Fear of humans disrupts trophic interactions that underpin animal-mediated ecosystem functions

Keystone trophic interactions have disproportionate impacts on the structure and function of ecosystems by regulating the flow of nutrients through food webs.¹ However, human disturbances alter animal habitat use and foraging behavior, with uncertain consequences for trophic interactions and the ecosystem functions they support.^2,3 We simulated human activities to manipulate a keystone process-animals transporting salmon-derived nutrients from aquatic to terrestrial ecosystems-and documented the effects on animal behaviors and associated ecosystem functions. We used acoustic playbacks to alter the landscape of fear at salmon streams, and we experimentally added salmon carcasses to identify the pathways linking human activities to salmon removal, soil biogeochemistry, and organic matter decomposition. We show that animals, including bears (Ursus spp.), were twice as likely to flee from off-highway vehicle sounds compared with control sounds (gull calls), and they were almost 10× more likely to flee from human vocalizations. We found no effect of treatments on soil biogeochemistry, but structural equation models revealed significant direct and indirect effects of bears on decomposition via trophic interactions. Bears significantly increased the removal of added salmon at both control and treatment sites (p < 0.001), which significantly reduced decomposition (p < 0.001); however, the total effect of bears on decomposition was 3.6× stronger at control sites compared with treatment sites. Our findings demonstrate how human activities alter the landscape of fear, thereby disrupting foraging behaviors and degrading animal-mediated ecosystem functions. Conserving and restoring complex trophic interactions in human-dominated landscapes will therefore be essential to maintaining resilient ecosystems in the Anthropocene.