Human effects on brown bear diel activity may facilitate subadults foraging on Pacific salmon

Abstract

Humans can markedly alter the temporal activity of wildlife. The ecological consequences of such temporal shifts are poorly understood but can lead to reduced fitness, increased competition, and induce trophic cascades under certain conditions. Furthermore, if individuals or species vary in their tolerance to human disturbance, then they may be able to exploit resources left vacant by more human-sensitive animals. Here, we explored the potential of a “temporal-shield” offered by human disturbance, using brown bears (Ursus arctos) foraging on sockeye salmon (O. nerka) in southwestern Alaska as a model system. We deployed motion-activated cameras on six salmon spawning streams over five summers (2013-2018), capturing footage of 1,935 independent bear encounters including single adult bears (n=1,612), females with cubs (n=197) and subadults (n=126). Using a von Mises circular kernel density estimation procedure, we estimated the overlap of activity for each bear group type relative to humans observed on our cameras (n=932 encounters; all researchers). All bears avoided peak times of human activity, but socially-subordinate subadults exhibited significantly higher overlap with humans (23%) as compared to females with cubs and single adults (11% and 12%, respectively). Furthermore, subadult bears increased their overlap with human activity over the course of each sampling year, while females with cubs and single adults generally decreased their overlap. These results highlight that the effects of human-disturbance on large carnivores can be complex and may allow for increased foraging opportunities for socially subordinate, but more human-tolerant, individuals.