Bears, eagles, and humans, oh my: How human-wildlife interactions impact forest nutrient cycles

Footprints from bears, eagles, and humans in the mud alongside a stream in the Héen Latinee Experimental Forest, Alaska. Forest Service photo by Philip Manlick.

The age-old question asks, “If a tree falls in a forest and no one is around to hear it, does it make 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’s forests, their reality is an acoustic soundscape where everything from falling trees, other animal noises, and of course, the sounds of humans, have important impacts on their behavior. That’s the basis of the question that 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 dispersing nutrients in forested ecosystems. In a typical, undisturbed forested landscape, bears, for example, will catch or scavenge for salmon as the fish return to freshwater spawning grounds. In times of relative abundance, bears will 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 by way of their feces. The remaining parts of the fish carcass are typically left along the near shore 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.

The acoustic camera system used to record animal reactions to various sounds in riparian areas of the Héen Latinee Experimental Forest, Alaska. Forest Service photo by Philip Manlick.

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 would activate 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 ten times more likely to flee from the sound of humans talking when compared to the control sites. In addition, the cameras documented the animals displaying varied behaviors at “human-disturbed” test sites, such as removing less salmon from streams and a bias toward nighttime foraging, when compared to 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 a key part 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 be used to help prioritize trail and recreation maintenance or to 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 both preserve recreation opportunities, but also to preserve areas “where bears can be bears,” as Manlick put it.

Planning around these outcomes would allow forests to soak up the nutrients that they need in the areas that need them most, supporting the many ways in which forests provide for the people and animals that depend on them every day.

Caption: Top left: A cinnamon-colored black bear and her cub flee after hearing the sound of human voices. Top right: Brown bears on alert after hearing the sound of engines from an off-highway vehicle. Bottom left: A bald eagle eats a freshly caught meal from the nearby stream, ignoring the control sounds of gull calls. Bottom right: A black bear continues to eat a salmon, ignoring the sound of gull calls at a control site. Forest Service photos.