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Polar bear (Ursus maritimus) foraging ecology in spring Open Access


Other title
polar bear
ringed seal
habitat selection
Type of item
Degree grantor
University of Alberta
Author or creator
Pilfold, Nicholas W
Supervisor and department
Derocher, Andrew E (Biological Sciences)
Examining committee member and department
Trites, Andrew (University of British Columbia)
Boyce, Mark (Biological Sciences)
Bayne, Erin (Biological Sciences)
Stirling, Ian (Biological Sciences)
Department of Biological Sciences
Date accepted
Graduation date
Doctor of Philosophy
Degree level
Polar bears (Ursus maritimus) enter a period of intensified feeding in the spring, which allows for the accumulation of energy stores critical to surviving the open water season. Study on polar bear predation has been limited by sample size and spatial extent, and hypotheses on the demographic composition of seal kills and the spatial distribution of polar bears and seals were incongruent. In this thesis, I used a long-term dataset (1985-2011) of seals killed by polar bears (n = 650) and predation attempts at ringed seal (Pusa hispida) subnivean lairs (n = 1396) in the Beaufort Sea, Canada, to link the habitats polar bears use and the seals that polar bears kill during hyperphagia. Using DNA and field observations, I determined that polar bears primarily killed ringed seals, but that bearded seals (Erignathus barbatus) contributed a significant portion of kill biomass. An increase in seal kill frequency was observed temporally over the spring, associated with the onset of ringed seal whelping. The influence of ringed seal whelping was also observable at inter-annual scales, with total kill frequency positively correlated to years of high ringed seal natality, while adults were killed in higher proportion in years when natality was low. Employing locations of seal kills and attempted hunts at ringed seal subnivean lairs, I examined the habitats in which polar bears hunt and ringed seals whelp. Polar bears selected for active areas of sea ice near the floe edge when hunting seals. Ringed seal whelping areas were located over a range of habitats, and the distribution was correlated with natality. In years of low natality pup kills were observed primarily in shorefast ice close to land, but during years of high natality the distribution widened, and pup kills were observed farther from land and more frequently near active ice areas. Results suggest that during periods of high natality, the habitats in which ringed seals whelp overlaps with areas preferred by polar bears for hunting. 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. Finally, I explored the assumptions of common analytical modelling approaches in ecology. I established 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 found to be insufficient when validated against independent data. The empirical analyses within this dissertation suggest that strong assumptions of ecological models may not always hold. Collecting biologically relevant data in the field, beyond simply recording events, can test model assumptions and validate results, increasing model portability and the relevance of the findings.
Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
Citation for previous publication
Pilfold NW, Derocher AE, Stirling I, Richardson E, Andriashek D. 2012. Age and sex composition of seals killed by polar bears in the eastern Beaufort Sea. PLoS One 7: e41429Pilfold NW, Derocher AE, Richardson E. 2014. Influence of interference competition on the distribution of a wide ranging non-territorial carnivore. Global Ecology and Biogeography 23: 425-435Pilfold NW, Derocher AE, Stirling I, Richardson E. 2014. Polar bear predatory behaviour reveals seascape distribution of ringed seal lairs. Population Ecology 56: 129-138Pilfold NW, Derocher AE, Stirling I, Richardson E. Multi-temporal factors influence spring predation for polar bears in a changing climate. Oikos. In Press.

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