Bias in Diet Determination: Incorporating Traditional Methods in Bayesian Mixing Models

Valentina Franco-Trecu,Federico G Riet-Sapriza,Andrew Parnell,Pablo Inchausti,Massimiliano Drago,Rosina Frau,Alan Hubbard

doi:10.1371/journal.pone.0080019

Valentina Franco-Trecu, Federico G Riet-Sapriza + Show 5 more

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https://doi.org/10.1371/journal.pone.0080019

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Abstract

There are not “universal methods” to determine diet composition of predators. Most traditional methods are biased because of their reliance on differential digestibility and the recovery of hard items. By relying on assimilated food, stable isotope and Bayesian mixing models (SIMMs) resolve many biases of traditional methods. SIMMs can incorporate prior information (i.e. proportional diet composition) that may improve the precision in the estimated dietary composition. However few studies have assessed the performance of traditional methods and SIMMs with and without informative priors to study the predators’ diets. Here we compare the diet compositions of the South American fur seal and sea lions obtained by scats analysis and by SIMMs-UP (uninformative priors) and assess whether informative priors (SIMMs-IP) from the scat analysis improved the estimated diet composition compared to SIMMs-UP. According to the SIMM-UP, while pelagic species dominated the fur seal’s diet the sea lion’s did not have a clear dominance of any prey. In contrast, SIMM-IP’s diets compositions were dominated by the same preys as in scat analyses. When prior information influenced SIMMs’ estimates, incorporating informative priors improved the precision in the estimated diet composition at the risk of inducing biases in the estimates. If preys isotopic data allow discriminating preys’ contributions to diets, informative priors should lead to more precise but unbiased estimated diet composition. Just as estimates of diet composition obtained from traditional methods are critically interpreted because of their biases, care must be exercised when interpreting diet composition obtained by SIMMs-IP. The best approach to obtain a near-complete view of predators’ diet composition should involve the simultaneous consideration of different sources of partial evidence (traditional methods, SIMM-UP and SIMM-IP) in the light of natural history of the predator species so as to reliably ascertain and weight the information yielded by each method.

Highlights

Trophic interactions determine the flow of energy among trophic levels that drives population dynamics, community and food web structure and most ecosystem processes
The goal of this paper is to evaluate the biases and precision of three commonly used methods to study predators’ diet composition using the South American fur seal (SAFS, Arctocephalus australis) and sea lions (SASL, Otaria flavescens) as a study system
The similarity of the diet compositions estimated by scat analysis and stable isotope mixing models (SIMM)-IP was greater than 96% for both species, whereas those of the SIMMs had a similarity of 83% and 92% for SAFS and SASL, respectively

Summary

Introduction

Trophic interactions determine the flow of energy among trophic levels that drives population dynamics, community and food web structure and most ecosystem processes. Ecologists must employ several indirect methods to determine diet composition These methods include the analyses of stomach contents [3,4], scats [5,6,7], regurgitations [8] and pellets [9,10]. All these indirect methods rely on the recovery and identification of hard items (e.g. otholites, beaks, bones) of consumed preys, but yield high-resolution information on the species composition, body length and mass of the prey consumed [10]. Scat analysis is perhaps the least invasive of the traditional, indirect methods to determine diet composition and probably the one yielding the highest sample size for the lowest effort and cost [13]

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