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Predicting fluctuations of reintroduced ibex populations: the importance of density dependence, environmental stochasticity and uncertain population estimates

Authors

  • BERNT-ERIK SÆTHER,

    1. Department of Biology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway; Department of Mathematical Sciences, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway; and
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  • MAGNAR LILLEGÅRD,

    1. Department of Biology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway; Department of Mathematical Sciences, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway; and
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  • VIDAR GRØTAN,

    1. Department of Biology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway; Department of Mathematical Sciences, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway; and
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  • FLURIN FILLI,

    1. Parc Naziunal Svizzer, Chasa dal Parc, PO Box 505, CH-7530 Zernez, Switzerland
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  • STEINAR ENGEN

    1. Department of Biology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway; Department of Mathematical Sciences, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway; and
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  • Re-use of this article is permitted in accordamce with the Creative Commons Deed, Attribution 2·5, which does not permit commercial exploitation.

B.-E. Sæther, Department of Biology, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway. Tel.: +4773590584. Fax: +4773596100. E-mail: Bernt-Erik.Sather@bio.ntnu.no

Summary

  • 1Development of population projections requires estimates of observation error, parameters characterizing expected dynamics such as the specific population growth rate and the form of density regulation, the influence of stochastic factors on population dynamics, and quantification of the uncertainty in the parameter estimates.
  • 2Here we construct a Population Prediction Interval (PPI) based on Bayesian state space modelling of future population growth of 28 reintroduced ibex populations in Switzerland that have been censused for up to 68 years. Our aim is to examine whether the interpopulation variation in the precision of the population projections is related to differences in the parameters characterizing the expected dynamics, in the effects of environmental stochasticity, in the magnitude of uncertainty in the population parameters, or in the observation error.
  • 3The error in the population censuses was small. The median coefficient of variation in the estimates across populations was 5·1%.
  • 4Significant density regulation was present in 53·6% of the populations, but was in general weak.
  • 5The width of the PPI calculated for a period of 5 years showed large variation among populations, and was explained by differences in the impact of environmental stochasticity on population dynamics.
  • 6In spite of the high accuracy in population estimates, the uncertainty in the parameter estimates was still large. This uncertainty affected the precision in the population predictions, but it decreased with increasing length of study period, mainly due to higher precision in the estimates of the environmental variance in the longer time-series.
  • 7These analyses reveal that predictions of future population fluctuations of weakly density-regulated populations such as the ibex often become uncertain. Credible population predictions require that this uncertainty is properly quantified.

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