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Human noise pushes Alaska predators toward night foraging, altering salmon nutrient pathways
The age-old question asks, "If a tree falls in a forest and no one is around to hear it, does it make a sound?" While philosophers and scientists alike have pondered this question for centuries, the more relevant question for today's recreation and wildlife managers is: Does it matter what makes the sound?
For keystone predators in Alaska forests, reality is an acoustic landscape where everything from falling trees and other animal noises to the sounds of humans affects their behavior. That's the question Philip Manlick, a research ecologist with the Pacific Northwest Research Station, set out to answer. As humans recreate, forage or otherwise spend time in forests, what effect do these sounds have on wildlife and, subsequently, nutrient distribution if keystone predators begin avoiding those areas?
Black bears, brown bears and eagles—all keystone predators—play important roles in dispersing nutrients in forested ecosystems. In a typical, undisturbed forested landscape, bears, for example, catch or scavenge salmon as the fish return to freshwater spawning grounds. In times of relative abundance, bears eat only the choice, fatty parts of the fish, such as the eggs, brains and skin. The bears then facilitate the spread of nutrients further into forested landscapes through their feces.
The remaining parts of the fish carcass are typically left along the nearshore area, either scavenged by other carrion eaters or left to decompose. Both outcomes provide outlets for nutrient-rich salmon carcasses to release critical forest-building blocks, such as nitrogen, into the ecosystem.
What happens, though, when humans are part of the equation? To answer this question, Manlick and his team set up motion-activated cameras and speakers in riparian areas of the Héen Latinee Experimental Forest near Juneau, Alaska. They programmed the speakers to play the sounds of off-highway vehicles, humans talking and, as a control, the natural sound of seabirds. The researchers then placed salmon in the riparian areas to attract opportunistic predators, which activated the sounds and cameras to document the animals' behaviors.
The cameras captured hundreds of interactions between the programmed sounds and opportunistic predators such as bald eagles, ravens, black bears and brown bears. The research team found that animals were twice as likely to flee from sounds from off-highway vehicles and almost 10 times more likely to flee from the sound of humans talking compared with the control sites. In addition, the cameras documented varied behaviors at "human-disturbed" test sites, such as removing less salmon from streams and a bias toward nighttime foraging, compared with control sites.
This study demonstrates that humans influence both the spatial and temporal foraging habits of keystone predators and, thus, to a degree, where key nutrients are deposited and which trees or plants have access to those nutrients.
Application
Tourism, recreation and human foraging are key parts of rural economies. Since the onset of COVID in 2020, land managers and forest planners have been contending with sharp increases in visitors to national and state forests.
This information can help prioritize trail and recreation maintenance or identify locations for new trail or recreation sites while also providing areas where animals can go undisturbed. The equation is not about humans or bears but rather maintaining productive forests for coexistence. This allows managers to preserve both recreation opportunities and areas "where bears can be bears," as Manlick put it.
Planning around these outcomes would allow forests to soak up the nutrients they need in the areas that need them most, supporting the many ways forests provide for the people and animals that depend on them every day.
Publication details
Philip J. Manlick et al, Fear of humans disrupts trophic interactions that underpin animal-mediated ecosystem functions, Current Biology (2026). DOI: 10.1016/j.cub.2026.04.063
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Citation: Human noise pushes Alaska predators toward night foraging, altering salmon nutrient pathways (2026, July 14) retrieved 14 July 2026 from https://phys.org/news/2026-07-human-noise-alaska-predators-night.html
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