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The role of predation in the decline and extirpation of woodland caribou

Abstract

To select appropriate recovery strategies for endangered populations, we must understand the dynamics of small populations and distinguish between the possible causes that drive such populations to low numbers. It has been suggested that the pattern of population decline may be inversely density-dependent with population growth rates decreasing as populations become very small; however, empirical evidence of such accelerated declines at low densities is rare. Here we analyzed the pattern of decline of a threatened population of woodland caribou (Rangifer tarandus caribou) in British Columbia, Canada. Using information on the instantaneous rate of increase relative to caribou density in suitable winter foraging habitat, as well as on pregnancy rates and on causes and temporal distribution of mortalities from a sample of 349 radiocollared animals from 15 subpopulations, we tested 3 hypothesized causes of decline: (a) food regulation caused by loss of suitable winter foraging habitat, (b) predation-sensitive foraging caused by loss of suitable winter foraging habitat and (c) predation with caribou being secondary prey. Population sizes of caribou subpopulations ranged from <5 to >500 individuals. Our results showed that the rates of increase of these subpopulations varied from −0.1871 to 0.0496 with smaller subpopulations declining faster than larger subpopulations. Rates of increase were positively related to the density of caribou in suitable winter foraging habitat. Pregnancy rates averaged 92.4% ±2.24 and did not differ among subpopulations. In addition, we found predation to be the primary cause of mortality in 11 of 13 subpopulations with known causes of mortality and predation predominantly occurred during summer. These results are consistent with predictions that caribou subpopulations are declining as a consequence of increased predation. Recovery of these woodland caribou will thus require a multispecies perspective and an appreciation for the influence of inverse density dependence on population trajectories.

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Acknowledgements

This project was funded by the British Columbia Ministries of Forests and Environment, Columbia Basin Fish and Wildlife Compensation Program, Parks Canada, Forest Renewal British Columbia, and numerous forestry companies that operate across the range of arboreal lichen feeding woodland caribou. H.U. Wittmer was supported by fellowships from the International Council for Canadian Studies, the University of British Columbia, and the Faculty of Agricultural Sciences at the University of British Columbia. We are thankful to the numerous people involved in data collection. C. Apps provided access to GIS basemaps and B. Lessard engaged in many stimulating discussions. P. Arcese, J. Brashares, F. Courchamp, M. Festa-Bianchet and D. Shackleton provided many helpful comments on earlier versions of this manuscript. Finally, we thank an anonymous reviewer whose comments greatly improved this manuscript.

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Correspondence to Heiko U. Wittmer.

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Communicated by John Fryxell

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Wittmer, H.U., Sinclair, A.R.E. & McLellan, B.N. The role of predation in the decline and extirpation of woodland caribou. Oecologia 144, 257–267 (2005). https://doi.org/10.1007/s00442-005-0055-y

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Keywords

  • Allee effect
  • Apparent competition
  • Inverse density dependence
  • Rangifer tarandus caribou
  • Population dynamics