Multi‐temporal factors influence predation for polar bears in a changing climate

Abstract

Predation is an ecological interaction infl uenced by abiotic and biotic factors acting on multiple temporal scales, yet multi-temporal comparisons are rare in empirical studies. For polar bears Ursus maritimus , the physical confi guration of the habitat and conditions in which seals are hunted may change on intra-and inter-seasonal scales. Additionally, while the eff ects of climate change on polar bears have focused on linking reductions in sea ice to body condition and survival, the potential changes to on-ice hunting conditions have not been examined. Employing observational counts of seals killed by polar bears between early-April and late-May 1985 -2011 (n ϭ 650), we modelled the likelihood of predation events in the Beaufort Sea, Canada at multi-temporal scales. We used the top model to estimate the expected kill rate of seals in the springs of 1985 -1986 and 2005 -2006 and integrated the result with fasting rates derived from physiological markers in blood samples. A log-likelihood ratio test suggested a multi-temporal approach fi t the seal kill data better than any single scale alone. Predation events were infl uenced by ringed seal Pusa hispida reproduction and haul-out behaviour, regional sea ice concentration and the phase of climatic indices. Th e expected kill rate from the top predation model and the estimated mean biomass of seal kills were signifi cant predictors of polar bear fasting rates. Results suggest that 50% less seal biomass was killed in 2005 -2006 than in 1985 -1986, which correlates with a signifi cant increase in the frequency of polar bears in a fasting state. We propose that the documented changes in polar bear fasting rates between 1985 -1986 and 2005 -2006 are due to a complex set of abiotic and biotic factors including underlying prey dynamics, rather than a single-scale environmental correlation.

Spring fasting behavior in a marine apex predator provides an index of ecosystem productivity

“…Pilfold et al (2015) similarly documented impacts of wind speed on polar bear predation success and wind has been documented as an important factor for successful predation in other species (Funston, Mills, & Biggs, 2001;Nevitt, Losekoot, & Weimerskirch, 2008). The probability of a predation event for polar bears and ringed seal body condition from the eastern Beaufort Sea have both been shown to have significant relationships with the AO and AOO (Pilfold et al, 2015;Nyugen et al, 2017). These results are consistent with our finding that the winter AO in the Beaufort Sea was correlated with fasting probability.…”

“…Although wind speed did not vary across subpopulations, speed increased between periods and could affect polar bear fasting behavior via declines in haul-out duration of seals (Carlens, Lydersen, Kraffft, & Kovacs, 2006) or disruption of polar bears' olfactory cues (Togunov, Derocher, & Lunn, 2017). Pilfold et al (2015) similarly documented impacts of wind speed on polar bear predation success and wind has been documented as an important factor for successful predation in other species (Funston, Mills, & Biggs, 2001;Nevitt, Losekoot, & Weimerskirch, 2008). The probability of a predation event for polar bears and ringed seal body condition from the eastern Beaufort Sea have both been shown to have significant relationships with the AO and AOO (Pilfold et al, 2015;Nyugen et al, 2017).…”

“…We did not employ model selection, based on reasons similar to those outlined in Hobbs, Andr en, Persson, Aronsson, and Chapron (2012). We were not seeking to create the most parsimonious model, but rather to include all parameters known to potentially affect fasting (e.g., mating and capture date, sex, and age; Pilfold, Derocher, Stirling, & Richardson, 2015) and to test hypothetical factors that may affect fasting based on polar bear and seal ecology (as described above). We assessed the importance of each variable based on whether the 95% credible interval (CI) overlapped zero, similar to Hobbs et al (2012).…”

Invariant polar bear habitat selection during a period of sea ice loss

“…Similar to our results, Mauritzen et al [12] found that the probability of using high sea ice concentrations decreased when bears were located in areas with high sea ice concentrations, but exhibited the opposite pattern when bears were in areas with low concentrations of sea ice. Polar bears live in a highly dynamic environment, requiring them to respond by constantly moving to areas with conditions suitable for finding and capturing prey [16,36]. The lack of change in functional responses between periods implies that polar bears are making similar decisions in response to their local conditions as they did before sea ice changes.…”

Polar bears: proceedings of the 18th working meeting of the IUCN/SSC Polar Bear Specialist Group, Anchorage, Alaska, 7-11 June 2016

“…The spatial overlap between polar bears and whelping ringed seals likely influences a change in the age-class proportions of kills, as polar bears respond to the availability of vulnerable pups. Exploring the assumptions of common analytical modelling approaches in ecology, Pilfold et al (2015) showed that including biologically relevant measures, such as the size of kills, provided significant improvement to the models in both fit and interpretation. Measuring only the occurrence of an ecological event, whether temporally or spatially was insufficient when validated against independent data.…”