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.     

Chytrids enhance Daphnia fitness by selectively retained chytrid-synthesised stearidonic acid and conversion of short-chain to long-chain polyunsaturated fatty acids

1. Chytrid fungal parasites convert dietary energy and essential dietary molecules, such as long-chain (LC) polyunsaturated fatty acids (PUFA), from inedible algal/cyanobacteria hosts into edible zoospores. How the improved biochemical PUFA composition of chytrid-infected diet may extend to zooplankton, linking diet quality to consumer fitness, remains unexplored.
2. Here, we assessed the trophic role of chytrids in supporting dietary energy and PUFA requirements of the crustacean zooplankton Daphnia, when feeding on the filamentous cyanobacterium Planktothrix.
3. Only Daphnia feeding on chytrid-infected Planktothrix reproduced successfully and had significantly higher survival and growth rates compared with Daphnia feeding on the sole Planktothrix diet. While the presence of chytrids resulted in a two-fold increase of carbon ingested by Daphnia, carbon assimilation increased by a factor of four, clearly indicating enhanced carbon transfer efficiency with chytrid presence.
4. Bulk carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes did not indicate any treatment-specific dietary effects on Daphnia, nor differences in trophic position among diet sources and the consumer. Compound-specific carbon isotopes of fatty acids (δ¹³C_FA), however, revealed that chytrids bioconverted short-chain to LC-PUFA, making it available for Daphnia. Chytrids synthesised the ω-3 PUFA stearidonic acid de novo, which was selectively retained by Daphnia. Values of δ¹³C_FA demonstrated that Daphnia also bioconverted short-chain to LC-PUFA.
5. We provide isotopic evidence that chytrids improved the dietary provision of LC-PUFA for Daphnia and enhanced their fitness. We argue for the existence of a positive feedback loop between enhanced Daphnia growth and herbivory in response to chytrid-mediated improved diet quality. Chytrids upgrade carbon from the primary producer and facilitate energy and PUFA transfer to primary consumers, potentially also benefitting upper trophic levels of pelagic food webs.

     

Experimentally Derived δ13C and δ15N Discrimination Factors for Gray Wolves and the Impact of Prior Information in Bayesian Mixing Models

Stable isotope analysis of diet has become a common tool in conservation research. However, the multiple sources of uncertainty inherent in this analysis framework involve consequences that have not been thoroughly addressed. Uncertainty arises from the choice of trophic discrimination factors, and for Bayesian stable isotope mixing models (SIMMs), the specification of prior information; the combined effect of these aspects has not been explicitly tested. We used a captive feeding study of gray wolves (Canis lupus) to determine the first experimentally-derived trophic discrimination factors of C and N for this large carnivore of broad conservation interest. Using the estimated diet in our controlled system and data from a published study on wild wolves and their prey in Montana, USA, we then investigated the simultaneous effect of discrimination factors and prior information on diet reconstruction with Bayesian SIMMs. Discrimination factors for gray wolves and their prey were 1.97‰ for δ¹³C and 3.04‰ for δ¹⁵N. Specifying wolf discrimination factors, as opposed to the commonly used red fox (Vulpes vulpes) factors, made little practical difference to estimates of wolf diet, but prior information had a strong effect on bias, precision, and accuracy of posterior estimates. Without specifying prior information in our Bayesian SIMM, it was not possible to produce SIMM posteriors statistically similar to the estimated diet in our controlled study or the diet of wild wolves. