Evidence of rapid shifts in the trophic base of lotic predators using experimental dietary manipulations and assimilation-based analyses

Assimilation-based techniques such as stoichiometric analysis, bulk tissue stable isotope analysis (SIA), fatty acid (FA) profiling and compound-specific SIA of FAs can be used to resolve ambiguities in consumer-resource relationships, but comparisons of their effectiveness are lacking. Feeding trials and concurrent application of these techniques were used to investigate the trophic base of two shrimps from Hong Kong streams: the atyid Caridina cantonensis, which is a primary consumer, and predatory Macrobrachium hainanense (Palaemonidae). Leaf litter and periphyton were fed to C. cantonensis (CC-LF and CC-PF, respectively) reared in the laboratory for 2 months, when C. cantonensis, that had fed on a mixed diet (CC-WC), were collected from the field. Atyids from each group (CC-LF, CC-PF and CC-WC) were fed to M. hainanense (MH-L, MH-P and MH-W, respectively) during a further 2-month trial, at the end of which M. hainanense were also collected from the field (MH-R). FA biomarkers present in CC-WC and CC-PF indicated that C. cantonensis depended primarily on autochthonous foods, and FA profiles of CC-WC were distinct from those of CC-LF. Differences in C/N ratios and isotope signatures of leaf litter and periphyton were not reflected in tissues of atyids or palaemonid predators. FA profiles of M. hainanense groups were similar, but FA stable C isotope ratio (δ¹³C) signatures of MH-R and MH-W were distinct from those of MH-L and leaf litter. FA δ¹³C signatures of MH-R were similar to those of MH-P and MH-W, indicating that autochthonous resources constituted the trophic base of production for this predator. This is the first study using compound-specific SIA to study stream food chains. Compound-specific SIA and FA profiling allowed accurate elucidation of consumer-resource relationships that were not revealed by stoichiometry or bulk tissue SIA. Compound-specific SIA was particularly sensitive for detecting rapid shifts in the predator trophic base. This tool will have wide applicability for investigating food webs in a range of ecosystems. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A non‐lethal approach to estimate whole‐body 13C and 15N stable isotope ratios of freshwater amphipods using walking legs

To analyze the stable isotope ratios of small‐bodied invertebrates, the entire animal is typically sacrificed and processed, which is problematic for threatened or endangered species. Appendages which are regenerated could be used to infer whole‐body isotope ratios, but differences in turnover rates and isotopic signatures among tissues may confound such an approach. We tested the hypothesis that the δ¹³C and δ¹⁵N of whole‐body tissue for freshwater amphipods could be predicted from the δ¹³C and δ¹⁵N of walking legs, with the goal of estimating body δ¹³C and δ¹⁵N of Gammarus acherondytes, a United States federally endangered species. To test this, we analyzed the δ¹³C and δ¹⁵N of walking legs and bodies of five species of amphipods from geographically distant areas (Idaho, Illinois, and Washington) in the United States. The general relationships of whole‐body isotope ratios of C and N as a function of leg isotope ratios were linear and had slopes of one. In the range of the data, leg δ¹³C was slightly lower than body δ¹³C, indicating some tissue‐specific fractionation, while δ¹⁵N was similar for legs and bodies. Our data suggest that legs can be used to predict body isotope ratios in freshwater amphipods. This approach provides an additional tool to help researchers understand the biology of small, endangered invertebrates without sacrificing individuals. This is especially useful in cave ecosystems where populations are naturally sparse.     

Morphological and trophic specialization in a subterranean amphipod assemblage

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Combining stable isotope analysis and conventional techniques to improve knowledge of the diet of the European Roller Coracias garrulus

