title: How large is large: estimating ecologically meaningful isotopic differences in observational studies of wild animals.
abstract: BACKGROUND
  In ecological studies of wildlife movements and foraging, bio-logging and isotopic data are routinely collected and increasingly analyzed in tandem. Such analyses have two shortcomings: (1) small sample size linked with the number of telemetric tags that can be deployed, and (2) the observational nature of isotopic gradients. Wildlife ecologists are thus put in a statistical conundrum known as the small n, large p problem.
  
  
  METHODS
  Using shrinkage regression, which directly addresses the issue of accurately estimating effects from sparse data, we studied what counts as a biologically meaningful isotopic difference (a prerequisite to delineate isoscapes) in the southern elephant seal (Mirounga leonina), a large and elusive marine predator.
  
  
  RESULTS
  Seals foraging in Antarctic waters had a lower carbon isotopic value (by ≈ 2‰) than seals foraging either in the interfrontal zone or on the Kerguelen Plateau. The latter two foraging strategies were indistinguishable on the sole basis of δ(13) C values with our data.
  
  
  CONCLUSIONS
  Shrinkage regression is a conservative statistical technique that has wide applicability in isotopic ecology to help separate robust biological signals from noise.
authors.affiliation: Centre d'Études Biologiques de Chizé, UPR 1934 du CNRS, 79 360, Villiers-en-Bois, France. authierm@gmail.com
authors.forename: Matthieu
authors.initials: M
authors.lastname: Authier
authors.forename: Anne-Cécile
authors.initials: AC
authors.lastname: Dragon
authors.forename: Yves
authors.initials: Y
authors.lastname: Cherel
authors.forename: Christophe
authors.initials: C
authors.lastname: Guinet
mesh_terms: D025041:Animal Migration; D000818:Animals; D000835:Animals, Wild / Q000097:blood*; D000864:Antarctic Regions; D002247:Carbon Isotopes / Q000097:blood*; D005247:Feeding Behavior; D013058:Mass Spectrometry / Q000379:methods*; D012044:Regression Analysis; D018401:Sample Size; D046048:Seals, Earless / Q000097:blood*; D012621:Seasons
chemical_list: D002247:Carbon Isotopes