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

Two distinct viability models are developed for Steller sea lions (Eumetopi as jubatus) to evaluate the sensitivity of extinction risk to various level s of stochasticity, spatial scale and density dependence. These models incl ude a metapopulation model, Analysis of the Likelihood of Extinction (ALEX; Possingham et al., 1992; Possingham, H., Davies, I.A., Noble, I. 1992. ALE X 2.2 Operation Manual. Department of Applied Mathematics, University of Ad elaide, Adelaide, SA 5005; Australia.), and a model that incorporates both sampling and process error in estimating population parameters from timeser ies 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 thr ee 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 Conserv ation. Island Press, Covelo, CA pp. 259-292). The combination of modeling a pproaches provides a basis for considering how model parameterization and t he selection of classification criteria affect both model results and poten tial status determinations. Results from the models generally agree with re gard 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 mo re than one viability model should be considered when listing decisions are made. On a more applied basis, the results of our analysis provide a quant itative assessment of extinction risk of Steller sea lions in the context o f its status pursuant to the US Endangered Species Act. (C) 2001 Elsevier S cience Ltd. All rights reserved.