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      Source Partitioning Using Stable Isotopes: Coping with Too Much Variation

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          Abstract

          Background

          Stable isotope analysis is increasingly being utilised across broad areas of ecology and biology. Key to much of this work is the use of mixing models to estimate the proportion of sources contributing to a mixture such as in diet estimation.

          Methodology

          By accurately reflecting natural variation and uncertainty to generate robust probability estimates of source proportions, the application of Bayesian methods to stable isotope mixing models promises to enable researchers to address an array of new questions, and approach current questions with greater insight and honesty.

          Conclusions

          We outline a framework that builds on recently published Bayesian isotopic mixing models and present a new open source R package, SIAR. The formulation in R will allow for continued and rapid development of this core model into an all-encompassing single analysis suite for stable isotope research.

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          Most cited references7

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          Erroneous behaviour of MixSIR, a recently published Bayesian isotope mixing model: a discussion of Moore & Semmens (2008).

          The application of Bayesian methods to stable isotopic mixing problems, including inference of diet has the potential to revolutionise ecological research. Using simulated data we show that a recently published model MixSIR fails to correctly identify the true underlying dietary proportions more than 50% of the time and fails with increasing frequency as additional unquantified error is added. While the source of the fundamental failure remains elusive, mitigating solutions are suggested for dealing with additional unquantified variation. Moreover, MixSIR uses a formulation for a prior distribution that results in an opaque and unintuitive covariance structure.
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            Individual-level diet variation in four species of Brazilian frogs.

            1. Many natural populations exploiting a wide range of resources are actually composed of relatively specialized individuals. 2. This interindividual variation is thought to be a consequence of the invasion of 'empty' niches in depauperate communities, generally in temperate regions. If individual niches are constrained by functional trade-offs, the expansion of the population niche is only achieved by an increase in interindividual variation, consistent with the 'niche variation hypothesis'. 3. According to this hypothesis, we should not expect interindividual variation in species belonging to highly diverse, packed communities. 4. In the present study, we measured the degree of interindividual diet variation in four species of frogs of the highly diverse Brazilian Cerrado, using both gut contents and delta(13)C stable isotopes. 5. We found evidence of significant diet variation in the four species, indicating that this phenomenon is not restricted to depauperate communities in temperate regions. 6. The lack of correlations between the frogs' morphology and diet indicate that trade-offs do not depend on the morphological characters measured here and are probably not biomechanical. The nature of the trade-offs remains unknown, but are likely to be cognitive or physiological. 7. Finally, we found a positive correlation between the population niche width and the degree of diet variation, but a null model showed that this correlation can be generated by individuals sampling randomly from a common set of resources. Therefore, albeit consistent with, our results cannot be taken as evidence in favour of the niche variation hypothesis.
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              Temporal and intrapopulation variation in prey choice of wintering geese determined by stable isotope analysis.

              1. Individual variability in prey preferences can have marked effects on many demographic parameters from individual survival and fecundity to the vital rates of entire populations. A population level response is ultimately determined by individual prey choices; however, the effect of individual dietary choice is often overlooked. 2. We determined prey choice by individual consumers, light-bellied Brent geese Branta bernicla, during the overwintering period. Two hundred and eighty-one individuals were sampled at distinct temporal points over two winters. Stable isotopic ratios of carbon and nitrogen for blood cells and blood plasma, from each sampled individual were measured. Isotopic ratios for potential prey items were also measured. 3. Delta15N and delta13C for blood samples were both significantly different between sample months. Generally we found a decrease in both isotopic ratios during the course of the winter. All potential prey items were also isotopically distinct. Multisource mixing models (isosource) were used to determine the range of possible contribution to the diet of individuals. 4. During early winter, diet consisted almost exclusively of sea grass Zostera spp. The level of Zostera spp. in the diet dropped until mid-winter, and was supplemented by the utilization of green algae Ulva lactuca, and Enteromorpha spp., and terrestrial grasses. Terrestrial grass comprised an increasing proportion of the diet in late winter, representing virtually the exclusive food source by April. 5. By examining intrapopulation variability in resource utilization we highlight a number of ecologically important factors not addressed by previous population level studies.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2010
                12 March 2010
                : 5
                : 3
                : e9672
                Affiliations
                [1 ]School of Mathematical Sciences, University College Dublin, Dublin, Ireland
                [2 ]Centre for Ecology and Conservation, School of Biosciences, University of Exeter, Penryn, Cornwall, United Kingdom
                [3 ]Department of Zoology, School of Natural Sciences, Trinity College Dublin, Dublin, Ireland
                University of Bristol, United Kingdom
                Author notes

                Conceived and designed the experiments: ACP RI SB ALJ. Performed the experiments: ACP. Analyzed the data: ACP ALJ. Contributed reagents/materials/analysis tools: ACP ALJ. Wrote the paper: ACP RI SB ALJ.

                Article
                09-PONE-RA-13690R1
                10.1371/journal.pone.0009672
                2837382
                20300637
                6b25aa6e-cc99-43a2-9a50-948a39b34801
                Parnell et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
                History
                : 20 October 2009
                : 19 February 2010
                Page count
                Pages: 5
                Categories
                Research Article
                Ecology
                Ecology/Community Ecology and Biodiversity
                Ecology/Population Ecology
                Ecology/Theoretical Ecology

                Uncategorized
                Uncategorized

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