Mountain lions have major ecological impact even in small preserves

A mountain lion photographed by a motion capturing camera on Stanford's Jasper Ridge Biological Preserve. Credit: Trevor Hébert/Stanford University.

Big cats have a big impact. A long-term study showed that when mountain lions began regular visits to a small suburban preserve about 45 miles (72 kilometers) south of San Francisco, they changed the behavior of many other animals.

Mountain lions (Puma concolor) started appearing with increasing frequency on trail cameras at Stanford's Jasper Ridge Biological Preserve ('Ootchamin 'Ooyakma) from 2015 to 2020. Researchers documented a corresponding drop in deer activity compared with the prior years of lower or absent puma activity. Vegetation surveys also showed that many woody plants deer like to eat or tend to trample, including young oak trees, began to thrive.

These types of multilevel effects, called trophic cascades, have been studied primarily in large wilderness areas, particularly cascades caused by apex predators such as wolves reintroduced into Yellowstone National Park. This research, published in Ecology and Evolution , indicates the effect can be found in smaller preserves as well.

A mountain lion and her cubs at night in Stanford's Jasper Ridge Biological Preserve. Credit: Trevor Hébert/Stanford University.

"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

The researchers analyzed data from motion-capturing cameras and vegetation surveys and found two types of trophic cascades: one that connected mountain lions, deer and vegetation, called a tri-trophic cascade; and another that involved smaller predators at Jasper Ridge. For the second, increased puma presence was associated with decreases in the activity of coyotes and bobcats, which were perhaps leaving the area or changing the time they are active to avoid the much larger pumas.

With fewer coyotes and bobcats on the landscape, foxes appear to have seized an opening and were seen more often, which then may have suppressed the activity of their primary prey: rabbits.

A deer photographed by motion capturing cameras at Stanford's Jasper Ridge Biological Preserve. When mountain lions started increasing their presence on the preserve in 2015, deer, their primary prey, decreased. Credit: Trevor Hébert/Stanford University.

These patterns involving an apex predator have been called the "ecology of fear," since the mere perception of a large predator can cause other animals to change their behavior, which then affects the other organisms that they rely on for food.

In this study, the findings at the lower level of the cascades—the indirect influence of mountain lions on vegetation, foxes and rabbits—are considered provisional, as other influences such as changes in fog and temperature could not be ruled out.

Yet the mountain lion presence had a clear impact on deer, coyotes and bobcats—and therefore, on the ecology of Jasper Ridge—underscoring the role of both apex predators and small preserves. In the U.S., 82% of protected areas are under 5 square kilometers (about 2 square miles), so they will likely be critical spaces for wildlife and plants as rapid urbanization continues, said Rodolfo Dirzo, study co-author and Stanford professor of biology in H&S.

"Maintaining sites where there is an entire community of animals, from predators to prey to the prey's resource base, is very important," he said. "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."

A bobcat photographed by motion capturing cameras at Stanford's Jasper Ridge Biological Preserve. When puma numbers increased at the preserve, the activity of mid-sized predators, including bobcats and coyotes, also went down. Credit: Trevor Hébert/Stanford University.

Mystery of mountain lion motivations

Why the mountain lions started to frequent Jasper Ridge is unknown. One theory is that the female mountain lions found the preserve to be a safe place to raise their young, as a mom with kittens has been spotted on camera during the study. Whatever the reason, they are only visitors. Mountain lions have a huge range in the Santa Cruz Mountains from 20 to 170 square kilometers (about 8 to 66 square miles). Jasper Ridge is far too small to have its own puma population.

Despite the occasional high-profile sighting of a mountain lion…