Trophic Discrimination Factors Research Articles

Species-specific trophic discrimination factors can reduce the uncertainty of stable isotope analyses

Species-specific trophic discrimination factors can reduce the uncertainty of stable isotope analyses

Integrated approach for the isotope trophic position of black-tailed gull (Larus crassirostris) eggs over a decade: Combining stable isotopes of amino acids and fatty acids composition

Recently, compound-specific isotope analysis (CSIA) using the amino acid nitrogen stable isotope ratio (δ15NAAs) has been widely used for accurate estimation of trophic position (TP). In addition, a quantitative fatty acid signature analysis (QFASA) offers insights into diet sources. In this study, we used these techniques to estimate the TP for seabirds that rely on diverse food sources across multiple ecosystems. This allows for the proper combination of factors used in TP calculation which are different for each ecosystem. The approach involved the application of a multi-mixing trophic discrimination factor (TDF) and mixing β which is a Δδ15N between trophic and source amino acid of primary producer. Since the black-tailed gulls (BTGs) are income-breeding seabirds, which rely on energy sources obtained around their breeding sites, they and their eggs could be useful bioindicators for environmental monitoring. However, the ecological properties of BTGs such as habitats, diets, and TP are not well known due to their large migration range for wintering or breeding and their feeding habits on both aquatic and terrestrial prey. In this study, the eggs were used for estimating TP and for predicting TP of mother birds to overcome difficulties such as capturing birds and collecting non-invasive tissue samples. Eggs, sampled over a decade from three Korean islands, showed spatial differences in diet origin. Considering both the food chain and physiology of BTG, the TP of eggs was estimated to be 3.3–4.0. Notably, the TP was significantly higher at site H (3.8 ± 0.1) than at site B (3.5 ± 0.2), which indicated a higher contribution of marine diet as confirmed by QFASA. Using a reproductive shift of δ15NAAs, the TP of the mother birds was predicted to be 3.6–4.3, positioning them as the top predator in the food web. The advanced integration of multiple approaches provides valuable insights into bird ecology.

Diet-tissue discrimination factors of three neotropical freshwater fishes and a comparison of the trophic position

The trophic discrimination factor (TDF) is a key parameter for stable isotope analysis and due to a lack of species-specific TDFs, mean universal values have been used, resulting in uncertainties about the trophic position of species and a call for more experiments. In this study, we have addressed the lack of experimental species-specific TDFs conducting three experiments of 128 days each to determine the TDF (muscle and liver) of three species, the piscivore Pseudoplatystoma corruscans (Spix & Agassiz, 1829), and the omnivores Piaractus mesopotamicus (Holmberg, 1887) and Astyanax lacustris (Lütken, 1875), tropical fishes native to the La Plata River basin. Then, we calculated the trophic position (TP) using the mean universal TDF from literature and the species-specific TDF produced in this study for Pseudoplatystoma corruscans. We estimated the TDFs for the three species through experiment and the values found differed from the mean universal TDF in the literature. Moreover, the TP was lower when using the species-specific TDFs. The TP is important for several analyses, including its use in functional diversity. Therefore, we recommend using species-specific TDF values for calculating TP once it differs from the results calculated with mean universal TDF.

Climate change-informed dietary modeling in Pacific cod: Experimentally-derived effects of temperature and dietary quality on carbon and nitrogen stable isotope trophic discrimination factors

