Using multiple tracers and directly accounting for trophic modification improves dietary mixing‐model performance

Abstract

Citation: Brett, M. T., M. E. Eisenlord, and A. W. E. Galloway. 2016. Using multiple tracers and directly accounting for trophic modification improves dietary mixing-model performance. Ecosphere 7(8):e01440. 10. 1002/ecs2.1440 Abstract. Stable isotope (SI) mixing models are one of the most common approaches used to infer resource pathways in consumers. However, SI-based analyses are often underdetermined, and consumer SI fractionation is often unknown. The use of fatty acid (FA) biomarkers in mixing models offers an alternative approach that can resolve the underdetermined constraint. A limitation to both methods is the considerable uncertainty regarding the "trophic modification" of dietary tracers, which occurs when consumers transform dietary resources into their own tissues. We tested the utility of SI and FA approaches for inferring the diets of the marine benthic isopod (Idotea wosnesenskii) fed various marine macroalgae in controlled feeding trials. Our analyses quantified how the accuracy and precision of Bayesian mixing models were influenced by the choice of algorithm (SIAR vs. MixSIR), trophic modification (assumed or known), and whether the model was under or overdetermined (seven sources and two vs. 26 tracers) for cases where isopods were fed an exclusive diet of one of the seven macroalgae. Using the conventional approach (two SI with assumed trophic modification) resulted in average model outputs, that is, the contribution from the exclusive resource =0.20 ± 0.23 (0.00-0.79), mean ± SD (95% credible interval), that only differed slightly from the prior assumption. This result was only somewhat improved by the use of measured trophic modification values for SI data. Using the FA-based approach with known trophic modification greatly improved model performance, that is, the contribution from the exclusive resource =0.91 ± 0.10 (0.58-0.99). The choice of algorithm (SIAR vs. MixSIR) made a clear difference when trophic modification was known, and the model was overdetermined. In this case, SIAR and MixSIR had outputs of 0.86 ± 0.11 (0.48-0.96) and 0.96 ± 0.05 (0.79-1.00), respectively. When using FA biomarkers, going from two to seven tracers dramatically improved model performance (i.e., from 0.55 to 0.89, respectively), whereas going from 13 to 26 tracers only improved performance by 0.01. This analysis shows the choice and number of dietary tracers and the trophic modification assumption greatly influence the performance of mixing-model dietary reconstructions and ultimately our understanding of what resources actually support aquatic consumers.

A multi‐biomarker approach supports the use of compound‐specific stable isotope analysis of amino acids to quantify basal carbon source use in a salt marsh consumer

“…The presence of bacterial FAs, for example, in seaside sparrow tissues suggests that the carbon flow pathways supporting these consumers may also involve microbial transformations of primary production. When trophic modification is well characterized, FAs offer the advantage of a large number of tracers to act as carbon source discriminators …”

“…This is of note as a growing body of literature suggests that intra‐specific niche variation has important evolutionary and ecological consequences . This increased precision found in CSIA‐AA may be due to two factors: differential uncertainty in source biomarker values and differential uncertainty in trophic discrimination . The CSIA‐AA‐based model used five tracers (δ 13 C‐EAA values) to define carbon sources, allowing for a more robust characterization than in the single‐tracer (bulk tissue δ 13 C values) SIA‐based model.…”

“…Fiddler crabs ( Uca spp., family Ocypodidae) represent microalgae/detritus‐derived carbon pathways, hereafter referred to as aquatic basal carbon sources. We expected that all biomarker results would be consistent with each other, but that CSIA‐AA would provide the highest resolving power from both qualitative and quantitative perspectives …”

Intrapopulation variability in wolf diet revealed using a combined stable isotope and fatty acid approach

“…, Brett et al. ). An advantage of diet biomarkers is that they provide insights into what an animal was eating over ecologically relevant time frames (Tiezen et al.…”

Trophic Transfer of Macroalgal Fatty Acids in Two Urchin Species: Digestion, Egestion, and Tissue Building

“…However, dietary FAs are often modified by the consumer, either by elongation, desaturation, or de novo synthesis of FAs from precursors present in dietary tissues (Cook et al, 2007;González-Durán et al, 2008;Brett et al, 2016). Our understanding of FA trophic modification by marine invertebrates is still developing, and sea urchins are an interesting case study for this question because they have been studied for the sake of aquaculture (Castell et al, 2004).…”