How large is large: estimating ecologically meaningful isotopic differences in observational studies of wild animals

RATIONALE 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. Copyright © 2012 John Wiley & Sons, Ltd.