Oecologia

, Volume 147, Issue 4, pp 606–614 | Cite as

Using behavioural and state variables to identify proximate causes of population change in a seabird

  • Sue Lewis
  • David Grémillet
  • Francis Daunt
  • Peter G. Ryan
  • Robert J.M. Crawford
  • Sarah Wanless
Population Ecology

Abstract

Changes in animal population size are driven by the interactions between intrinsic processes and extrinsic forces, and identifying the proximate mechanisms behind population change remains a fundamental question in ecology. Here we report on how measuring behavioural and state proxies of food availability among populations experiencing different growth rates can be used to rapidly identify proximate drivers of population trends. In recent decades, the Cape gannet Morus capensis has shown a major distributional shift with historically large colonies in Namibia decreasing rapidly, whilst numbers at South African colonies have increased, suggesting contrasting environmental conditions in the two regions. We compared per capita growth rates of five of the six extant colonies with foraging range (using miniaturised Global Positioning System loggers), foraging work rate, food delivery rates and body condition of breeding adults. We found significant associations between the rate of population change, individual behaviour, energetic gain and body condition that indicate that recent population changes are associated with extrinsic effects. This study shows that behavioural and state data can be used to identify important drivers of population change, and their cost-effectiveness ensures that they are an appealing option for measuring the health of animal populations in numerous situations.

Keywords

Population regulation Foraging behaviour Distribution shift Fishery interactions Cape gannet 

Notes

Acknowledgements

This study was funded by grants from the Association for the Study of Animal Behaviour, the British Council, the UK Embassy Paris, Centre National de la Recherche Scientifique, the South African National Research Foundation and the University of Cape Town. The authors sincerely thank P. Bartlett, Y. Ropert-Coudert, B. Dundee, M. P. Harris, J. James, R. Jones, N. Uhongora and S. Wepener for their help with fieldwork and logistics. Thanks also to G. Peters and G. Dell’Omo for support with the loggers and to S. Albon, P. Becker, M. Frederiksen, S. Hatch, M. Marquiss, F. Mougeot, K. Peard, J-P. Roux, T. Sherratt and an anonymous referee for commenting on various drafts of this paper, and for very helpful discussions. The South African National Parks, Cape Nature Conservation and the Namibian Ministry of Fisheries & Marine Resources granted access to the islands and provided permission to attach loggers to the gannets.

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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  • Sue Lewis
    • 1
    • 2
  • David Grémillet
    • 3
  • Francis Daunt
    • 2
  • Peter G. Ryan
    • 4
  • Robert J.M. Crawford
    • 5
  • Sarah Wanless
    • 2
  1. 1.Department of ZoologyUniversity of AberdeenAberdeenUK
  2. 2.Centre for Ecology and Hydrology BanchoryNERCBanchoryUK
  3. 3.Centre d’Ecologie et Physiologie EnergétiquesCentre National de la Recherche ScientifiqueStrasbourg Cedex 02France
  4. 4.DST/NRF Centre of Excellence at the Percy FitzPatrick Institute of African OrnithologyUniversity of Cape TownRondeboschSouth Africa
  5. 5.Department of Environmental Affairs and TourismMarine and Coastal ManagementRogge BaySouth Africa

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