Influence of intraspecific competition on the distribution of a wide‐ranging, non‐territorial carnivore

Abstract

Aim It is generally held that the dominant competitors in a population will occupy high‐quality habitat while forcing subordinates into lower‐quality habitats through interference competition. We examined the distribution of a non‐territorial apex carnivore relative to foraging habitat to assess the effect of two types of interference: competitive asymmetries in predatory ability and conspecific predation risk. Location Beaufort Sea, Canada. Methods The quality of foraging habitat was modelled using resource selection functions to relate the locations of seals killed by polar bears (Ursus maritimus) to attributes of the sea ice. We used estimated seal biomass as a sample weight to reflect the energetic return of different sizes of kills. To test the effect of sample weighting, locations at which polar bears were captured were used to compare habitat quality modelled with kills weighted equally and kills weighted by their biomass. The distributions of different demographic classes of polar bears were then compared with the general predictions of unequal‐competitor models. Results Polar bear distribution was correlated with the quality of the foraging habitat as determined by the kill biomass model (rs = 0.90, P = 0.04), but not in the unweighted design (P = 0.75). No difference was detected in use of the highest‐quality foraging habitat by subadults and adults. Females with cubs‐of‐the‐year used lower‐quality foraging habitat relative to the rest of the population. Main conclusions Weighting the habitat model with biologically relevant information improved its fit to species distribution, and suggested that density of use alone was insufficient to define habitat quality. Intraspecific competition had a varying influence on the distribution: unequal competitors coexisted, while the avoidance of conspecific predation risk resulted in semi‐truncation.

Migratory response of polar bears to sea ice loss: to swim or not to swim

“…In spring, subadults select for active ice that is near areas of open water (Stirling et al , Pilfold et al ), which may increase the need for short‐distance swims. Stirling () observed subadults swimming more frequently than other age‐classes in summer in the High Arctic.…”

“…Studies have suggested that prolonged exposure to frigid water may have negative consequences for offspring survival (Blix and Lentfer , Durner et al , Pagano et al ). Females with offspring‐of‐the‐year avoid habitats with open water in spring (Stirling et al , Freitas et al , Pilfold et al ) and may swim with cubs on their back to reduce offspring immersion (Aars and Plumb ). Reducing the frequency of swims is likely the main mechanism adult females use to protect their offspring from the high energetic cost associated with submergence in water.…”

“…Results suggest that swims in the BS are primarily related to polar bears following the pack ice edge as it retreats. During spring, polar bears of the BS preferentially hunt seals in active ice, selecting for areas near the shorefast ice boundary (Pilfold et al ). As sea ice begins to breakup, polar bears may try to extend their time over the continental shelf where seal densities are highest (Stirling et al , Frost et al ).…”

“…Demographic variables were used to assess individual variation in the likelihood of a swim. As offspring are at risk of hypothermia if exposed to water for extended periods (Blix and Lentfer , Aars and Plumb ), and adult females with cubs‐of‐the‐year tend to avoid habitats with open water (Stirling et al , Pilfold et al ), we hypothesized that females with offspring would be less likely to swim. Additionally, we used body condition metrics to assess how individual condition may influence the likelihood of a potentially energetically costly movement strategy (Durner et al ).…”

Influence of sea ice phenology on the movement ecology of ringed seals across their latitudinal range

“…In the Amundsen Gulf and Sanikiluaq during winter, ringed seals were resident in areas of stable shore-fast ice. Shore-fast ice with adequate snow cover allows ringed seals to minimise risk of predation by polar bears, who prefer to forage in areas of active sea ice further away from shore (Smith & Hammill 1981, Pilfold et al 2014. During winter at one of the most southern sampling locations (i.e.…”