Cougars Save lives, New Study Finds

Cougars save lives. That is the counterintuitive finding of a recent study published in Current Biology (paywall). Researchers working with several local tribes on Washington’s Olympic Peninsula tracked cougar and deer movements and then overlaid those data with automobile collision data. They found that the presence of cougars influenced the movement of deer, resulting in substantially fewer crashes —a striking example of how the ecological effects of predators can translate into direct benefits for people.

Warren Cornwall interviewed the study’s lead author, Justin Suraci, and quotes him saying: “Deer alter their use of both space and time where pumas are present, overlapping less with roads and reducing activity at night, when some of the most dangerous collisions occur…Increased road safety thus represents an ecosystem service that large predators like pumas can provide to human communities.” You can read Cornwall’s article on the study in Anthropocene Magazine.

How predators change ecosystems

This finding is a fascinating example of a trophic cascade, an ecological process in which a predator reduces the number of its prey or changes their behavior, setting off effects that ripple through the ecosystem. The concept grew out of work by University of Washington marine ecologist Robert Paine, who also introduced the concept of the keystone species. Paine removed a species of sea star from one stretch of Pacific shoreline and left it in another. He found that in the absence of the sea star, its main prey increased and crowded out a number of other species, resulting in a substantial loss of biodiversity.

James Estes took this line of research and scaled it up. He discovered that, in the absence of sea otters, sea urchin populations could decimate kelp beds and reduce both the abundance and diversity of nearshore marine life. Paine and Estes, together with other researchers, demonstrated that top-down control could be an important ecological regulatory force. This contributed to an ongoing debate over the relative importance of top-down and bottom-up control in ecosystems.

From Yellowstone wolves to urban coyotes

The best-known example of a trophic cascade is the reintroduction of wolves to Yellowstone National Park. Christina Eisenberg tells this story in The Wolf’s Tooth. The argument is that the reintroduction of wolves reduced elk browsing, allowing aspen, willow, and other heavily browsed vegetation to recover. More abundant mature vegetation reduced streamside erosion, provided habitat for birds, and supplied food for beavers. Beavers, themselves ecosystem engineers, went about building dams, which further reduced streambank erosion, improved water quality, and created habitat for other species. This idea was popularized in the video “How Wolves Change Rivers,” which has been viewed more than 46 million times.

The Yellowstone story, however, may have been blown out of proportion. Daniel Stahler, Yellowstone’s lead wolf biologist, has said that the idea of a trophic cascade there is exaggerated. “You could argue a trophic trickle maybe,” he said in a 2024 New York Times story on Yellowstone’s wolves.

Whether Yellowstone’s wolves produced a cascade or a trickle, the capacity of predators to organize ecosystems indirectly through their effects on prey is now well established. I found one particularly interesting example from an urban context. Kevin Crooks and Michael Soulé studied the fragmented landscape of southern California. They found that large habitat fragments that included coyotes had lower populations of smaller predators—raccoons, opossums, and domestic cats—and a greater diversity of birds. Smaller fragments without coyotes showed the inverse condition: abundant small predators and lower diversity of scrubland birds.

When trophic cascades reach people

What is especially fascinating is that coyote predation on cats was only one cause of lower cat abundance. Another was the decision by cat owners to keep their cats indoors when coyotes were present. The ecological effects of the predator therefore operated not only through predation, but through changes in human behavior.

Typically, we think of a trophic cascade as flowing down the food chain. In both the cougar and coyote examples, however, the effects eventually reached the species that considers itself at the top: us.

These are compelling examples of why policymakers, planners, and designers are increasingly exploring ways to make room for wildlife in the built environment. We may benefit far more from coexistence, despite the occasional friction it creates, than we would from eradicating medium and large predators.