Diet studies are crucial for understanding the ecology and evolution of species, as well as for establishing appropriate conservation and management strategies. However, they remain methodologically challenging due to variation between seasons, sites, sexes or age groups and even variation between individuals. Due to method‐specific characteristics and biases, a combination of existing techniques can overcome the inherent limitations of each technique and provide a more accurate and broad picture of species’ food preferences. Here, we examine diet information obtained using three different assessment methods to better understand the trophic ecology of the European Roller Coracias garrulus, a species targeted by conservation measures in Europe. First, we analysed regurgitated pellets and video‐recordings to report the diet composition of adult and nestling Rollers, respectively. Secondly, we used stable isotope analysis (SIA) to investigate adult sexual diet segregation as well as to confirm the main findings regarding adult and nestling diets obtained through conventional methods. Based on the analysis of pellets, the diet of adult Rollers was dominated by Coleoptera, whereas camera images revealed that the diet of nestlings was dominated by Orthoptera, mainly grasshoppers and bush crickets. Blood isotopic signatures of adult and nestling Rollers confirmed the results obtained through pellet and video‐recording techniques. Of the three methods, pellet analysis provided the most comprehensive trophic information regarding the detectable prey spectrum and prey species contribution, as well as basic diet information for the SIA. Our results also highlight the potential of SIA for assessing intraspecific variation in diet by sampling individuals of known age and sex, which is often unfeasible through conventional approaches. SIA analysis showed no differences in δ¹³C and δ¹⁵N ratios of blood between males and females and a high degree of overlap amongst isotopic niches, suggesting no sex‐specific partitioning in resource use. Overall, we showed that the combination of different methods could be used to gain new and clearer insights into avian trophic ecology that are essential for informing habitat management aiming to improve availability of foraging resources.     

Tracing biogeochemical subsidies from glacier runoff into Alaska's coastal marine food webs

Nearly half of the freshwater discharge into the Gulf of Alaska originates from landscapes draining glacier runoff, but the influence of the influx of riverine organic matter on the trophodynamics of coastal marine food webs is not well understood. We quantified the ecological impact of riverine organic matter subsidies to glacier‐marine habitats by developing a multi‐trophic level Bayesian three‐isotope mixing model. We utilized large gradients in stable (δ¹³C, δ¹⁵N, δ²H) and radiogenic (Δ¹⁴C) isotopes that trace riverine and marine organic matter sources as they are passed from lower to higher trophic levels in glacial‐marine habitats. We also compared isotope ratios between glacial‐marine and more oceanic habitats. Based on isotopic measurements of potential baseline sources, ambient water and tissues of marine consumers, estimates of the riverine organic matter source contribution to upper trophic‐level species including fish and seabirds ranged from 12% to 44%. Variability in resource use among similar taxa corresponded to variation in species distribution and life histories. For example, riverine organic matter assimilation by the glacier‐nesting seabirds Kittlitz's murrelet (Brachyramphus brevirostris) was greater than that of the forest‐nesting marbled murrelet (B. marmoratus). The particulate and dissolved organic carbon in glacial runoff and near surface coastal waters was aged (12100–1500 years BP ¹⁴C‐age) but dissolved inorganic carbon and biota in coastal waters were young (530 years BP ¹⁴C‐age to modern). Thus terrestrial‐derived subsidies in marine food webs were primarily composed of young organic matter sources released from glacier ecosystems and their surrounding watersheds. Stable isotope compositions also revealed a divergence in food web structure between glacial‐marine and oceanic sites. This work demonstrates linkages between terrestrial and marine ecosystems, and facilitates a greater understanding of how climate‐driven changes in freshwater runoff have the potential to alter food web dynamics within coastal marine ecosystems in Alaska.     

New insights into the trophic ecology of the scalloped hammerhead shark,Sphyrna lewini,in the eastern tropical Pacific Ocean

Scalloped hammerhead shark, Sphyrna lewini, is considered a generalist predator, showing individual specializations and different trophic levels. This species inhabits both oceanic islands and coastal nursery areas in the eastern tropical Pacific (ETP) Ocean. Existing trophic ecology studies on S. lewini have been focused on stomach contents. This study provides new insights into its foraging preferences, habitat use, and trophic position, using stable isotope analysis (SIA) of muscle tissue. Stable isotope signatures of δ¹³C and δ¹⁵N were determined in S. lewini muscles (n?=?29) from the Ecuadorian Pacific in 2013. Trophic position (TP), isotopic niches, and overlap of S. lewini were estimated by sex, age, and maturity stages to infer their dietary habits throughout life stages. SIA revealed complex movement patterns related to sex and age classes of S. lewini, highlighting high degrees of dietary plasticity and habitat use, with a stronger relation to coastal regions than previously reported. This study provides crucial information regarding essential areas and the related migration behavior of S. lewini, with important implications for their conservation and management in the ETP.

