology Department at 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.
Hadley is also the Paul S. and Billy Achilles Professor Emeritus of Environmental Biology at H&S. Professor Emeritus of Earth System Science at the Doerr School of Sustainability. A member of Stanford BioX. Senior Researcher at Woods Research Institute.
Other Stanford co-authors include Trevor Hébert, an academic technology specialist at Jasper Ridge; Kevin Leempoel, former postdoctoral fellow in biology; Nicole Nova and Jordana Meyer were both doctoral students in biology. Amelia Zuckerweis is a former biology undergraduate.
This research received support from the National Science Foundation.
