Modeling polar bear (Ursus maritimus) snowdrift den habitat on Alaska's Beaufort Sea coast using SnowDens-3D and ArcticDEM data

Abstract

Pregnant polar bears (Ursus maritimus) excavate maternal dens in seasonal snowdrifts during fall along Alaska's Beaufort Sea coast to shelter their altricial young during birth and development. With recent sea ice decreases, bears are denning more frequently on land. Each year, the weather and blowing-snow conditions control the creation of snowdrifts across the landscape. Therefore, available snowdrift den habitat can vary widely from one year to the next, depending on the late fall and early winter air temperature, snowfall, and wind speed and direction. We implemented a physics-based, spatiotemporal, polar bear snowdrift den habitat model (SnowDens-3D) across the eastern Alaska Beaufort Sea coast (an area of approximately 17,000 km²). High-resolution (2.0 m) topography data were provided by the ArcticDEM, and daily meteorological forcings were provided by NASA's MERRA-2 reanalysis. In many areas across the Arctic Alaska simulation domain, the raw ArcticDEM data contained physically unrealistic topographic anomalies (bumps and depressions) of similar magnitude (± 1.5 m) to the topographic variations that underlie potential den habitat (height differences of approximately 1.5 m). To create an ArcticDEM dataset for this den habitat model, considerable pre-processing of the ArcticDEM data was required; we implemented numerous filters to remove the topographic anomalies while preserving those topographic features capable of creating snowdrifts deep enough to provide viable polar bear den habitat. A 21-year (2000–2020) SnowDens-3D simulation was performed, and model outputs were compared with 91 historical polar bear den locations. The year-specific simulations identified viable den habitat for 98% of the observed den locations. The interannual variation in den habitat area over the 21-year period ranged by approximately a factor of three from the minimum year (2001; 554 km²) to the maximum year (2017; 1,566 km²). The ability to identify viable polar bear snowdrift den habitat in near-real time, as demonstrated here, will help wildlife managers and industry personnel identify potential polar bear maternity den sites and minimize disturbance to occupied dens.