     

Mesograzer identity,not host algae,determines consumer stable isotope ratios

Species-specific foraging habits or feeding preferences influence the overall trophic functioning in consumer assemblages. We set out to assess the in situ trophic diversity in a mesograzer assemblage (isopods, amphipods and gastropods) associated with marine littoral macroalgae. More specifically, we set out to establish whether stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) are dependent on primary consumer identity or expressed in relation to the identity of the algal host, i.e. the habitat of the consumer. Consumer locations in bivariate isotope space revealed significant differences among grazer taxa, but no effect of the algal host. This suggests that grazer-specific foraging is more important in driving the qualitative range of feeding in the consumer assemblage than host diversity per se within a macrohabitat, such as a bay or a lagoon. The stable isotope ratios of the mesoherbivores were in line with expectations based on the known feeding ecology of the grazers. However, the trophic diversity suggested by stable isotope analysis implies that even an established concept such as the mesograzer guild may encompass simplifications in terms of functional group membership.     

Relationships between growth traits and scale stable isotopes (δ13C, δ15N) of adult chum salmon Oncorhynchus keta in Hokkaido,Japan

Stable isotope analysis (SIA) in combination with growth analysis using scales collected from adult chum salmon Oncorhynchus keta migrating back to Hokkaido, Japan, was performed to describe the variation of isotopic composition of carbon (δ¹³C) and nitrogen (δ¹⁵N) in scales and to examine relationships with growth traits [age, fork length (FL), and relative growth ratio in the last growth period [(RGR^last)]. Scale stable isotope (SI) values in 3‐ to 6‐year‐old fish were highly variable, ranging from ?17.6‰ to ?14.3‰ for δ¹³C and 9.5‰ to 13.4‰ for δ¹⁵N. The δ¹⁵N was positively correlated with FL, and this tendency may indicate changes in trophic level with growth. Significant effect was not detected between δ¹⁵N and RGR^last, it can be inferred that factors potentially yielding high δ¹⁵N may not necessarily result in higher growth rates. No trend found between FL and δ¹³C may imply that there is no clear segregation in feeding locations between the 3‐ to 6‐year groups. This study provided basic information for scale SI values of chum salmon adults and indicated that SIA using scales could be a new approach to elucidating the trophic ecology of chum salmon.     

Carbon and nitrogen isotope fractionation of amino acids in an avian marine predator,the gentoo penguin (Pygoscelis papua)

Compound‐specific stable isotope analysis (CSIA) of amino acids (AA) has rapidly become a powerful tool in studies of food web architecture, resource use, and biogeochemical cycling. However, applications to avian ecology have been limited because no controlled studies have examined the patterns in AA isotope fractionation in birds. We conducted a controlled CSIA feeding experiment on an avian species, the gentoo penguin (Pygoscelis papua), to examine patterns in individual AA carbon and nitrogen stable isotope fractionation between diet (D) and consumer (C) (Δ¹³C_C‐D and Δ¹⁵N_C‐D, respectively). We found that essential AA δ¹³C values and source AA δ¹⁵N values in feathers showed minimal trophic fractionation between diet and consumer, providing independent but complimentary archival proxies for primary producers and nitrogen sources respectively, at the base of food webs supporting penguins. Variations in nonessential AA Δ¹³C_C‐D values reflected differences in macromolecule sources used for biosynthesis (e.g., protein vs. lipids) and provided a metric to assess resource utilization. The avian‐specific nitrogen trophic discrimination factor (TDF_Glu‐Phe = 3.5 ± 0.4‰) that we calculated from the difference in trophic fractionation (Δ¹⁵N_C‐D) of glutamic acid and phenylalanine was significantly lower than the conventional literature value of 7.6‰. Trophic positions of five species of wild penguins calculated using a multi‐TDF_Glu‐Phe equation with the avian‐specific TDF_Glu‐Phe value from our experiment provided estimates that were more ecologically realistic than estimates using a single TDF_Glu‐Phe of 7.6‰ from the previous literature. Our results provide a quantitative, mechanistic framework for the use of CSIA in nonlethal, archival feathers to study the movement and foraging ecology of avian consumers.     