Stable isotope analysis is a powerful tool for dietary modeling and trophic ecology research. A crucial piece of information for isotopic dietary modeling is the accurate estimation of trophic discrimination factors (TDFs), or the isotopic offset between a consumer’s tissue and its diet. In order to parameterize stable isotope dietary models for future climate scenarios, we investigated the effect of water temperature and dietary protein and lipid content on TDFs in juvenile Pacific cod (Gadus macrocephalus). Pacific cod are a commercially and ecologically important species, with stock numbers in the northeast Pacific recently having dropped by more than 70%. We tested four water temperatures (6, 8, 10, and 12°C) and two dietary regimens (low and high lipid content), representing a range of potential ocean temperature and prey quality scenarios, in order to determine carbon and nitrogen TDFs in juvenile Pacific cod. Additionally, we assessed dietary intake and proximate composition of the experimental fish in order to estimate consumption, assimilation, and retention of dietary nutrients. The results of this study suggest that dietary protein catabolism is a primary driver of nitrogen TDF variability in juvenile Pacific cod. Across all temperature treatments from 6 to 12°C, fish reared on the lower quality, lower lipid content diet had higher nitrogen TDFs. The mean TDFs for fish raised on the higher lipid, lower protein diet were +3.40 ‰ for nitrogen (Δ15N) and +0.36 ‰ for lipid-corrected carbon (Δ LC 13C). The mean TDFs for fish raised on the lower lipid, higher protein diet were +4.09 ‰ for nitrogen (Δ15N) and 0.00 ‰ for lipid-corrected carbon (Δ LC 13C). Lipid-corrected carbon isotope data showed that, regardless of temperature, fish consuming the lower lipid diet had essentially no trophic discrimination between diet and bulk tissues. We found no ecologically meaningful differences in TDFs due to water temperature across the 6°experimental range. The results of this experiment demonstrate that dietary quality, and more specifically the use of dietary protein for energetic needs, is a primary driver of trophic discrimination factors. The TDFs determined in this study can be applied to understanding trophic ecology in Pacific cod and closely related species under rapidly changing prey availability and ocean temperature conditions.

Trophic Transfer of Halogenated Organic Pollutants in a Wetland Food Web: Insights from Compound-Specific Nitrogen Isotope of Amino Acids and Food Source Analysis.

A trophic position (TP) model (TPmix model) that simultaneously considered trophic discrimination factor and βGlu/Phe variations was developed in this study and was first applied to investigate the trophic transfer of halogenated organic pollutants (HOPs) in wetland food webs. The TPmix model characterized the structure of the wetland food web more accurately and significantly improved the reliability of TMF compared to the TPbulk, TPAAs, and TPsimmr models, which were calculated based on the methods of stable nitrogen isotope analysis of bulk, traditional AAs-N-CSIA, and weighted βGlu/Phe, respectively. Food source analysis revealed three interlocking food webs (kingfisher, crab, and frogs) in this wetland. The highest HOP biomagnification capacities (TMFmix) were found in the kingfisher food web (0.24-82.0), followed by the frog (0.08-34.0) and crab (0.56-11.7) food webs. The parabolic trends of TMFmix across combinations of log KOW in the frog food web were distinct from those of aquatic food webs (kingfisher and crab), which may be related to differences in food web composition and HOP bioaccumulation behaviors between aquatic and terrestrial organisms. This study provides a new tool to accurately study the trophic transfer of contaminants in wetlands and terrestrial food webs with diverse species and complex feeding relationships.

Isotopic Turnover and Fractionation of δ15N and δ13C in Captive Pseudopleuronectes americanus (Walbaum)

Stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) are ubiquitous ecological tracers used to elucidate an organism’s diet and habitat. However, the application of stable isotope ratios to reconstruct a consumer’s ecology relies upon accurate rates for isotopic turnover at both a tissue and species-specific level. This study estimated isotope turnover rates and trophic discrimination factors in four different tissues (liver, digestive tissue, muscle, and skin) with variable metabolic activity in winter flounder Pseudopleuronectes americanus using a controlled diet-switch experiment. Differences in half-lives were noted among the tissues and between the experimental diets for both δ15N and δ13C. The experimental diets of krill and mysis had variability in nutritional composition, resulting in similar turnovers in δ15N but slower turnovers in δ13C for fish fed krill. Turnovers in both δ15N and δ13C were strongly influenced by metabolism, with the contribution reaching up to 98%, as fish exhibited minimal overall growth. The results of this study demonstrate the importance of considering differences in the catabolic activity of tissue maintenance for fish exhibiting minimal growth, as well as differences in metabolic assimilation of dietary sources that vary in their protein and lipid contents.

Trophic discrimination factors and stable isotope variability in a captive feeding trial of the southern rock lobster Jasus edwardsii () (Decapoda: Palinuridae) in Tasmania, Australia

