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13 results for “Trophic Discrimination Factors”
Meta-analysis and critical review of trophic discrimination factors (Δ13C and Δ15N): importance of tissue, trophic level, and diet source
<ol> <li>Robustly quantifying dietary resource use and trophic position using stable isotopes requires accurate trophic discrimination factors (TDF; Δ<sup>13</sup>C and Δ<sup>15</sup>N for carbon and nitrogen, respectively), defined as the isotopic difference between consumer and diet. Early TDF studies converged on values of around 1.0‰ for Δ<sup>13</sup>C and 3.4‰ for Δ<sup>15</sup>N, but more recent work indicates that TDF values may be more nuanced, depending on taxa, tissues, trophic level, and diets. Yet, the relative importance of these factors remains unclear.</li> <li>Focusing on vertebrates (birds, fish, herptiles, and mammals), we conducted a literature review of 279 studies that estimated TDF values and used a Bayesian framework to determine how tissue type, trophic level, and diet source influence variation in Δ<sup>13</sup>C and Δ<sup>15</sup>N. Additionally, we reviewed 358 trophic ecology studies to determine if studies accounted for these factors during their TDF selection process.</li> <li>For Δ<sup>13</sup>C, vertebrates showed consistent patterns among tissue types (likely influenced by amino acid composition) and between trophic levels and diet sources (likely a result of dietary protein content and metabolic routing). Comparatively, for Δ<sup>15</sup>N, vertebrates showed considerable variation among tissue types and trophic levels, likely due to differences in tissue synthesis and physiological capabilities. Overall, Δ<sup>13</sup>C ranged from -5.1‰ to 9.1‰ and Δ<sup>15</sup>N from -3.3‰ to 9.7‰, underscoring that 1.0‰ for Δ<sup>13</sup>C and 3.4‰ for Δ<sup>15</sup>N are not universally appropriate. Moreover, both Δ<sup>13</sup>C and Δ<sup>15</sup>N varied by more than 9‰ within a single species and tissue type, demonstrating that using TDF values from the same, or similar, species may not be appropriate if diet and trophic level are not considered.</li> <li>Despite the importance of diet source on TDF values, most trophic ecology studies did not account for it. Further, most fish studies relied on literature review values that failed to account for tissue type, trophic level, and diet source. To aid ecologists in diet and trophic assessments of vertebrates, we used our meta-analysis to model taxon-specific TDF estimates (mean ± SD) for each tissue type, trophic level, and diet source combination. These more refined TDF values should improve ecological assessments that use stable isotopes.</li> </ol>
Meta-analysis and critical review of trophic discrimination factors (Δ13C and Δ15N): importance of tissue, trophic level, and diet source
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Metabolic rate data and code for: Intra-specific differences in metabolic rates shape carbon stable isotope trophic discrimination factors of muscle tissue in the common teleost Eurasian perch (Perca fluviatilis)
<p>Metabolic rate data and code for: Intra-specific differences in metabolic rates shape carbon stable isotope trophic discrimination factors of muscle tissue in the common teleost Eurasian perch (Perca fluviatilis)</p> <p> </p> <p>The raw data and R-code used to calculate SMR used in the analysis. </p>
Dataset: Testing for effects of growth rate on isotope trophic discrimination factors and evaluating the performance of Bayesian stable isotope mixing models experimentally: a moment of truth?
