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

Abstract

Summary1. Stable isotopes are increasingly used in ecology to investigate ontogenetic shifts in foraging habitat (via d 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.

Hitting the moving target: modelling ontogenetic shifts with stable isotopes reveals the importance of isotopic turnover

“…; Authier et al . ). However, simultaneously analysing δ 15 N and δ 13 C allows a better understanding of shifts in isotopic niche space (Layman et al .…”

“…In using SIA to study ontogeny, researchers often analyse d 15 N and d 13 C separately against body size (Graham et al 2007;Authier et al 2012). However, simultaneously analysing d 15 N and d 13 C allows a better understanding of shifts in isotopic niche space Turner, Collyer & Krabbenhoft 2010).…”

Foraging Fidelity as a Recipe for a Long Life: Foraging Strategy and Longevity in Male Southern Elephant Seals

“…There was in fact a small (in magnitude) positive correlation between the (positive) slope before and the (negative) slope after the shift [41]. The positive slope before the shift was expected because weaned pups rely exclusively on maternal milk before weaning.…”

Estimating continuous body size-based shifts in δ15N–δ13C space using multivariate hierarchical models

“…Multivariate statistics have seen wide use in the marine ecological literature, but the application of hierarchical multivariate models to stable isotope data has tended to focus primarily on mixing models and diet estimation (Semmens et al 2009;Parnell et al 2013). Examples of hierarchical multivariate models of stable isotope data are available in the literature (e.g., Authier et al 2012), but they remain underutilized by marine ecologists. As we show using a typical stable isotope data set, such methods can offer insight into key sources of variation and, moreover, may reduce parameter uncertainty and improve inferences.…”

“…Multivariate hierarchical models are routinely employed to estimate different prey contributions to predator diets within and among populations (Semmens et al 2009;Parnell et al 2013) and have seen use in other specialized applications including estimation of change-points in stable isotope time series (Authier et al 2012). However, given the occurrence of grouping structure within stable isotope data sets and the common desire to examine joint shifts in δ 15 N-δ 13 C space related to continuous predictor variables, multivariate hierarchical models remain underutilized.…”