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Classifying the effects of human disturbance on denning polar bears

Susannah P. Woodruff*, Erik M. Andersen, Ryan R. Wilson, Lindsey S. Mangipane, Susanne B. Miller, Kimberly J. Klein, Patrick R. Lemons

*Corresponding author:

ABSTRACT: Climate change is resulting in decreased sea ice extent and increased industrial activity in Arctic regions. In northern Alaska, USA, sea ice loss has increased the frequency of land-based polar bear Ursus maritimus maternal dens, leading to increased potential for overlap between industrial activities and denning bears. Responses of denning bears to human disturbance could result in costly reproductive outcomes, although observation of these responses is logistically challenging and expensive. We developed a method to standardize the process of classifying the response of denning polar bears to disturbance using decision rules based on polar bear biology and denning chronology. We applied this method to 46 maternal polar bear dens exposed to human activity (e.g. vehicle traffic, ground-based monitoring). Because the timing of disturbance influences the response and subsequent fitness consequences, we determined outcomes specific to 4 denning periods: (1) den establishment (den excavation to cub birth); (2) early denning (cub birth to 60 d old); (3) late denning (60 d old to den emergence); and (4) post-emergence (den emergence to den site departure). We classified outcomes as ‘no documented effect’ (no observed response), ‘behavioral’ response (observed behavioral disruption), ‘early emergence’ (den emergence occurring earlier than an undisturbed emergence), ‘early departure’ (den site abandonment post-emergence earlier than if undisturbed), or ‘cub mortality’ (death or abandonment of ≥1 cub). We classified the outcomes to 79 exposures as 37 having ‘no documented effect’, 7 as ‘behavioral’, 17 as ‘early emergence’, 14 as ‘early departure’, and 4 as ‘cub mortality’. Outcomes with potential fitness consequences occurred in every denning period. Our classification method facilitated a standardized approach that can be used in the future to classify the outcome of den disturbance. Determining outcomes in relation to a specific denning period may facilitate improved implementation of mitigation strategies to reduce disturbance to denning bears.