Abstract Trophic discrimination or fractionation factors (TDFs), such as ∆15N and ∆13C, are used in stable isotope mixing models to account for differences between source tissues (diet/prey) and consumer tissues (predator). We aimed firstly to obtain TDF values for a spiny lobster, the southern rock lobster Jasus edwardsii (Hutton, 1875), to better understand lobster diet in the wild and secondly to investigate variability in isotope signature within tissues of individuals and across a temporal scale to test if non-lethal sampling can be used in an ecological context. We conducted an 18-mo captive feeding trial with juvenile lobsters using three diet treatments and analysed dorsal and leg muscle, along with dorsal and leg exoskeleton for δ13C and δ15N values. Average TDFs for the three diet treatments were 3.86 ± 0.98‰ (∆13C) and 5.06 ± 0.65‰ (∆15N) for leg muscle, and 4.45 ± 1.04‰ (∆13C) and 4.36 ± 0.6‰ (∆15N) for dorsal muscle. When tested against wild lobsters and prey, these TDFs outperformed multi-taxa TDFs found in the literature. Isotope values from lobster leg muscle were not identical to associated dorsal muscle but the two were highly correlated, indicating that non-lethal sampling is acceptable. Values for exoskeleton isotope were significantly different from muscle, likely due to the exoskeleton not being in a constant state of growth and replacement, unlike the muscle tissue, which constantly incorporates new material. We conclude that our experimentally derived TDFs are suitable for mixing model analysis for J. edwardsii and when tested on a wild sample of lobsters they outperformed other TDFs reported in the literature. We show that non-lethal sampling using leg muscle is an appropriate sampling method, since this tissue is highly correlated to the commonly used dorsal muscle. This option for non-lethal sampling enhances the potential to widely sample wild populations or sample during industrial processing without the need to sacrifice whole animals.

Meta‐analysis and critical review of trophic discrimination factors (Δ13C and Δ15N): Importance of tissue, trophic level and diet source

Abstract Robustly quantifying dietary resource use and trophic position using stable isotopes requires accurate trophic discrimination factors (TDF; Δ13C and Δ15N 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 Δ13C and 3.4‰ for Δ15N 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. 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 Δ13C and Δ15N. Additionally, we reviewed 358 trophic ecology studies to determine if studies accounted for these factors during their TDF selection process. For Δ13C, 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 Δ15N, vertebrates showed considerable variation among tissue types and trophic levels, likely due to differences in tissue synthesis and physiological capabilities. Overall, Δ13C ranged from −5.1‰ to 9.1‰ and Δ15N from −3.3‰ to 9.7‰, underscoring that 1.0‰ for Δ13C and 3.4‰ for Δ15N are not universally appropriate. Moreover, both Δ13C and Δ15N 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. 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. Read the free Plain Language Summary for this article on the Journal blog.

Food web differences between two neighboring tropical high mountain lakes and the influence of introducing a new top predator.

High mountain lakes (HMLs) are considered unique and comparable ecosystems for monitoring global climate change. The food web structure can indicate the response of these ecosystems to ecological threats, such as fish introduction, by analyzing the trophic dynamics. Nonetheless, the food webs of tropical HMLs are less well-studied than temperate HMLs. The present study assessed the food webs of two neighboring (600 m apart) tropical HMLs, El Sol and La Luna, inside the crater of the Nevado de Toluca volcano, Mexico. It used stable isotopes (δ13C and δ15N) and Bayesian mixing models with different trophic discrimination factors and priors to assess the impacts of introduced rainbow trout, persisting only in the larger lake, El Sol. The food web in Lake El Sol was more complex than in Lake La Luna, mainly due to its larger size, extensive vegetated littoral zone, and being fueled by autochthonous primary production. In contrast, the smaller and fishless Lake La Luna has a reduced and bare littoral zone that harbored a simple food web substantially sustained by allochthonous carbon inputs. The persistence of introduced rainbow trout in Lake El Sol but not in Lake La Luna accentuated the differences between the lakes. The models suggested that rainbow trout fed on key consumers of littoral macroinvertebrates (70-80%) and pelagic zooplankton (20-30%), increasing the linkage between sub-networks. In both tropical HMLs, the species richness and herbivorous fraction were elevated compared with temperate HMLs, while the linkage density and omnivorous fraction were lower. Basal nodes dominated these tropical HMLs, and the vegetated littoral zone of Lake El Sol had more intermediate (omnivore) nodes. Our results showed the convenience of food web analysis to compare the effects of introduced fish in originally fishless lakes in different latitudes.

