As apex predators, large carnivores can shape ecosystems by suppressing herbivores and smaller carnivores, initiating trophic cascades that may promote biodiversity and ecosystem function. However, most of our knowledge regarding the ecological role of large carnivores comes from relatively pristine habitats (e.g., national parks), while a growing number of large carnivore populations exist in or close to human-dominated landscapes. Humans are a major source of mortality for many wildlife populations, and are often particularly lethal to large carnivores, meaning that fear of humans maybe widespread among these species. Yet it remains unknown whether and how the fear of humans as apex predators will reshape terrestrial food webs. We conducted a landscape-scale field experiment to test the effects of the fear of humans on terrestrial food webs, presenting human or control vocalizations over replicate 1-km2 patches of habitat in the Santa Cruz Mountains of central California. We tested for changes in large carnivore (Puma, Puma concolor) habitat use in response to perceived human presence using GPS collars on seven individuals, and monitored for changes in local abundance and behavior among lower trophic level species using camera traps (mescarniovres, large herbivores) and live trapping grids (small mammals).
Results/Conclusions
Pumas significantly altered their habitat use during human treatments relative to controls, avoiding areas of high perceived human presence. We found similar suppressive effects of perceived human presence on mesocarnivore behavior, with species variously reducing overall activity levels, reducing foraging intensity, and shifting towards more nocturnal activity during the human treatment, relative to controls. Small mammal trapping results suggest that changes in mesocarnivore behavior in response to perceived human presence may additionally affect their interactions with their small mammal prey. This work provides the first experimental evidence that the perceived presence of human, independent of habitat alteration or other anthropogenic impacts, substantially alters the behavior of and interactions between species across terrestrial food webs. Our results indicate that the fear of humans is widespread among terrestrial wildlife species, affecting the habitat use and behavior of large carnivores and smaller predators alike, and suggest that the mere presence of humans may have cascading effects across terrestrial communities.