<p><span>Discerning assimilated diets of wild animals using stable isotopes is well established where potential dietary items in food webs are isotopically distinct. With the advent of mixing models, and Bayesian extensions of such models (Bayesian Stable Isotope Mixing Models, BSIMMs), statistical techniques available for these efforts have been rapidly increasing. The accuracy with which BSIMMs quantify diet, however, depends on several factors including uncertainty in tissue discrimination factors (TDFs; <em>Δ</em>) and identification of appropriate error structures. Whereas performance of BSIMMs has mostly been evaluated with simulations, here we test the efficacy of BSIMMs by raising domestic broiler chicks (<em>Gallus gallus domesticus</em>) on four isotopically distinct diets under controlled environmental conditions, ideal for evaluating factors that affect TDFs and testing how BSIMMs allocate individual birds to diets that vary in isotopic similarity. For both liver and feather tissues,<em> δ</em><sup>13</sup>C and <em>δ </em><sup>15</sup>N values differed among dietary groups. <em>Δ</em><sup>13</sup>C of liver, but not feather, was negatively related to the rate at which individuals gained body mass. For <em>Δ</em><sup>15</sup>N, we identified effects of dietary group, sex, and tissue type, as well as an interaction between sex and tissue type</span><span><span>, </span></span><span><span>with f</span></span><span>emales having higher liver <em>Δ</em><sup>15</sup>N relative to males. For both tissues, BSIMMs allocated most chicks to correct dietary groups, especially for models using combined TDFs rather than diet specific TDFs, and those applying a multiplicative error structure. These findings provide new information on how biological processes affect TDFs and confirm that adequately accounting for variability in consumer isotopes is necessary to optimize performance of BSIMMs. Moreover, they demonstrate experimentally that these types of models reliably characterize consumed diets when appropriately parameterized.<span> </span></span></p>
Reevaluating trophic discrimination factors (Δδ13C and Δδ15N) for diet reconstruction
<p>Stable isotope analysis is increasingly used to assess diet and trophic positions of animals. Such assessments require estimates of trophic discrimination factors (TDF — offset between the isotopic composition of diet and animal tissues), with imprecise applications of TDFs leading to biased conclusions in resource use. Because TDFs are unavailable for most species, ecologists often apply values from taxonomically similar species or use trophic step increases of about 1‰ for carbon (TDF-δ<sup>13</sup>C) and 3‰ for nitrogen (TDF-δ<sup>15</sup>N). Such practices may be inaccurate since TDFs vary greatly, even within a species. To better understand the factors that influence TDFs, we conducted a meta-analysis of TDF-δ<sup>13</sup>C and TDF-δ<sup>15</sup>N for mammals and quantified variation in relation to consumer type (herbivore, omnivore, carnivore) and diet source (C3-based, C4-based, marine-based, mixture). Additionally, to guide TDF choice, we used an isotopic dataset of small mammal tissues and diet items to assess how predicted dietary contributions vary with TDFs estimated using (1) taxonomic relatedness, (2) consumer type and diet source, or (3) values derived from wild animals eating natural diets. Our meta-analysis revealed that metabolic routing and interactions between consumer class, dietary source, and the protein versus energy content of diets best explained variation in TDF-δ<sup>13</sup>C values (-1.5 to 7.3‰), whereas consumer class best explained variation in TDF-δ<sup>15</sup>N values (-0.5 to 7.1‰). Our test of methods to estimate TDFs indicated that ecologists should avoid relying on taxonomic relatedness when selecting TDF-δ<sup>13</sup>C, as mixed-diet lab studies may cause misleading results for herbivores and omnivores. Additionally, field-derived estimates could help fill TDF gaps where diets within a consumer class are absent. Overall, we suggest that using standard TDF trophic step values should be abandoned, as feeding studies are often poor proxies for natural diets, particularly for herbivores and omnivores. Instead, we provide recommendations on how to select TDFs, along with a range of TDF-δ<sup>13</sup>C and TDF-δ<sup>15</sup>N values depending on diet source, consumer class, and tissue type. Use of these more refined recommendations and TDF values in isotopic assessments will improve estimates of diets and trophic interactions in natural systems, leading to a better understanding of ecological interactions and communities.</p>
Reevaluating trophic discrimination factors (Δδ13C and Δδ15N) for diet reconstruction
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Evaluating Bayesian stable isotope mixing models of wild animal diet and the effects of trophic discrimination factors and informative priors
<blockquote> <p>1. Ecologists quantify animal diets using direct and indirect methods, including analysis of faeces, pellets, prey items and gut contents. For stable isotope analyses of diet, Bayesian stable isotope mixing models (BSIMMs) are increasingly used to infer the relative importance of food sources to consumers. Although a powerful approach, it has been hard to test BSIMM performance for wild animals because precise, direct dietary data are difficult to collect.<br> 2. We evaluated the performance of BSIMMs in quantifying animal diets when using δ13C and δ15N stable isotope ratios from the feathers and red blood cells of common buzzard Buteo buteo chicks. We analysed mixing model outcomes with various trophic discrimination factors (TDFs), with and without informative priors, and compared these to direct observations of prey provisioned to chicks by adults at nests, using remote cameras. <br> 3. Although BSIMMs with different TDFs varied markedly in their performance, the statistical package SIDER generated TDFs for both feathers and blood that resulted in model outputs that accorded well with direct observations of prey provisioning. Using feather TDFs derived from captive peregrines Falco peregrinus resulted in estimates of diet composition that were also similar to provisioned prey, though blood TDFs from the same study performed poorly. The inclusion of informative priors, based on conventional analysis of pellet and prey remains, markedly reduced model performance.<br> 4. BSIMMs can provide accurate assessments of diet in wild animals. TDF estimates from the SIDER package performed well. The inclusion of informative priors from conventional methods in Bayesian mixing models can transfer biases into model outcomes, leading to erroneous results.</p> </blockquote>
