Breaking the sticks: a hierarchical change-point model 1 for estimating ontogenetic shifts with stable isotope 2 data

• Published in: Methods in Ecology and Evolution
• Main Authors: Authier, Matthieu, Martin, Céline, Ponchon, Aurore, Steelandt, Stéphanie, Bentaleb, Ilham, Guinet, Christophe
• Other Authors: Centre d'études biologiques de Chizé (CEBC), Centre National de la Recherche Scientifique (CNRS), École Doctorale Sciences pour l'Environnement Gay Lussac, PRES Limousin Poitou-Charentes, Institut des Sciences de l'Evolution de Montpellier (UMR ISEM), Centre de Coopération Internationale en Recherche Agronomique pour le Développement (Cirad)-École pratique des hautes études (EPHE), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Montpellier (UM)-Institut de recherche pour le développement IRD : UR226-Centre National de la Recherche Scientifique (CNRS), Centre d’Ecologie Fonctionnelle et Evolutive (CEFE), Université Paul-Valéry - Montpellier 3 (UPVM)-Institut National de la Recherche Agronomique (INRA)-Centre international d'études supérieures en sciences agronomiques (Montpellier SupAgro)-École pratique des hautes études (EPHE), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Montpellier (UM)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD France-Sud )-Institut national d’études supérieures agronomiques de Montpellier (Montpellier SupAgro), Institut national d'enseignement supérieur pour l'agriculture, l'alimentation et l'environnement (Institut Agro)-Institut national d'enseignement supérieur pour l'agriculture, l'alimentation et l'environnement (Institut Agro)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2012
• Subjects: Bayesian methods; linear models; modelling; population ecology; statistics; [SDE]Environmental Sciences; Elephant Seals; Mirounga leonina; Southern Elephant Seals
• Online Access: https://hal.archives-ouvertes.fr/hal-00627200; https://hal.archives-ouvertes.fr/hal-00627200/document; https://hal.archives-ouvertes.fr/hal-00627200/file/Authier_Breaking_the_stick.pdf; https://doi.org/10.1111/j.2041-210X.2011.00162.x

International audience 1. Stable isotopes are increasingly used in ecology to investigate ontogenetic shifts in foraging habitat (via 13C) and in trophic level (via 15N). These shifts are in essence an individual-level phenomenon, requiring repeated measures throughout the life of individuals, that is 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 13C and 15N 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 from maternal 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.