Breaking the sticks: a hierarchical change-point model for estimating ontogenetic shifts with stable isotope data

Matthieu Authier*, Celine Martin, Aurore Ponchon, Stephanie Steelandt, Ilham Bentaleb, Christophe Guinet

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

18 Citations (Scopus)

Abstract

1. Stable isotopes are increasingly used in ecology to investigate ontogenetic shifts in foraging habitat (via d13C) and in trophic level (via d15N). These shifts are in essence an individual-level phenomenon, requiring repeated measures throughout the life of individuals, i.e. longitudinal data. Longitudinal data require in turn specifying an appropriate covariance structure. Here we present a hierarchical model to jointly investigate individual ontogenetic shifts in d13C and d15N values. 2. In a Bayesian framework, we used a Cholesky decomposition for estimating a moderately-sized covariance matrix, thereby directly estimating correlations between parameters describing time-series of isotopic measurements. We offer guidelines on how to select the covariance structure. 3. The approach is illustrated with a hierarchical change-point (or broken stick) model applied to a data set collected on Southern Elephant Seals, Mirounga leonina. Ontogenetic shifts in foraging habitat, following a juvenile and variable stage, were detected and interpreted as fidelity to a foraging strategy; while ontogenetic shifts in trophic level were more likely the result of complete independence frommaternal resources followed by a gradual increase in trophic level as seals aged. 4. Specifying both an appropriate covariance and mean structure enabled us to draw strong inferences on the ecology of an elusive marine predator, and has wide applicability for isotopic ecology provided repeated isotopic measurements are available.

Original languageEnglish
Pages (from-to)281-290
Number of pages10
JournalMethods in Ecology and Evolution
Volume3
Issue number2
DOIs
Publication statusPublished - Apr 2012

Bibliographical note

Acknowledgements
We would like to thank all volunteers who helped collecting teeth from dead animals found on iles Kerguelen. We thank the Museum National d’Histoire Naturelle (Paris) for kindly providing teeth collected before the population crash. Wethank Hubert Vonhof and Els Ufkes for discussing the results. We are alsoindebted to Christophe Barbraud, Emmanuelle Cam, Luca Borger, two anonymous reviewers and the associate editor for helpful and constructive comments that greatly improved the manuscript. This study is part of a national research program (no. 109, H. Weimerskirch and the observatory Mammiferes Explorateurs du Milieu Oceanique, MEMO SOERE CTD 02) supported by the French Polar Institute (Institut Paul Emile Victor, IPEV). The Territoire des Terres Australes et Antarctiques Francaises (TAAF), the TOTAL Foundation and ANR-VMC 07 IPSOS-SEAL program contributed to this study. The ethics committee of the French Polar Institute (IPEV) approved this study. All animals in this study were cared for in accordance with its guidelines. This is Publicatio ISE-Mno. ISEM2011–127.

Keywords

  • Bayesian methods
  • linear models
  • modelling
  • population ecology
  • statistics
  • SOUTHERN ELEPHANT SEALS
  • LONGITUDINAL DATA
  • TROPHIC ECOLOGY
  • CARBON
  • DELTA-C-13
  • NICHE
  • NITROGEN
  • VARIANCE
  • OCEAN
  • PLANKTON

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