Sandwiched between the Californian cities of San Francisco, Santa Cruz and San Jose are the Santa Cruz Mountains. This biodiverse region contains redwood forests, watersheds – and mountain lions.
Also known as pumas, these big cats roam territories in the mountain range that typically span 20 to 170 square kilometres (around 8 to 66 square miles). But scientists from Stanford University discovered that they were expanding their range into more suburban areas, around 45 miles south of San Francisco.
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A trophic cascade
Stanford University’s Jasper Ridge Biological Reserve ('Ootchamin 'Ooyakma) stretches 1,193 acres to the east of the Santa Cruz Mountains, and acts as a natural laboratory for students and researchers.
Between 2015 and 2020, researchers noticed that mountain lions were appearing more frequently on trail cameras in the reserve – including in the day. Mountain lions are usually nocturnal creatures and will generally avoid people wherever possible.
During the same period, they also observed a decline in deer activity compared with previous years when mountain lions were rarely seen.
Vegetation surveys revealed that woody plants commonly eaten by deer (such as young oak trees) showed signs of recovery and even growth. These findings, which are published in the journal Ecology and Evolution, point to a trophic cascade in the area.
This is when changes at the top of a food chain create ripples through multiple levels of an ecosystem. One of the most-cited examples of a trophic cascade is the reintroduction of grey wolves to Yellowstone National Park – although later research has challenged these claims.
Ecology of fear
Researchers identified another trophic cascade which involved smaller predators that are found within the reserve. As mountain lions were observed more frequently, coyote and bobcat activity decreased.
The scientists suggest that these animals may have avoided the area, or shifted their activity patterns, to reduce encounters with the larger predators.
Fox activity consequently increased as their predators posed less of a threat, which could then in turn reduce activity among rabbits. This is known as the ecology of fear: when a top predator can influence the behaviour and activity of other animals, even without direct predation.
However, the researchers stress that the lower-level impacts (such as vegetation, foxes and rabbits) in the study remain tentative due to possible influence from environmental factors.
But they noted that the evidence linking mountain lion activity to that in deer, coyotes and bobcats was significantly stronger, suggesting that even smaller ecosystems can be affected by the arrival of large predators.
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“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 a doctoral student at Stanford University.
Further research is needed to identify why mountain lions began to use Jasper Ridge between 2015 and 2020. One suggestion is that the reserve provides a safer place to raise young, with trail cameras capturing images of a mother with kittens.
The team also stress the increasing role that smaller preserves may play in supporting wildlife.
“Maintaining sites where there is an entire community of animals, from predators to prey to the prey’s resource base, is very important,” explained Rodolfo Dirzo, the study’s co-author and a Stanford University biology professor.
“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.”
Read the full paper here: Mammal Community Responses to Increasing Puma Activity in a Suburban Preserve
Top image: a mountain lion photographed by a motion-activated cameras on Stanford's Jasper Ridge Biological Preserve. Credit: Trevor Hébert/Stanford University