Effects of coexistence on habitat use and trophic ecology of interacting native and invasive amphipods

1. Invasive species in aquatic systems are major drivers of changes in biodiversity. Amphipods are key species in freshwaters, with invasive amphipods either replacing or coexisting with native species and often damaging local biodiversity. However, the consequences of interactions among native and invasive amphipods for their habitat use and feeding ecology and ecosystem function are not yet well understood. 2. We examined a number of streams in Brittany and Northern Ireland, with native and invasive amphipods, to evaluate the consequences of species interactions for both habitat use and diet. Our field studies centred on testing two proposed models: a cohabitation model without competition between two native species (Gammarus pulex vs Echinogammarus berilloni), and a competition model between an invasive and a native species (Gammarus pulex vs Gammarus duebeni celticus). For these three species, alone and in combination, we assessed their habitat use and feeding patterns, the latter through gut contents and stable C and N isotope analyses of their tissues. 3. When existing as single‐species populations, all three species used stream habitats broadly similarly, although G. pulex was more strongly associated with leaf litter and vegetation compared to pebble substrata than the other species. When G. pulex coexisted with either E. berilloni or G. d. celticus, the latter two changed to using all habitats equally, whereas the former retained its habitat preferences. 4. Similarly, all three species when alone had similar gut contents, with inorganic material predominating, followed by leaf and woody material and more rarely algae and invertebrates. When G. pulex coexisted with E. berilloni, the diet of the latter did not change; however, the frequency of inorganic matter, leaves and wood declined in the gut contents of G. pulex. When G. pulex coexisted with G. d. celticus, the pattern of gut contents did not change in either species. 5. When existing as single‐species populations, G. pulex had a broader range of isotopic signatures, both for δ¹³C and for δ¹⁵N, than the two other species, indicating a more variable diet among individuals. When G. pulex coexisted with either E. berilloni or G. d. celticus, the latter two had similar ranges of δ¹³C and δ¹⁵N, whereas for G. pulex the range was much less for δ¹³C and δ¹⁵N, suggesting a less diverse diet. 6. Our results infer two different modes of coexistence between native and non‐native amphipods. We have shown that the native species, which coexist stably, appear to show interference competition, leading to spatial habitat segregation, whereas competition for food and possible intraguild predation by G. pulex on G. d. celticus would explain why the distribution and density of the latter is affected by G. pulex. However, since all the species have a similar diet and feeding habit, we expect no great overall effect on ecosystem processes as a consequence of species interactions and displacements.     

Effects of ethanol storage and lipid and urea extraction on δ15N and δ13C isotope ratios in a benthic elasmobranch,the bluespotted maskray Neotrygon kuhlii

Ethanol storage and lipid and urea extraction had no effect on bluespotted maskray Neotrygon kuhlii muscle δ¹³C values whereas urea‐removal and ethanol storage increased δ¹⁵N values. Results presented here show a significant δ¹⁵N increase post‐urea removal and provide additional support for this approach in future elasmobranch stable‐isotope analysis (SIA) studies. Further experimental work on other elasmobranch species is needed to assess extraction and preservation effects on stable‐isotope (SI) values.     

There is more than one ‘killer shrimp’: trophic positions and predatory abilities of invasive amphipods of Ponto‐Caspian origin

