When the presence of humans is palpable, pumas and medium-sized carnivores keep a lower profile, according to a new study. The research suggests the change in predator behavior allows rodents to take a more brazen approach to foraging.

The findings, published this week in the journal Ecology Letters, highlight the ripple effects both humans and fear can have on ecosystem dynamics.

"Humans are sufficiently scary to pumas and smaller predators that they suppressed their behavior and changed the way they used their habitats when they thought we were around," lead author Justin Suraci, a postdoctoral researcher at the University of California, Santa Cruz, said in a news release. "The most surprising part was seeing how those changes benefit rodents."

For the study, researchers placed a grid-like pattern of 25 speakers across a one-square-kilometer-sized swath of the Santa Cruz Mountains. Some of the speakers broadcast human voices when pumas came near, while others, the control group, played the calls of tree frogs.

Researchers used telemetry to track the movement of pumas and motion-sensor cameras to observe their behaviors in response to the different sounds.

"When the frog recordings played, they would move right through the landscape," said Suraci. "But when they heard human voices, they went out of their way to avoid the grid."

Human voices caused mountain lions to slow down, reduce their activity and limit their movements. The presence of humans caused bobcats to become more nocturnal and skunks to reduce their activity levels. Possums also reduced their foraging activity by 66 percent when human voices were played through the speakers.

"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."

As humans continue to encroach on what remains of the wilderness predators like pumas rely on, the mountain lions could struggle to find enough to eat. Previous studies have shown human development has made it increasingly difficult for mountain lions to find genetically distinct mates and maintain the amount of genetic diversify needed for population health.

Predators weren't the only ones impacted by the presence of humans. Research showed the change in predator behavior opened up the landscape for smaller species. Rodents, including deer, mice and wood rats, significantly increased their daytime foraging activity levels.

"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," Suraci said.

As the presence becomes a reality for more and more wild animals, researchers are keen to understand how ecosystem dynamics will shift and how already vulnerable species will fare.

"Just the fear of humans can affect how wildlife use the landscape and how they interact with each other," said Suraci. "It turns out, the mere perceived presence of humans triggers a disruption of natural predator-prey interactions -- and rodents really benefit."

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