Tissue‐ and diet‐dependent stable carbon and nitrogen isotope discrimination: a calibration study in a captive shorebird species

In ecology, stable‐isotope ratios are widely used to determine diets of organisms and reconstruct food webs. This is usually done by analyzing the stable‐isotope ratios of nitrogen (δ15N), which increase with increasing trophic level, and those of carbon (δ13C), which correlate with the δ13C value of food source(s) and generally differ between terrestrial and marine food sources. Assimilation of food changes stable‐isotope ratios, resulting in different values between the food source and its consumer. These differences are known as isotope trophic discrimination factors and, if known, can be used to determine from the stable‐isotope ratios in the consumer's tissue what the consumer has been eating. What is often ignored is that discrimination factors can differ between consumer's food sources and also between tissue types. Therefore, we performed a controlled feeding study in red knots Calidris canutus to determine discrimination factors between different food sources and red knot tissues. We kept two groups of red knots in captivity on a stable diet, one group feeding on mudsnails and the other on Trouvit pellets, for several months, during which the birds molted their feathers. We analyzed δ13C and δ15N in both food sources and in five red knot tissues (blood cells, blood plasma and three feather types) and subsequently calculated the isotope discrimination factors. We confirmed that the discrimination factors differed between tissues, and also between diets. Our values deviated from general averages reported in reviews on a wide range of animals/birds, but were very similar to values from previous red knot and dunlin studies. We therefore think that our discrimination factors can be used in future stable isotope studies, not only on red knots, but also on other marine shorebird species and plea for careful consideration of using the right discrimination factors.Keywords: δ13C, δ15N, discrimination factor, red knot, shorebird, stable isotope

Isotopic ecology of Hawaiian green sea turtles (Chelonia mydas) and reliability of δ13C, δ15N, and δ34S analyses of unprocessed bone samples for dietary studies

This study conducted stable isotope analysis (δ13C, δ15N, and δ34S) on the epidermis and two skeletal elements (rib and squamosal bones) of Hawaiian green turtles (Chelonia mydas) and putative diet items obtained from two neritic sites: the Kona/Kohala coast and Oahu. Turtle tissues were collected in 2018–2020 and diet samples in 2018, 2019, and 2021. The effect of body size and sampling locality on individual bulk tissue isotope values was evaluated, and stable isotope mixing models based on δ13C, δ15N, and δ34S values from those tissues and four groups of food sources were used to reconstruct diet histories of the turtles. Mixing models indicated that green turtles along the Kona/Kohala coast consumed an omnivorous diet, whereas those from Oahu had an herbivorous diet. These diet make-ups are consistent with published gut content analyses. However, mixing models using the stable isotope ratios in rib and squamosal bone failed to yield reasonable diet histories, probably due to inadequacies of the applied trophic discrimination factor (TDF), a key model parameter. These results further establish that stable isotope ratios in the epidermis can be used effectively to study green turtle diet, but also reveal that more validation—and establishment of appropriate TDFs—is needed before bone can be used reliably to assess green turtle diet.

Isotopic turnover in polar cod (Boreogadus saida) muscle determined through a controlled feeding experiment.

Polar cod (Boreogadus saida) is an important trophic link within Arctic marine food webs and is likely to experience diet shifts in response to climate change. One important tool for assessing organism diet is bulk stable isotope analysis. However, key parameters necessary for interpreting the temporal context of stable isotope values are lacking, especially for Arctic species. This study provides the first experimental determination of isotopic turnover (as half-life) and trophic discrimination factors (TDFs) of both δ13 C and δ15 N in adult polar cod muscle. Using a diet enriched in both 13 C and 15 N, we measured isotopic turnover times of 61 and 49 days for δ13 C and δ15 N, respectively, with metabolism accounting for >94% of the total turnover. These half-life estimates are valid for adult polar cod (>3 years) experiencing little somatic growth. We measured TDFs in our control of 2.6‰ and 3.9‰ for δ13 C and δ15 N, respectively, and we conclude that applying the commonly used TDF of ~1‰ for δ13 C for adult polar cod may lead to misrepresentation of dietary carbon source, while the use of 3.8‰ for δ15 N is appropriate. Based on these results, we recommend that studies investigating seasonal shifts in the diet of adult polar cod sample at temporal intervals of at least 60 days to account for isotopic turnover in polar cod muscle. Although isotopic equilibrium was reached by the fish in this study, it was at substantially lower isotope values than the diet. Additionally, the use of highly enriched algae in the experimental feed caused very high variability in diet isotope values which precluded accurate calculation of TDFs from the enriched fish. As a result of the challenges faced in this study, we discourage the use of highly enriched diets for similar experiments and provide recommendations to guide the design of future isotopic turnover experiments.