1. Biological invasions are regarded as one of the greatest threats to biological diversity. One of the macroinvertebrate groups with the largest number of invasive species in fresh water are gammarid amphipods. Their omnivorous (including predatory) feeding behaviour may facilitate their spread and establishment in new areas. 2. Dikerogammarus villosus, the ‘killer shrimp’, is a well‐known example of a Ponto‐Caspian gammarid that is a very effective predator and successful coloniser in Europe. There are, however, other invasive Ponto‐Caspian amphipods, which have spread successfully in Northern, Central and Western Europe. Our aim here was to test whether two of such invaders (Pontogammarus robustoides and Dikerogammarus haemobaphes) are also more predacious than a native species (Gammarus fossarum). 3. Stable isotope analysis (δ¹⁵N and δ¹³C) of Ponto‐Caspian amphipods coexisting in a reservoir demonstrated that the trophic positions of P. robustoides and D. haemobaphes were similar to that of D. villosus. Echinogammarus ischnus and Chelicorophium curvispinum occupied the lowest position in the food web, while the native Gammarus fossarum (collected from another waterbody) had an intermediate trophic position. 4. Stomach content analysis of P. robustoides, D. haemobaphes and G. fossarum collected in the field, as well as laboratory feeding experiments, was used to compare diet and feeding preferences among the two invasive and one native species. All three species were omnivorous and predacious. However, the two invasive species (P. robustoides and D. haemobaphes) were more effective predators than G. fossarum and showed a clear preference for animal prey and tissue. 5. Pontogammarus robustoides and D. haemobaphes may, like D. villosus, also be called ‘killer shrimps’ and could have a similar impact as invaders of European freshwater and brackish waterbodies.     

Dual‐tracer‐based isotope turnover rates in a highly invasive mysid Limnomysis benedeni from Lake Constance

Understanding the ecological patterns of invasive species and their habitats require an understanding of the species’ foraging ecology. Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values provide useful information into the study of animal ecology and evolution, since the isotope ratios of consumers reflect consumer's dietary patterns. Nevertheless, the lack of species‐ and element‐specific laboratory‐derived turnover rates could limit their application. Using a laboratory‐based dual stable isotope tracer approach (Na¹⁵NO₃ and NaH¹³CO₃), we evaluated the δ¹⁵N and δ¹³C isotope turnover rates in full‐grown adult invasive Limnomysis benedeni from Lake Constance. We provide δ¹⁵N and δ¹³C turnover rates based on nonlinear least‐squares regression and posterior linear regression models. Model precisions and fit were evaluated using Akaike's information criterion. Within a couple of days, the δ¹⁵N and δ¹³C of mysids began to change. Nevertheless, after about 14 days, L. benedeni did not reach equilibrium with their new isotope values. Since the experiment was conducted on adult subjects, it is evident that turnover was mainly influenced by metabolism (in contrast to growth). Unlike traditional dietary shifts, our laboratory‐based dual stable isotope tracer approach does not shift the experimental organisms into a new diet and avoids dietary effects on isotope values. Results confirm the application of isotopic tracers to label mysid subpopulations and could be used to reflect assimilation and turnover from the labeled dietary sources. Field‐based stable isotope studies often use isotopic mixing models commonly assuming diet‐tissue steady state. Unfortunately, in cases where the isotopic composition of the animal is not in equilibrium with its diet, this can lead to highly misleading conclusions. Thus, our laboratory‐based isotopic incorporation rates assist interpretation of the isotopic values from the field and provide a foundation for future research into using isotopic tracers to investigate invasion ecology.     

Niche overlap and trophic resource partitioning of two sympatric batoids co‐inhabiting an estuarine system in southeast Australia

Elasmobranchs play an important role within the trophic structure of marine ecosystems, but there are relatively few studies published on the feeding ecology of these species. Reported herein is the feeding ecology and trophic resource partitioning of two sympatric batoid species, Urolophus cruciatus and Narcine tasmaniensis from southeast Australia. The diet of males and females of both species was similar, suggesting no sex‐specific dietary preferences. Ontogenetic changes in diet were observed from the diets of both species: as the body size increased, the proportion consumed of crustacea to polychaeta decreased. A relatively high degree of niche overlap (70%) was detected between the trophic resources of the two species. The way in which the predators partitioned the resources, however, was significantly different. U. cruciatus fed predominately on small benthic crustaceans (amphipods and decapods), while N. tasmaniensis displayed a preference towards Maldanidae polychaetes. Therefore, although U. cruciatus and N. tasmaniensis both feed predominately on benthic invertebrates, they specialise on different taxa. This trophic resource partitioning contributes to the biodiversity of the region by facilitating the coexistence of these sympatric species.     

