Steep and deep: Terrain and climate factors explain brown bear (Ursus arctos) alpine den site selection to guide heli-skiing management

Abstract

Winter recreation and tourism continue to expand worldwide, and where these activities overlap with valuable wildlife habitat, there is greater potential for conservation concerns. Wildlife populations can be particularly vulnerable to disturbance in alpine habitats as helicopters and snowmachines are increasingly used to access remote backcountry terrain. Brown bears (Ursus arctos) have adapted hibernation strategies to survive this period when resources and energy reserves are limited, and disturbance could negatively impact fitness and survival. To help identify areas of potential conflict between helicopter skiing and denning brown bears in Alaska, we developed a model to predict alpine denning habitat and an associated data-based framework for mitigating disturbance activities. Following den emergence in spring, we conducted three annual aerial surveys (2015-2017) and used locations from three GPS-collared bears (2008-2014) to identify 89 brown bear dens above the forest line. We evaluated brown bear den site selection of land cover, terrain, and climate factors using resource selection function (RSF) models. Our top model supported the hypothesis that bears selected dens based on terrain and climate factors that maximized thermal efficiency. Brown bears selected den sites characterized by steep slopes at moderate elevations in smooth, well-drained topographies that promoted vegetation and deep snow. We used the RSF model to map relative probability of den selection and found 85% of dens occurred within terrain predicted as prime denning habitat. Brown bear exposure to helicopter disturbance was evident as moderate to high intensities of helicopter flight tracking data overlapped prime denning habitat, and we quantified where the risk of these impact was greatest. We also documented evidence of late season den abandonment due to disturbance from helicopter skiing. The results from this study provide valuable insights into bear denning habitat requirements in subalpine and alpine landscapes. Our quantitative framework can be used to support conservation planning for winter recreation industries operating in habitats occupied by denning brown bears.

Fig 1. Study area used to describe brown bear alpine den site selection in Southeast Alaska, U.S.A., 2008–2017.

Generalized brown bear den locations are denoted by black circles, study area is delineated by the solid yellow line, approved heli-skiing areas are shaded violet, and dominant land cover classes are detailed, republished from [57] under a CC BY license, with permission from Commission for Environmental Cooperation, original copyright 2020.

Fig 2. Photographs taken during aerial surveys of brown bear den site selection in alpine and subalpine habitats near Haines, Alaska, U.S.A.

a) View of the study area from fixed-wing aircraft while scanning for dens, b) multiple den sites excavated in earth and surrounded by shrub habitat at 983 m elevation on 36° slope, c) bear observed near den site after emergence, d) female with two cubs observed at den entrance.

Fig 3. Brown bear resource selection function model (RSF) based on terrain and climate factors important to brown bears denning in alpine and subalpine habitats near Haines, Alaska, U.S.A., 2008–2017.

Generalized bear den locations are depicted by black circles, the study area is outlined by a solid yellow line, approved heli-skiing areas are shaded violet, and relative probability of den use is color coded. The map inset displays a zoomed in portion of the study area to detail the high-resolution detail of the RSF surface. Land cover information is republished from [57] under a CC BY license, with permission from Commission for Environmental Cooperation, original copyright 2020.

Fig 4. Average effect of individual continuous factors from resource selection function model of brown bear alpine den site selection near Haines, Alaska, U.S.A., 2008–2017.

Black lines represent the relative probability of den site selection and gray shading signifies 95% confidence intervals.

Fig 5. Heli-skiing impact risk to prime brown bear denning habitat near Haines, Alaska, U.S.A., 2008–2017.

Areas where prime denning habitat was overlapped by high helicopter intensity are colored red, other areas are color coded to show the potential risk to denning habitat from heli-skiing. Generalized brown bear den locations are denoted by black circles, study area is delineated by the solid yellow line, approved heli-skiing areas are outlined in violet, and authorized heliports are identified. Land cover information is republished from [57] under a CC BY license, with permission from Commission for Environmental Cooperation, original copyright 2020.