No abstract

The interaction between brown bears (Ursus arctos) and Pacific salmon (Oncorhynchus spp.) is important to the population dynamics of both species and a celebrated example of consumer‐mediated nutrient transport. Yet, much of the site‐specific information we have about the bears in this relationship comes from observations at a few highly visible but unrepresentative locations and a small number of radio‐telemetry studies. Consequently, our understanding of brown bear abundance and behavior at more cryptic locations where they commonly feed on salmon, including small spawning streams, remains limited. We employed a noninvasive genetic approach (barbed wire hair snares) over four summers (2012–2015) to document patterns of brown bear abundance and movement among six spawning streams for sockeye salmon, O. nerka, in southwestern Alaska. The streams were grouped into two trios on opposite sides of Lake Aleknagik. Thus, we predicted that most bears would forage within only one trio during the spawning season because of the energetic costs associated with swimming between them or traveling around the lake and show fidelity to particular trios across years because of the benefits of familiarity with local salmon dynamics and stream characteristics. Huggins closed‐capture models based on encounter histories from genotyped hair samples revealed that as many as 41 individuals visited single streams during the annual 6‐week sampling season. Bears also moved freely among trios of streams but rarely moved between these putative foraging neighborhoods, either during or between years. By implication, even small salmon spawning streams can serve as important resources for brown bears, and consistent use of stream neighborhoods by certain bears may play an important role in spatially structuring coastal bear populations. Our findings also underscore the efficacy of noninvasive hair snagging and genetic analysis for examining bear abundance and movements at relatively fine spatial and temporal scales.

Predators may alter niche overlap between prey species by eliciting divergent anti‐predator behavior. Accordingly, we exploited heterogeneous gray wolf Canis lupus presence in Washington, USA, to contrast patterns of resource and dietary overlap between mule Odocoileus hemionus and white‐tailed deer O. virginianus at sites with and without resident packs. Mule deer run (stot) in a way that is less effective as a means of fleeing from predators than the galloping gait of white‐tailed deer. Consequently, mule deer manage risk from coursing predators like wolves by avoiding encounters, whereas white‐tailed deer respond to such predators by exploiting areas where they are most likely to escape pursuit. Thus, under the ‘refuge partitioning hypothesis' whereby predators reduce prey niche overlap by eliciting use of different refugia, we predicted wolf exposure to 1) decrease resource and dietary overlap between these ungulates, and 2) induce segregation consistent with each species using different parts of the landscape to reduce their wolf risk. At the home range scale, the ways in which resource overlap diminished in the wolf areas were consistent with the prey species reducing their respective risks, particularly with respect to slope, with mule deer separating from white‐tailed deer by seeking steeper areas where wolf encounters are less likely. At the within‐home range scale, the manner in which spatial overlap decreased in relation to forest cover was consistent with species‐specific risk management, with mule deer avoiding wolf encounters by shifting toward this resource. Reduced resource overlap between the deer in areas occupied by wolves did not correspond with dietary divergence. Our findings suggest that wolf risk mediates spatial but not necessarily dietary overlap between sympatric ungulates, divergent anti‐predator behavior is a non‐consumptive pathway by which predators can reduce interspecific competition among prey, and use of disparate refugia by prey may not result in dietary divergence.