Geochemically distinct carbon isotope distributions in Allochromatium vinosum DSM 180T grown photoautotrophically and photoheterotrophically

Anoxygenic, photosynthetic bacteria are common at redox boundaries. They are of interest in microbial ecology and geosciences through their role in linking the carbon, sulfur, and iron cycles, yet much remains unknown about how their flexible carbon metabolism—permitting either autotrophic or heterotrophic growth—is recorded in the bulk sedimentary and lipid biomarker records. Here, we investigated patterns of carbon isotope fractionation in a model photosynthetic sulfur‐oxidizing bacterium, Allochromatium vinosum DSM180^T. In one treatment, A. vinosum was grown with CO₂ as the sole carbon source, while in a second treatment, it was grown on acetate. Different intracellular isotope patterns were observed for fatty acids, phytol, individual amino acids, intact proteins, and total RNA between the two experiments. Photoautotrophic CO₂ fixation yielded typical isotopic ordering for the lipid biomarkers: δ¹³C values of phytol > n‐alkyl lipids. In contrast, growth on acetate greatly suppressed intracellular isotopic heterogeneity across all molecular classes, except for a marked ¹³C‐depletion in phytol. This caused isotopic “inversion” in the lipids (δ¹³C values of phytol < n‐alkyl lipids). The finding suggests that inverse δ¹³C patterns of n‐alkanes and pristane/phytane in the geologic record may be at least in part a signal for photoheterotrophy. In both experimental scenarios, the relative isotope distributions could be predicted from an isotope flux‐balance model, demonstrating that microbial carbon metabolisms can be interrogated by combining compound‐specific stable isotope analysis with metabolic modeling. Isotopic differences among molecular classes may be a means of fingerprinting microbial carbon metabolism, both in the modern environment and the geologic record.     

The influence of lipid‐extraction and long‐term DMSO preservation on carbon (δ13C) and nitrogen (δ15N) isotope values in cetacean skin

Stable isotope analysis (SIA) has rapidly become a useful tool to study the ecology of wild animal populations, especially for elusive, wide‐ranging predators like marine mammals. The development of projectile biopsy techniques resulted in the collection of thousands of cetacean tissue samples that were archived in a dimethyl sulfoxide (DMSO) solution for long‐term, multidecadal preservation. Here we examine the influence of DMSO preservation on carbon (δ¹³C) and nitrogen (δ¹⁵N) values by comparing a set of paired delphinid skin samples stored frozen without preservative and in DMSO for up to 22 yr. Treatment of paired frozen and DMSO‐preserved skin in a 2:1 chloroform:methanol solution yielded similar δ¹³C and δ¹⁵N values, revealing that DMSO and lipid contamination have similar isotopic effects on skin, and that these effects can be removed using routine lipid‐extraction methods. Further, amino acid concentrations in DMSO‐preserved and frozen skin tissue were similar, providing independent evidence of minimal protein alteration due to preservation. Access to a rich archive of skin samples preserved in DMSO will expand our ability to examine temporal and spatial variability in the isotope values of cetaceans, which will aid our understanding of how their ecology has been influenced by historical changes in environmental conditions.     

Non-lethal sampling for stable isotope analysis of pike Esox lucius: how mucus,scale and fin tissue compare to muscle

Stable isotope analysis (SIA) was used to examine the isotopic relationships between dorsal muscle and fin, scale and epidermal mucus in pike Esox lucius. δ¹³C and δ¹⁵N varied predictably within each tissue pairing, with conversion factors calculated for the surrogate tissues, enabling their application to the non-lethal sampling of E. lucius for SIA.     

Challenges using stable isotopes for estimating trophic levels in marine amphipods

