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Marine Ecology Progress Series

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MEPS 288:273-283 (2005)  -  doi:10.3354/meps288273

Comparison of southern elephant seal populations, and observations of a population on a demographic knife-edge

Clive R. McMahon1,2,4, Mark A. Hindell3, Harry R. Burton1, Marthán N. Bester2

1Australian Antarctic Division, Channel Highway, Kingston, 7050 Tasmania, Australia
2Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, 0002 Pretoria, Gauteng,Republic of South Africa
3Antarctic Wildlife Research Unit, School of Zoology, University of Tasmania, GPO Box 252-05, Hobart, 7001 Tasmania, Australia
4Present address: School of Biological Sciences, Institute of Environmental Sustainability, University of Wales Swansea,Singleton Park, Swansea SA2 8PP, UK

ABSTRACT: The dynamics of animal populations are determined by several key demographic parameters, which vary over time with resultant changes in the status of the population. When managing declining populations, the identification of the parameters that drive such change are a high priority, but are rarely achieved for large and long-lived species. Southern elephant seal populations in the South Indian and South Pacific oceans have decreased by as much as 50% during the past 50 yr. The reasons for these decreases remained unknown. This study used a projected stochastic Leslie-matrix model based on long-term demographic data to examine the potential role of several life-history parameters in contributing to the declines. The models simulated the observed population trends that were independently derived from annual abundance surveys. Small changes in survival and fecundity had dramatic effects on population growth rates. At Macquarie Island for example, a small change (ca. 5%) in survival and fecundity rates resulted in the population reverting from a decreasing one to a population that increased. The vital rates that had the greatest impact on fitness were, in order of importance: (1) juvenile survival, (2) adult survival, (3) adult fecundity and (4) juvenile fecundity. Population viability analysis (PVA) for each of the 2 decreasing populations revealed that there was a high probability of the Marion Island population becoming extinct within the next 150 yr, while the probability of extinction at Macquarie Island was low. The estimated mean times to extinction for each population was 134 yr (95% confidence intervals: 105 to 332 yr) at Marion Island and 564 yr at Macquarie Island (the earliest time to extinction was 307 yr).

KEY WORDS: Elasticity · Population fitness · Population trajectories · Vital rates

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