Multi-taxa marine isoscapes provide insight into large-scale trophic dynamics in the North Pacific

Quantifying large scale trophic dynamics and interactions in marine systems remains a key challenge in ecological research and conservation, particularly in remote habitats which are difficult and costly to access. Stable isotope (SI) analysis has emerged as an effective tool for studying ecosystem interactions, especially when used to construct isotopic landscapes, or “isoscapes”. We used a combination of empirical SI data, global modeled phytoplankton isoscapes, and oceanographic data to investigate how large SI datasets might be harnessed to describe and predict patterns of regional food web dynamics and trophic structuring. We collated data from 99 published and author-contributed datasets of carbon and nitrogen SI data for open-ocean and neritic nektonic fish and invertebrates in the North Pacific Ocean to estimate mean SI values for successive trophic level groups within Longhurst biogeographic provinces. We found that these estimates reflected geographic patterns predicted by modeled phytoplankton isoscapes, suggesting that baseline SI ratios are consistently propagated up through regional food webs. This was further corroborated by significant correlations between our estimated SI values with the same oceanographic and geographic variables known to influence phytoplankton SI ratios. However, we observed variable amounts of isotopic offset between successive trophic level groups within and across provinces, indicating that the underlying drivers of trophic dynamics differ across regions. We found evidence of decreased isotopic differences between higher level trophic groups, suggesting the possibility of trophic compression and highlighting the importance of using taxa- and tissue-specific trophic discrimination factors (TDFs) when comparing or predicting consumer SI ratios. We used stable isotopic niche analyses to demonstrate that coupled use of δ13C and δ15N data increased capacity to differentiate province-specific consumer groups, particularly where adjacent provinces had similar ranges of values for a single SI. We also found that baseline isoscapes may be a viable tool for predicting broad trends of consumer SI ratios by calculating ‘predicted’ δ13C and δ15N values for successive trophic groups across provinces using modeled phytoplankton data and diet-dependent discrimination factors. These predicted values were significantly correlated with our estimates based on empirical data, with similarity of predicted δ13C values increasing for higher trophic level groups. We conclude that regional SI patterns remain consistent across diverse ecosystems. With increased spatiotemporal resolution of SI data across ocean basins and increased availability of species-specific life history traits and physiologies, multi-taxa isoscapes hold great potential for studying large scale marine trophodynamics and movement patterns of highly mobile marine species.

TDFCAM: A method for estimating stable isotope trophic discrimination in wild populations.

Stable isotope mixing models (SIMMs) are widely used for characterizing wild animal diets. Such models rely upon using accurate trophic discrimination factors (TDFs) to account for the digestion, incorporation, and assimilation of food. Existing methods to calculate TDFs rely on controlled feeding trials that are time-consuming, often impractical for the study taxon, and may not reflect natural variability of TDFs present in wild populations.We present TDFCAM as an alternative approach to estimating TDFs in wild populations, by using high-precision diet estimates from a secondary methodological source-in this case nest cameras-in lieu of controlled feeding trials, and provide a framework for how and when it should be applied.In this study, we evaluate the TDFCAM approach in three datasets gathered on wild raptor nestlings (gyrfalcons Falco rusticolus; peregrine falcons Falco perigrinus; common buzzards Buteo buteo) comprising contemporaneous δ13C & δ15N stable isotope data and high-quality nest camera dietary data. We formulate Bayesian SIMMs (BSIMMs) incorporating TDFs from TDFCAM and analyze their agreement with nest camera data, comparing model performance with those based on other relevant TDFs. Additionally, we perform sensitivity analyses to characterize TDFCAM variability, and identify ecological and physiological factors contributing to that variability in wild populations.Across species and tissue types, BSIMMs incorporating a TDFCAM outperformed any other TDF tested, producing reliable population-level estimates of diet composition. We demonstrate that applying this approach even with a relatively low sample size (n < 10 individuals) produced more accurate estimates of trophic discrimination than a controlled feeding study conducted on the same species. Between-individual variability in TDFCAM estimates for ∆13C & ∆15 N increased with analytical imprecision in the source dietary data (nest cameras) but was also explained by natural variables in the study population (e.g., nestling nutritional/growth status and dietary composition).TDFCAM is an effective method of estimating trophic discrimination in wild animal populations. Here, we use nest cameras as source dietary data, but this approach is applicable to any high-accuracy method of measuring diet, so long as diet can be monitored over an interval contemporaneous with a tissue's isotopic turnover rate.