In marine food web studies, stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C) are widely used to estimate organisms’ trophic levels (TL) and carbon sources, respectively. For smaller organisms, whole specimens are commonly analyzed. However, this “bulk method” involves several pitfalls since different tissues may fractionate stable isotopes differently. We compared the δ¹⁵N and δ¹³C values of exoskeleton versus soft tissue, in relation to whole specimens, of three common Arctic amphipods in Svalbard waters: the benthic Anonyx nugax, the sympagic (ice-associated) Gammarus wilkitzkii and the pelagic Themisto libellula. The δ¹⁵N values of the exoskeletons were significantly lower than those of the soft tissues for A. nugax (10.5 ± 0.7‰ vs. 15.7 ± 0.7‰), G. wilkitzkii (3.3 ± 0.3‰ vs. 8.3 ± 0.4‰) and T. libellula (8.6 ± 0.3‰ vs.10.8 ± 0.3‰). The differences in δ¹³C values between exoskeletons and soft tissues were insignificant, except for A. nugax (−21.2 ± 0.2‰ vs. −20.3 ± 0.2‰, respectively). The δ¹⁵N values of whole organisms were between those of the exoskeletons and the soft tissues, being similarly enriched in ¹⁵N as the exoskeletons (except G. wilkitzkii) and depleted in ¹⁵N by 1.2–3.7‰ compared to the soft tissues. The δ¹⁵N-derived TLs of the soft tissues agreed best with the known feeding preferences of the three amphipods, which suggest a potential underestimation of 0.5–1.0 TL when stable isotope analyses are performed on whole crustaceans with thick exoskeletons. The insignificant or small differences in δ¹³C values among exoskeletons, soft tissues and whole specimens, however, suggest low probability for misinterpretations of crustaceans’ primary carbon source in marine ecosystems with distinctly different δ¹³C-carbon sources.     

Stable isotope records of sei whale baleens from Chilean Patagonia as archives for feeding and migration behavior

Carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope variations in baleen plates of sei whales (Balaenoptera borealis) stranded after a mass mortality event in Chilean Patagonia were investigated to assess potential dietary and migratory patterns. Carbon and nitrogen isotope ratios of seven baleens from six individuals were analyzed. The δ¹³C values ranged from ? 19.1 to ? 15.9‰ and the δ¹⁵N values from 8.7 to 15.4‰. Variations of up to 2.9‰ for δ¹³C and 5.3‰ for δ¹⁵N were observed within one baleen. Carbon and nitrogen isotope records of each baleen were significantly correlated and showed recurring oscillations confirmed by wavelet analyses. Oscillations slightly differed in periodicity indicating variable baleen growth rates between 10.0 and 16.5 cm/year. Food sources of the whales are discussed in terms of available isotope data for potential prey taxa and potential migratory behavior on the basis of latitudinal isotope gradients of particulate organic matter. Cyclicity could be explained by regular migrations of the sei whales from subtropical calving areas to high‐latitude foraging grounds. δ¹⁵N records of baleens differed between individuals eventually pointing to diverse feeding and migratory preferences among sei whale individuals.     

Using Bayesian stable isotope mixing models to estimate wolf diet in a multi-prey ecosystem

Stable isotope analysis (SIA) of wolf (Canis lupus) tissues can be used to estimate diet and intra-population diet variability when potential prey have distinct δ¹³C and δ¹⁵N values. We tested this technique using guard hairs collected from 44 wolves in 12 northwestern Montana packs, summer 2009. We used hierarchical Bayesian stable isotope mixing models to determine diet and scales of diet variation from δ¹³C and δ¹⁵N of wolves and potential prey, white-tailed deer (Odocoileus virginianus), mule deer (Odocoileus hemionus), elk (Cervus canadensis), moose (Alces alces), snowshoe hare (Lepus americanus), and other prey. As a check on SIA results, we conducted a separate diet analysis with temporally matched scats (i.e., collected in summer 2008) from 4 of the same packs. Wolves were centered on the ungulate prey in the isotope mixing space. Both methods revealed differences among pack diets and that wolves may consume moose in greater proportions than predicted by available biomass. Stable isotope analysis, and scat results were not entirely concordant; assumptions related to tissues of use in SIA, hair growth period in wolves, and scat sampling may have contributed to a mismatch between methods. Incorrect fractionation values, insufficient separation of prey in the isotope mixing space, choice of prior information in the Bayesian mixing models, and unexplained factors may have distorted diet estimates. However, the consistently high proportion of moose in pack diets suggests that increased population monitoring would benefit management of moose and wolves. Our results also support suggestions of other researchers that species-specific fractionation values should be used whenever possible, and that SIA may sometimes only provide indices of use for general groups of prey (e.g., large ungulates). © 2012 The Wildlife Society.