Implications of three viability models for the conservation status of the western population of Steller sea lions (Eumetopias jubatus)

Two distinct viability models are developed for Steller sea lions (Eumetopias jubatus) to evaluate the sensitivity of extinction risk to various levels of stochasticity, spatial scale and density dependence. These models include a metapopulation model, Analysis of the Likelihood of Extinction (ALEX; Possingham et al., 1992; Possingham, H., Davies, I.A., Noble, I. 1992. ALEX 2.2 Operation Manual. Department of Applied Mathematics, University of Adelaide, Adelaide, SA 5005; Australia.), and a model that incorporates both sampling and process error in estimating population parameters from timeseries data (Gerber and DeMaster, 1999; Gerber, L.R., DeMaster, D.P. 1999. An approach to endangered species act classification of long-lived vertebrates: a case study of north Pacific humpback whales. Conservation Biology 13 (5);1203-1214.). Results are compared with a third model that encompasses three different geographic scales (York et al., 1996; York, A.E., Merrick, R.L., Loughlin, T.R. 1996. An analysis of the Steller Sea lion metapopulation in Alaska. In: McCullough, D.R. (Ed.), Metapopulations and Wildlife Conservation. Island Press, Covelo, CA pp. 259-292). The combination of modeling approaches provides a basis for considering how model parameterization and the selection of classification criteria affect both model results and potential status determinations. Results from the models generally agree with regard to central tendency, 25th and 75th percentile times to extinction. For Steller sea lions, the distributions of time to extinction for each model were narrower than the range of extinction distributions between models. If this finding applies generally to listed species, it would suggest that more than one viability model should be considered when listing decisions are made. On a more applied basis, the results of our analysis provide a quantitative assessment of extinction risk of Steller sea lions in the context of its status pursuant to the US Endangered Species Act.