Pumas, Bobcats Hiding Out While Rodents Flourish: UCSC Study

SANTA CRUZ COUNTY, CA — A new study out of the University of California, Santa Cruz, finds that humans are quite scary to local pumas and medium-sized predators. These animals are hiding out when they sense mankind in their territory, and that means potentially higher populations of mice and other small rodents.

"Humans are sufficiently scary" to pumas and smaller predators, so much so that they change their behavior, said lead author Justin Suraci, a postdoctoral researcher at UC Santa Cruz. "The most surprising part was seeing how those changes benefit rodents."

Fear of humans suppresses the movement and activity of pumas, bobcats, skunks, and opossums, which benefits small mammals. As their own predators respond to their fear of humans, deer mice and wood rats perceive less risk and in turn forage for food farther away and more intensively, the researchers found.

Humans are top predators of many wildlife species, and our mere presence can create a "landscape of fear," according to the researchers.

The new study, "Fear of Humans as Apex Predators has Landscape-scale Impacts from Mountain Lions to Mice," appears in the July 17 online edition of the journal Ecology Letters.

"We've spent 10 years learning how fear of humans drives mountain lion physiology, behavior, and ecology," said senior author Chris Wilmers, a professor of environmental studies at UC Santa Cruz and director of the Santa Cruz Puma Project. "This is the first large-scale experiment I'm aware of that documents how fear cascades through the food web from top predators to the smallest prey."

For their recent study, Suraci and Wilmers used recordings of human voices at two remote research sites in the Santa Cruz Mountains that are closed to public access. They placed 25 speakers in a grid pattern across a one-square-kilometer area. The speakers broadcast human voices, and recordings of tree frogs as a control.

Pumas, the researchers found, responded to the sound of human voices by significantly reducing their activity, keeping their distance, and slowing their movements. "When the frog recordings played, they would move right through the landscape," Suraci said. "But when they heard human voices, they went out of their way to avoid the grid."

Medium-sized predators changed their behavior in significant ways, too: Bobcats became much more nocturnal; skunks reduced their overall activity by 40 percent; and opossums reduced their foraging activity by a stunning 66 percent. "Bobcats pretty much gave up on daytime activity, shifting almost entirely to the night, when they presumably feel safer," said Suraci. "These predators aren't necessarily leaving the area, they're just less active, presumably because they're hiding more."

Over time, these behavioral changes could have dire consequences for pumas and the other predators if their food intake drops, Suraci said.

By contrast, deer mice increased their range by 45 percent, and the intensity of foraging by mice and wood rats increased by 17 percent. "They were apparently responding to the reduced activity of everybody else," Suraci added. "They're feeling braver, so they're moving around more and finding more food. They're not too averse to people, so they're taking advantage of the opportunity. It turns out, the mere perceived presence of humans triggers a disruption of natural predator-prey interactions—and rodents really benefit."

Read the full story from UCSC.