The role of predation in the decline and extirpation of woodland caribou |
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Authors: | Email author" target="_blank">Heiko?U?WittmerEmail author Anthony?R?E?Sinclair Bruce?N?McLellan |
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Institution: | (1) Agroecology, Faculty of Agricultural Sciences, University of British Columbia, 2357 Main Mall, Vancouver, BC, V6T 1Z4, Canada;(2) Centre for Biodiversity Research, University of British Columbia, 6270 University Boulevard, Vancouver, BC, V6T 1Z4, Canada;(3) Ministry of Forests, Research Branch, PO Box 9158, RPO #3, Revelstoke, BC, V0E 3K0, Canada;(4) Present address: Department of Environmental Science, Policy and Management, University of California, Berkeley, CA 94720-3110, USA |
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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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Keywords: | Allee effect Apparent competition Inverse density dependence Rangifer tarandus caribou Population dynamics |
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