The presence of people in remote areas of the Santa Cruz Mountains turns mountain lions into veritable fraidy-cats and strikes so much fear in bobcats, skunks and opossums that they change their behavior to avoid detection, a new study has found.
Rats and mice, on the other hand, actually forage more in areas where homo sapien voices are heard, probably because they know fewer rodent-eating predators are around, the UC Santa Cruz study concluded.
The paper, published Wednesday in the journal Ecology Letters, describes how humans create a “landscape of fear” among both large and small predators just by being around, a situation that is ripe for exploitation by rodents and, potentially, other pests like ticks.
“We already know that humans are incredibly lethal predators. We kill other predators at much higher rates than any other predator kills predators,” said Chris Wilmers, an associate professor of environmental studies who co-authored the paper with doctoral students. “What we didn’t know was the impact of just our presence in the forest.”
The findings, part of the university-managed Santa Cruz Puma Project, came two years after a previous study by Wilmers showed cougars on trail cameras abandoning deer carcasses and turning tail and running when recorded human voices suddenly started playing near them.
This time Wilmers wanted to find out the wider impact of the human presence, so his research team selected two remote locations closed to the public that some of the more than 40 cougars fitted with GPS and radio telemetry collars are known to frequent.
Twenty-five speakers were spaced evenly in five rows of five in each of the two square-kilometer grids, one inside the Sierra Azul Preserve, just south of Los Gatos, and the other in the San Vicente Redwoods, east of the coastal town of Davenport. There were about 200 meters between each speaker.
Between May 29 and Aug. 31, 2017, the speakers alternated broadcasts of human voices and Pacific tree frog vocalizations for five weeks each, with long silences in between. The researchers then compared the behavior and responses of the various animals.
The seven mountain lions they observed changed their behavior dramatically when the human voices were playing, becoming more cautious and avoiding the area where they perceived there was a human presence. The cougars increased their distance from the nearest speaker by 29% and were detected inside the test areas 30% less often when human voices were being broadcast.
“They both avoided the grid and changed their behavior,” said Justin Suraci, a post doctoral student in Wilmers’ lab and the lead author of the study. “They slowed down their movement speed, which we interpreted as increased caution.”
Bobcats reduced their daytime activity by 31%, skunk activity decreased 40%and opossums foraged 66% less when people were talking. All the medium-sized carnivores were detected less on camera at feed stations when the human voices were within earshot.
“All three of the meso predators were behaviorally suppressed by the presence of humans,” Suraci said. “As it turns out humans are sufficiently scary that it was better to be more cautious and avoid a risky human encounter.”
The opposite happened with mice and rats. Deer mice expanded their range by 45% when people were heard talking in the forest. Both mice and woodrats increased their foraging activities by 17% compared to times when human voices weren’t playing, according to the study.
None of the animals in the experiment changed their behavior or reacted in any noticeable way to the sound of tree frogs.
Wilmers said the sudden boldness of rodents is probably as significant to us as the fear displayed by the mountain lions. It could mean more tick and insect-borne diseases, like Lyme disease, are being spread by rodents and other prey species when predators aren’t around.
Previous studies of cougars in the Santa Cruz Mountains showed that they kill more deer in residential areas, but spend less time feeding when they are near humans. Researchers believe they abandon prey more often around people and then must kill more deer because they are hungry.
Wilmers said human-cougar encounters in the Bay Area mostly occur because mountain lion travel corridors have been blocked by development. One such incident occurred in May 2014 when a large male puma hid behind a small hedge on a busy street in Mountain View for nine hours as pedestrians and bicyclists passed only a few feet away.
The confused cat eventually was tranquilized amid a community furor and released in the hills, but he was later killed trying to cross Interstate 280.