For Wildlife, Human Noise Is the Downside of Outdoor Fun

For hikers, bikers, and dog walkers, the proposed Skyline Trail was a no-brainer—another way to clear their heads and enjoy views of the Grand Tetons near Jackson, Wyoming. But as the U.S. Forest Service gathered public input on its plan in 2011, wildlife advocates argued that the fast-developing region didn’t need another trail, let alone one that would pulse more people through increasingly rare intact habitat where mule deer calved and American Goshawks nested.

The supporters ultimately prevailed, but the debate wasn’t over or confined to Jackson. Around the country, land management agencies are building more trails for a public that wants to get outside. The pandemic sent more people into the woods: National forests hosted 18 million more visits in 2020 than in 2019, a 12 percent increase. Visitation has remained above pre-pandemic levels since.

Trail running and backcountry skiing may leave a smaller mark on the landscape than, say, mining or drilling, but such activities still impact wildlife. In 2016—a year before the Skyline Trail was completed—scientists surveyed the existing research and found abundant evidence that outdoor recreation can have a negative influence on wild animals. The most severe effects include less species diversity, lower reproduction, and reduced survival.

That has public lands officials trying to find the right balance. “How do we provide great trail systems close to communities,” says Linda Merigliano, recreation planner for the Bridger-Teton National Forest, “while leaving big wild areas undisturbed?”

To help answer that question, the Forest Service and conservation groups in 2020 launched the Jackson Hole Recreation-Wildlife Co-Existence Project, a research effort across northwest Wyoming. Its aim is to better understand where, when, and how fun-seeking humans encroach on wild animals that are trying to find mates or keep themselves and their offspring fed—and then minimize disturbance. Some of the project’s most recent work seeks to understand a particularly pesky form of intrusion: sound.

Not surprisingly, previous research had demonstrated that the din of roaring tourist aircraft and whining snowmobiles changes how wildlife behave. But studies also show that people driving ordinary vehicles down highways to a favorite trailhead or fishing hole are enough to cause a disturbance. For a 2015 study, researchers played the recorded sounds of automobile traffic in a roadless area of Idaho where many birds stop to feed during fall migration. Avian abundance dropped by 25 percent. 

“The animals are using sounds of humans to avoid the area,” says Jesse Barber, curator of conservation science at the American Museum of Natural History and an author of that study. “It’s adding information to the landscape, and the information is, ‘There’s people there and I don’t want to be by them.’” Among those birds that stuck around despite the noise, MacGillivray’s Warblers and other species were so stressed out by this “phantom road” that they had trouble gaining the weight needed to continue their southward journeys. 

In Wyoming, wildlife biologists Mark Ditmer and Kathy Zeller with the Forest Service’s Rocky Mountain Research Station decided to go further and study whether animals react similarly to the sounds of trail users. They played audio of mountain bikes and chatty hikers from speakers and, using trail cameras, observed how wildlife reacted. Their 2024 paper reported that mammals such as elk, pronghorn, and black bears were far more likely to flee from those human sounds than from natural ones. Now the pair are studying how birds respond to the same noises, this time by recording avian vocalizations and identifying changes in the soundscape. 

The results won’t be clear for months, but they could eventually help convince outdoors enthusiasts to accept some restrictions that minimize disturbance from their favorite pastimes. “We need to make sure, for those species that remain, we’re doing it right—that the habitat isn’t getting degraded in ways we haven’t thought about before,” Ditmer says. In Jackson, the Forest Service closes the Skyline Trail for portions of spring to benefit nesting birds and calving deer. While some locals have protested, most people understand that humans need to give up a little to help wildlife, Merigliano says. 

Experts agree that the best way forward will involve compromises, not kicking people out of the woods. On top of its physical and mental health benefits, going outside may also encourage people to use their dollars, votes, or voices to support wildlife. A 2020 study showed that when Barber posted signs at Muir Woods National Monument in California asking people to remain quiet, visitors reported finding more birds. In follow-up surveys, they said they would be more likely to support conservation measures. 

Smarter management of wildlife habitat starts with acknowledging a truth that is becoming increasingly evident, Barber says: “When you build a trail, you change a place.” 

This story originally ran in the Winter 2024 issue as “Sounds Like Fun.” To receive our print magazine, become a member by making a donation today.