Data from: SIDER: an R package for predicting trophic discrimination factors of consumers based on their ecology and phylogenetic relatedness
Stable isotope mixing models (SIMMs) are an important tool used to study species' trophic ecology. These models are dependent on, and sensitive to, the choice of trophic discrimination factors (TDF) representing the offset in stable isotope delta values between a consumer and their food source when they are at equilibrium. Ideally, controlled feeding trials should be conducted to determine the appropriate TDF for each consumer, tissue type, food source, and isotope combination used in a study. In reality however, this is often not feasible nor practical. In the absence of species-specific information, many researchers either default to an average TDF value for the major taxonomic group of their consumer, or they choose the nearest phylogenetic neighbour for which a TDF is available. Here, we present the SIDER package for R, which uses a phylogenetic regression model based on a compiled dataset to impute (estimate) a TDF of a consumer. We apply information on the tissue type and feeding ecology of the consumer, all of which are known to affect TDFs, using Bayesian inference. Presently, our approach can estimate TDFs for two commonly used isotopes (nitrogen and carbon), for species of mammals and birds with or without previous TDF information. The estimated posterior probability provides both a mean and variance, reflecting the uncertainty of the estimate, and can be subsequently used in the current suite of SIMM software. SIDER allows users to place a greater degree of confidence on their choice of TDF and its associated uncertainty, thereby leading to more robust predictions about trophic relationships in cases where study-specific data from feeding trials is unavailable. The underlying database can be updated readily to incorporate more stable isotope tracers, replicates and taxonomic groups to further increase the confidence in dietary estimates from stable isotope mixing models, as this information becomes available.
Figure 3 in Trophic discrimination factors and stable isotope variability in a captive feeding trial of the southern rock lobster Jasus edwardsii (Hutton, 1875) (Decapoda: Palinuridae) in Tasmania, Australia
Figure 3. Isoplot showing the fit of different trophic discrimination factors (TDFs) to a wild population of Jasus edwardsii (■) and associated wild prey sources: Haliotis rubra (▲), Heliocidaris erythrogramma (■), and Lunella undulata (●), demonstrating that our experimentally derived TDFs provide the best fit. Black dots represent the isotope signatures of individual dorsal-muscle samples of lobsters. Mean and standard deviation values refer to the three source signatures with no TDF correction, with correction using multi-taxa TDFs from the literature (∆13C 0.4‰ and ∆15N 3.4‰ (Post, 2002)) and previously determined TDFs for J. edwardsii (∆13C 0.84‰ and ∆15N 3.28‰ (Suring & Wing, 2009)), with correction using our experimentally determined dorsal muscle TDFs (∆13C 4.45‰ and ∆15N 4.36‰), and correction with our experimentally determined leg muscle TDFs (∆13C 3.86‰ and ∆15N 5.05‰).
Figure 1 in Trophic discrimination factors and stable isotope variability in a captive feeding trial of the southern rock lobster Jasus edwardsii (Hutton, 1875) (Decapoda: Palinuridae) in Tasmania, Australia
Figure 1. Correlation plots showing the relationship between samples of captive Jasus edwardsii leg muscle and dorsal muscle (N = 34) for δ13C (A) and δ15N (B).
Figure 2 in Trophic discrimination factors and stable isotope variability in a captive feeding trial of the southern rock lobster Jasus edwardsii (Hutton, 1875) (Decapoda: Palinuridae) in Tasmania, Australia
Figure 2. Change in mean (± SE) isotopic signatures over time for captive Jasus edwardsii in three diet treatments: sea urchin (●), fish (▲), and mussel (■). Plots show change in δ13C (A) and change in δ15N (B). Solid lines represent lobster muscle tissue as represented by a leg-muscle icon, and dashed lines represent lobster exoskeleton as represented by a red-leg exoskeleton icon. Dotted lines represent the mean diet isotope + trophic discrimination factor (TDF) calculated as of 12 months. All mean values have a minimum sample size of five replicates except for the following samples: 4-month mussel (N = 4), 18-month fish (N = 4), and 18-month mussel (N = 3). For lethal samples (initial, 12, and 18 monts), exoskeleton samples are the mean of leg and dorsal samples where both were collected. Exoskeleton samples are only leg samples for non-lethal samples.
Data from: Evaluating Bayesian stable isotope mixing models of wild animal diet and the effects of trophic discrimination factors and informative priors
Open the record for dataset details and reuse information.
Data from: SIDER: an R package for predicting trophic discrimination factors of consumers based on their ecology and phylogenetic relatedness
Open the record for dataset details and reuse information.
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