Toxicity and Starvation Induce Major Trophic Isotope Variation in Daphnia Individuals: A Diet Switch Experiment Using Eight Phytoplankton Species of Differing Nutritional Quality.

Stable isotope values can express resource usage by organisms, but their precise interpretation is predicated using a controlled experiment-based validation process. Here, we develop a stable isotope tracking approach towards exploring resource shifts in a key primary consumer species Daphnia magna. We used a diet switch experiment and model fitting to quantify the stable carbon (δ13C) and nitrogen (δ15N) isotope turnover rates and discrimination factors for eight dietary sources of the plankton species that differ in their cellular organization (unicellular or filamentous), pigment and nutrient compositions (sterols and polyunsaturated fatty acids), and secondary metabolite production rates. We also conduct a starvation experiment. We evaluate nine tissue turnover models using Akaike's information criterion and estimate the repetitive trophic discrimination factors. Using the parameter estimates, we calculate the hourly stable isotope turnover rates. We report an exceedingly faster turnover value following dietary switching (72 to 96 h) and a measurable variation in trophic discrimination factors. The results show that toxic stress and the dietary quantity and quality induce trophic isotope variation in Daphnia individuals. This study provides insight into the physiological processes that underpin stable isotope patterns. We explicitly test multiple alternative dietary sources and fasting and discuss the parameters that are fundamental for field- and laboratory-based stable isotope studies.

Calibrating bulk and amino acid δ13C and δ15N isotope ratios between bivalve soft tissue and shell for paleoecological reconstructions

Ecological isotope proxies measured in ancient bivalve shell matrix protein have great promise for paleoecological reconstruction. Compound-specific isotopes of amino acids (CSI-AA) may be an ideal tool for developing paleoecological proxies, as initial work has indicated that CSI-AA is less subject to alteration under geologic conditions relative to bulk isotope values. While CSI-AA proxies have been applied in modern bivalve soft tissues and a few shell studies, they have yet to be systematically investigated in shell matrix protein. Here, we measured stable isotope values of carbon (δ13C) and nitrogen (δ15N) in both bulk and individual amino acids (AA), comparing soft tissue and shell organic fractions to test the fidelity of a suite of ecological isotope proxies in shell. We sampled three ubiquitous bivalve species in four seasons for one year from two common coastal environments: littoral rocky intertidal and estuarine delta ecosystems. Particulate organic matter (POM) was simultaneously collected to investigate relationships between tissue types and local POM isotope signatures. The ecological proxies tested include trophic niche breadth from bulk isotopes, and baseline δ13C and δ15N values, resource contribution, and trophic level from CSI-AA data. We found niche breadth corresponded well between tissue types, but that bulk isotope values were significantly higher in shell compared to soft tissue. In the CSI-AA record, there were no differences in baseline δ13C and δ15N proxies between tissue types. While no consistent seasonal trends were observed in the δ13C record, summer bulk and baseline δ15N values were lowest in both ecosystems in both tissues. Food resource contribution estimates from shell also closely matched soft tissue, however trophic level estimates were consistently higher in shell, attributed to a systematic offset in Glutamic acid δ15N values. We therefore propose here a new mollusk-specific trophic discrimination factor, and corresponding shell isotope enrichment factor to correct for isotopic routing, both required for accurate paleoecological reconstructions.

Reevaluating trophic discrimination factors (Δδ13C and Δδ15N) for diet reconstruction

AbstractStable isotope analysis is increasingly being used to assess diet and trophic positions of animals. Such assessments require estimates of trophic discrimination factors (TDFs)—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 approximately 1‰ for carbon (TDF‐δ13C) and 3‰ for nitrogen (TDF‐δ15N). Such practices may yield inaccuracies since TDFs vary greatly, even within a species. To better understand the factors that influence TDFs, we conducted a meta‐analysis of TDF‐δ13C and TDF‐δ15N 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 data set 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‐δ13C values (−1.5‰ to 7.3‰), whereas consumer class best explained variation in TDF‐δ15N 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‐δ13C because mixed‐diet lab studies may produce 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, because feeding studies are often poor proxies for natural diets, particularly for herbivores and omnivores. Instead, we make recommendations on how to select TDFs, along with a range of TDF‐δ13C and TDF‐δ15N 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.

