Fast carnivores and slow herbivores: differential foraging strategies among grizzly bears in the Canadian Arctic

Abstract

Categorizing animal populations by diet can mask important intrapopulation variation, which is crucial to understanding a species’ trophic niche width. To test hypotheses related to intrapopulation variation in foraging or the presence of diet specialization, we conducted stable isotope analysis (δ13C, δ15N) on hair and claw samples from 51 grizzly bears (Ursus arctos) collected from 2003 to 2006 in the Mackenzie Delta region of the Canadian Arctic. We examined within-population differences in the foraging patterns of males and females and the relationship between trophic position (derived from δ15N measurements) and individual movement. The range of δ15N values in hair and claw (2.0–11.0‰) suggested a wide niche width and cluster analyses indicated the presence of three foraging groups within the population, ranging from near-complete herbivory to near-complete carnivory. We found no linear relationship between home range size and trophic position when the data were continuous or when grouped by foraging behavior. However, the movement rate of females increased linearly with trophic position. We used multisource dual-isotope mixing models to determine the relative contributions of seven prey sources within each foraging group for both males and females. The mean bear dietary endpoint across all foraging groups for each sex fell toward the center of the mixing polygon, which suggested relatively well-mixed diets. The primary dietary difference across foraging groups was the proportional contribution of herbaceous foods, which decreased for both males and females from 42–76 to 0–27% and 62–81 to 0–44%, respectively. Grizzlies of the Mackenzie Delta live in extremely harsh conditions and identifying within-population diet specialization has improved our understanding of varying habitat requirements within the population.

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Acknowledgments

Funding for this study was provided by the University of Alberta, Government of Northwest Territories, Department of Environment and Natural Resources (Inuvik region), the Inuvialuit Land Claim Wildlife Studies Implementation Fund, ConocoPhillips (North) Canada Limited, Alberta Cooperative Conservation Research Unit, Western Biophysical Program of the Government of Northwest Territories, Polar Continental Shelf Project, Endangered Species Recovery Fund—World Wildlife Fund, Lorraine Allison Scholarship Trust Fund (M.A.E.), Circumpolar/Boreal Alberta Research Grant, Indian and Northern Affairs Canada Northern Scientific Training Program, and the Natural Sciences and Engineering Research Council of Canada (A.E.D.). We appreciate the support of the Wildlife Management Advisory Council (Northwest Territories), the Inuvialuit Game Council, the Inuvik Hunters’ and Trappers’ Committee, and the Tuktoyaktuk Hunters’ and Trappers’ Committee. This study would not have been possible without support from Ray Case. We thank Elizabeth Gordon for her assistance in collecting grizzly bear prey items and James Reist from Fisheries and Oceans Canada for providing isotope values of fish in the Mackenzie River. Funding for Jacqui Neilson and Heather Nelson, who assisted in the preparation of stable isotope samples, was provided by the Woman in Scholarship, Engineering, Science, and Technology Program. We thank Seth Cherry for his helpful discussions throughout the development of this study and Janice Cooke for her suggestions and the use of her lab and equipment.

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Communicated by Ilpo Kojola.

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Edwards, M.A., Derocher, A.E., Hobson, K.A. et al. Fast carnivores and slow herbivores: differential foraging strategies among grizzly bears in the Canadian Arctic. Oecologia 165, 877–889 (2011). https://doi.org/10.1007/s00442-010-1869-9

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Keywords

  • Diet specialization
  • Grizzly bear
  • Niche width
  • Trophic position
  • Ursus arctos