Dietary composition and spatial patterns of polar bear foraging on land in western Hudson Bay

Abstract

BackgroundFlexible foraging strategies, such as prey switching, omnivory and food mixing, are key to surviving in a labile and changing environment. Polar bears (Ursus maritimus) in western Hudson Bay are versatile predators that use all of these strategies as they seasonally exploit resources across trophic levels. Climate warming is reducing availability of their ice habitat, especially in spring when polar bears gain most of their annual fat reserves by consuming seal pups before coming ashore in summer. How polar bears combine these flexible foraging strategies to obtain and utilize terrestrial food will become increasingly important in compensating for energy deficits from lost seal hunting opportunities. We evaluated patterns in the composition of foods in scat to characterize the foraging behaviors that underpin the diet mixing and omnivory observed in polar bears on land in western Hudson Bay. Specifically, we measured diet richness, proportions of plant and animal foods, patterns in co-occurrence of foods, spatial composition and an index of temporal composition.ResultsScats contained between 1 and 6 foods, with an average of 2.11 (SE = 0.04). Most scats (84.9%) contained at least one type of plant, but animals (35.4% of scats) and both plants and animals occurring together (34.4% of scats) were also common. Certain foods, such as Lyme grass seed heads (Leymus arenarius), berries and marine algae, were consumed in relatively higher proportions, sometimes to the exclusion of others, both where and when they occurred most abundantly. The predominance of localized vegetation in scats suggests little movement among habitat types between feeding sessions. Unlike the case for plants, no spatial patterns were found for animal remains, likely due the animals’ more vagile and ubiquitous distribution.ConclusionsOur results suggest that polar bears are foraging opportunistically in a manner consistent with maximizing intake while minimizing energy expenditure associated with movement. The frequent mixing of plant-based carbohydrate and animal-based protein could suggest use of a strategy that other Ursids employ to maximize weight gain. Further, consuming high rates of certain vegetation and land-based animals that may yield immediate energetic gains could, instead, provide other benefits such as fulfilling vitamin/mineral requirements, diluting toxins and assessing new foods for potential switching.

Can polar bears use terrestrial foods to offset lost ice‐based hunting opportunities?

“…Observations of polar bears feeding on bird eggs near Hudson Bay and Foxe Basin, and bears mixing meat and vegetation, represent a small proportion of the population (Smith et al . 2010; Gormezano and Rockwell 2013b). At best, these behaviors may offset some ice‐based foraging opportunities for some individuals.…”

“…2007), despite well‐documented terrestrial foraging (Rockwell and Gormezano 2009; Smith et al . 2010; Gormezano and Rockwell 2013a, b).…”

“…Updated data on sex-and agespecific activity patterns would inform whether bears are devoting more time to foraging. n Nutritional value of terrestrially based foods Some authors suggest that polar bears may benefit from consuming vegetation and land-based animals to fulfill vitamin and mineral requirements (Derocher 2012;Gormezano and Rockwell 2013b). However, if terrestrial foraging is essential for trace element intake, bears in all populations would historically have spent time foraging in terrestrial habitats; yet this has not been the case.…”

“…For terrestrial foods to stabilize declines in polar bear survival and abundance, observed behaviors in some bears must provide net energetic gain, be indicative of behavior that is or soon will be displayed by much of the rest of the population, and will become widespread throughout the entire polar bear range. Observations of polar bears feeding on bird eggs near Hudson Bay and Foxe Basin, and bears mixing meat and vegetation, represent a small proportion of the population (Smith et al 2010;Gormezano and Rockwell 2013b). At best, these behaviors may offset some icebased foraging opportunities for some individuals.…”

Inuit perspectives of polar bear research: lessons for community-based collaborations

“…food sources even in the presence of preferred ones (Gormezano and Rockwell 2013b). Bears foraging for land-based foods have been reported in the literature prior to recent concerns over climate change (Russell 1975;Derocher and others 1993;Gormezano and Rockwell 2013a). Observations of bears consuming garbage are not uncommon (Russell 1975;Lunn and Stirling 1985;Gormezano and Rockwell 2013b) and bears are probably more aggressive at sites where resources are defendable and predictable (Elfström and others 2014), such as garbage dumps and Inuit hunting caches, which might explain aggressive behaviour of bears near communities.…”