Survival and abundance of polar bears in Alaska’s Beaufort Sea, 2001–2016

Abstract

The Arctic Ocean is undergoing rapid transformation toward a seasonally ice‐free ecosystem. As ice‐adapted apex predators, polar bears (Ursus maritimus) are challenged to cope with ongoing habitat degradation and changes in their prey base driven by food‐web response to climate warming. Knowledge of polar bear response to environmental change is necessary to understand ecosystem dynamics and inform conservation decisions. In the southern Beaufort Sea (SBS) of Alaska and western Canada, sea ice extent has declined since satellite observations began in 1979 and available evidence suggests that the carrying capacity of the SBS for polar bears has trended lower for nearly two decades. In this study, we investigated the population dynamics of polar bears in Alaska's SBS from 2001 to 2016 using a multistate Cormack–Jolly–Seber mark–recapture model. States were defined as geographic regions, and we used location data from mark–recapture observations and satellite‐telemetered bears to model transitions between states and thereby explain heterogeneity in recapture probabilities. Our results corroborate prior findings that the SBS subpopulation experienced low survival from 2003 to 2006. Survival improved modestly from 2006 to 2008 and afterward rebounded to comparatively high levels for the remainder of the study, except in 2012. Abundance moved in concert with survival throughout the study period, declining substantially from 2003 and 2006 and afterward fluctuating with lower variation around an average of 565 bears (95% Bayesian credible interval [340, 920]) through 2015. Even though abundance was comparatively stable and without sustained trend from 2006 to 2015, polar bears in the Alaska SBS were less abundant over that period than at any time since passage of the U.S. Marine Mammal Protection Act. The potential for recovery is likely limited by the degree of habitat degradation the subpopulation has experienced, and future reductions in carrying capacity are expected given current projections for continued climate warming.

Modeling the spatial and temporal dynamics of land‐based polar bear denning in Alaska

“…To estimate den abundance, we drew on published datasets from previous long‐term studies (Bromaghin 2015, USGS Alaska Science Center 2018, Durner 2020). The abundance of the SBS subpopulation was stable following a sharp decline at the beginning of this period (Bromaghin et al 2015, 2021).…”

“…For each year i : $$${{NAFC}0\text{\_}{obs}}_{i}\unicode{x0007E}\text{binomial}(p={pAFC}0,n={{Nbears}\text{\_}{obs}}_{i}){{Ndens}\text{\_}{success}}_{i}=\text{binomial}(p={pDensuccess},n={{Ndens}\text{\_}{obs}}_{i})$$$ The den abundance model also included priors describing the total population size ( Nbears ) as a normal distribution with a mean of 908 bears and a standard deviation of 163.8 bears, based on a population estimate for 2015 (Bromaghin et al 2015). These were reasonable priors because the SBS subpopulation was stable from 2006 to at least 2015 (Bromaghin et al 2021). Using this distribution for Nbears , we estimated the expected number of adult females with cubs ( NAFC0 ) from a binomial process (Equation ): $$${Nbears}\unicode{x0007E}\text{Normal}({\rm{\mu }}=908,{\rm{\sigma }}=163.8){NAFC}0\unicode{x0007E}\text{binomial}(p={pAFC}0,n={Nbears})$$$ We modeled the number of failed dens as a negative binomial process (Equation ), where p = pDensuccess and constrained it to not exceed the number of adult females who did not have age zero cubs.…”

“…The den abundance model also included priors describing the total population size (Nbears) as a normal distribution with a mean of 908 bears and a standard deviation of 163.8 bears, based on a population estimate for 2015 (Bromaghin et al 2015). These were reasonable priors because the SBS subpopulation was stable from 2006 to at least 2015 (Bromaghin et al 2021). Using this distribution for Nbears, we estimated the expected number of adult females with cubs (NAFC0) from a binomial process (Equation 3):…”

“…Sixty percent of the SBS occurred within Alaska and its marine waters, while the remainder occurred in Canada. The IUCN has recently redefined the eastern SBS boundary as 133° W (Bromaghin et al 2021), but we chose to use the SBS boundary as defined by Obbard et al (2009) rather than the newer boundary because the bulk of den observations occurred prior to the boundary shift.…”

Drivers of polar bear behavior, and the possible effects of prey availability on foraging strategy

“…Greater habitat fragmentation has increased polar bear path tortuosity (Biddlecombe et al, 2021), more open water has increased the frequency of long-distance swimming events (Pilfold et al, 2016; Pagano et al, 2020), and increased ice drift speed has increased the cost of station-keeping (Mauritzen et al, 2003; Auger-Méthé et al, 2016; Durner et al, 2017). Further, polar bears have exhibited shifts in distribution (Lone et al, 2018), reduced access to prey (Stirling et al, 2008; Ware et al, 2017; Florko et al, 2020b), a longer fasting period (Rode et al, 2021), increased exposure to zoonotic pathogens (Pilfold et al, 2021), higher levels of cortisol (Boonstra et al, 2020), reduced body condition (Stirling and Parkinson, 2006; Rode et al, 2021), reduced access to denning habitat (Rode et al, 2021; Merkel and Aars, 2022), reduced reproduction (Stirling et al, 1999), and consequently reduced abundance in several populations (Regehr et al, 2007; Lunn et al, 2016; Regehr et al, 2016; Obbard et al, 2018; Bromaghin et al, 2021). Many of the effects of climate change on polar bears are associated with behavioral shifts including changes in foraging (Galicia et al, 2016), migration (Pilfold et al, 2016; Pagano et al, 2020), and denning strategies (Olson et al, 2017; Escajeda et al, 2018).…”