Experimentally derived biochemical modelling parameters to improve understanding of aquaculture's effect on marine food webs

To construct robust biogeochemical models for application to marine-based aquaculture settings, careful selection of appropriate model parameters is necessary. This study used an experimental approach to establish biomarkers of farm and marine-derived organic matter, and to derive isotopic turnover rates, and trophic discrimination factors specific to aquaculture associated food webs.A shift towards a farm-derived resource base resulted in consumer tissues more depleted in the carbon-13 isotope (indicated by more negative δ13C values) and a higher proportion of oleic acid, linoleic acid, and alpha-linoleic acid in the fatty acid profile of consumers over time. Measured trophic discrimination factors between dietary sources and consumer tissues demonstrated high variability among species and tissue types, ranging from −0.25‰ to 0.82‰ for Δ13C and from −0.77‰ to 6.8‰ for Δ15N. Stable isotope half-lives were also diverse among species and tissue types, ranging from <7 days to 462 days.Results demonstrated that construction of robust models for tracing assimilation of farm-derived organic matter through marine food webs requires the use of taxa and tissue specific parameters. Turnover rates have applications for understanding assimilative capacity of communities and for managing populations within the ecological footprint of farms.

Leaf consumption and experimental discrimination of stable isotopes between mangrove leaves and the tree-climbing crab (Brachyura: Sesarmidae: Aratus pisonii)

The mangrove tree-climbing crab Aratus pisonii (Sesarmidae) is one of the most abundant and iconic arboreal brachyurans in the Neotropics. Despite being traditionally regarded as an herbivorous consumer leaving visible rasping marks on Rhizophora mangle leaves, gut content analyses, field observations and experimental feeding trials also suggest facultative omnivory. In addition, there are still questions regarding the nutritional challenge of leaf consumption, particularly whether the relatively nutrient-poor mangrove leaf litter is assimilated when offered as the sole component of the diet. This study aimed to: 1) experimentally compare consumption rates of fresh (green) and senescent (yellow) Rhizophora mangle leaves during a 26 d experiment; 2) estimating stable isotope trophic discrimination of carbon and nitrogen between leaves and muscle tissue after a 58 d controlled feeding experiment with A. pisonii from the Southwestern Caribbean Sea (Atrato River Delta, Colombia). A. pisonii consumed a total of 17.11 ± 20.75 mg leaf dry wt (g crab wet wt) −1 d−1 of R. mangle leaves during the experimental trial. However, the consumption was greater for green leaves (12.05 ± 13.07 mg) than for yellow leaves (5.06 ± 13.85 mg). Our experimental estimates of mean ± SD TDFs (trophic discrimination factors) were Δ13C 5.4 ± 0.9‰ and Δ15N 3.9 ± 0.5‰, close to those suggested for other sesarmid crabs. These results can be used in future estuarine food webs studies from Neotropical mangrove habitats, and provide new insights into the importance of A. pisonii as a key herbivore converting low-quality mangrove material into nutritious secondary production (biomass), potentially linking primary producers with intermediate and top predators (both aquatic and terrestrial).

Diet/Hair and Diet/Faeces Trophic Discrimination Factors for Stable Carbon and Nitrogen Isotopes, and Hair Regrowth in the Yellow-Necked Mouse and Bank Vole

We conducted a nonlethal feeding experiment to measure diet/hair and diet/faeces trophic discrimination factors (TDFs) in two rodent species fed two diet types. The mean TDFs between diet and hair (expressed as δ13C/δ15N) were 2.14‰/3.44‰ for C3 plant-based fodder and 2.65‰/3.25‰ for fodder with 30% animal (insect) matter in yellow-necked mice (Apodemus flavicollis) and 1.66‰/4.67‰ for C3 plant-based fodder and 1.94‰/4.49‰ for fodder with 30% animal (insect) matter in bank voles (Myodes glareolus). As compared with the diet, faeces were depleted in 13C by about 0.75‰ and enriched in 15N by about 2‰. All bank voles and only half of yellow-necked mice had regrown hair at the end of the study (day 95). We analysed archived skins of those species and confirmed, that moult in bank voles is more intensive. Our results can be used for trophic studies of temperate-forest rodents and for meta-analyses on trophic discrimination.