, Volume 159, Issue 1, pp 69–82 | Cite as

Shifting trends: detecting environmentally mediated regulation in long-lived marine vertebrates using time-series data

  • Clive R. McMahon
  • Marthán N. Bester
  • Mark A. Hindell
  • Barry W. Brook
  • Corey J. A. Bradshaw
Population Ecology - Original Paper


Assessing the status and trends in animal populations is essential for effective species conservation and management practices. However, unless time-series abundance data demonstrate rapid and reliable fluctuations, objective appraisal of directionality of trends is problematic. We adopted a multiple-working hypotheses approach based on information-theoretic and Bayesian multi-model inference to examine the population trends and form of intrinsic regulation demonstrated by a long-lived species, the southern elephant seal. We also determined the evidence for density dependence in 11 other well-studied marine mammal species. (1) We tested the type of population regulation for elephant seals from Marion Island (1986–2004) and from 11 other marine mammal species, and (2) we described the trends and behavior of the 19-year population time series at Marion Island to identify changes in population trends. We contrasted five plausible trend models using information-theoretic and Bayesian-inference estimates of model parsimony. Our analyses identified two distinct phases of population growth for this population with the inflexion occurring in 1998. Thus, the population decreased between 1986 and 1997 (−3.7% per annum) and increased between 1997 and 2004 (1.9% per annum). An index of environmental stochasticity, the Southern Oscillation Index, explained some of the variance in r and N. We determined analytically that there was good evidence for density dependence in the Marion Island population and that density dependence was widespread among marine mammal species (67% of species showed evidence for population regulation). This approach demonstrates the potential functionality of a relatively simple technique that can be applied to short time series to identify the type of regulation, and the uncertainty associated with the phenomenon, operating in populations of large mammals.


Demography Density dependence Climate change Antarctica Elephant seal Population change Mammal 



We thank all the seal counters at Marion Island from 1986 to 2004 for their dedication and hard work, in particular we thank P. Pistorius and G. Hofmeyr for collating the Marion Island data for us. We thank C. Barbraud, J.-M. Gaillard and an anonymous reviewer for helpful comments to improve the manuscript. We also thank G.-J. Yang, R. Lande, T. Coulson, S. Engen and B.-E. Sæther for valuable advice and discussion. The Republic of South Africa Department of Environmental Affairs and Tourism (formerly the Department of Environmental Affairs) provided financial and logistical support on the advice of the South African Scientific Committee for Antarctic Research. The project conformed to the requirements of the Ethics Committee, Faculty of Natural and Agricultural Sciences, University of Pretoria (ref. no. 990112-002) and was accomplished under permit from the Department of Environmental Affairs and Tourism. CRM and MAH conceived the idea for the paper, CRM, CJAB and BWB wrote the paper, CJAB and BWB did the analyses, MNB initiated and maintained the annual censuses of the Marion Island southern elephant seal population, and all co-authors contributed intellectual content.


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Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  • Clive R. McMahon
    • 1
    • 2
    • 3
  • Marthán N. Bester
    • 1
  • Mark A. Hindell
    • 3
  • Barry W. Brook
    • 4
  • Corey J. A. Bradshaw
    • 4
    • 5
  1. 1.Department of Zoology and Entomology, Mammal Research InstituteUniversity of PretoriaPretoriaRepublic of South Africa
  2. 2.School for Environmental Research, Institute of Advanced StudiesCharles Darwin UniversityDarwinAustralia
  3. 3.Antarctic Wildlife Research Unit, School of ZoologyUniversity of TasmaniaHobartAustralia
  4. 4.Research Institute for Climate Change and Sustainability, School of Earth and Environmental SciencesUniversity of AdelaideAdelaideAustralia
  5. 5.South Australian Research and Development InstituteAdelaideAustralia

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