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Complex interplay between intrinsic and extrinsic drivers of long-term survival trends in southern elephant seals

de Little, Siobhan C., Bradshaw, Corey J. A., McMahon, Clive R. and Hindell, Mark A. (2007). Complex interplay between intrinsic and extrinsic drivers of long-term survival trends in southern elephant seals. BMC Ecology,7(3).

Document type: Journal Article
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Title Complex interplay between intrinsic and extrinsic drivers of long-term survival trends in southern elephant seals
Author de Little, Siobhan C.
Bradshaw, Corey J. A.
McMahon, Clive R.
Hindell, Mark A.
Journal Name BMC Ecology
Publication Date 2007
Volume Number 7
Issue Number 3
ISSN 1472-6785   (check CDU catalogue open catalogue search in new window)
Scopus ID 2-s2.0-34247482381
Total Pages 12
Place of Publication London, United Kingdom
Publisher Biomed Central Ltd
Field of Research 0602 - Ecology
HERDC Category C1 - Journal Article (DEST)
Abstract Background
Determining the relative contribution of intrinsic and extrinsic factors to fluctuations in population size, trends and demographic composition is analytically complex. It is often only possible to examine the combined effects of these factors through measurements made over long periods, spanning an array of population densities or levels of food availability. Using age-structured mark-recapture models and datasets spanning five decades (1950–1999), and two periods of differing relative population density, we estimated age-specific probabilities of survival and examined the combined effects of population density and environmental conditions on juvenile survival of southern elephant seals at Macquarie Island.

Results
First-year survival decreased with density during the period of highest population size, and survival increased during years when the Southern Oscillation Index (SOI) anomaly (deviation from a 50-year mean) during the mother's previous foraging trip to sea was positive (i.e., El Niño). However, when environmental stochasticity and density were considered together, the effect of density on first-year survival effectively disappeared. Ignoring density effects also leads to models placing too much emphasis on the environmental conditions prevailing during the naïve pup's first year at sea.

Conclusion Our analyses revealed that both the state of the environment and population density combine to modify juvenile survival, but that the degree to which these processes contributed to the variation observed was interactive and complex. This underlines the importance of evaluating the relative contribution of both the intrinsic and extrinsic factors that regulate animal populations because false conclusions regarding the importance of population regulation may be reached if they are examined in isolation.
DOI http://dx.doi.org/10.1186/1472-6785-7-3   (check subscription with CDU E-Gateway service for CDU Staff and Students  check subscription with CDU E-Gateway in new window)


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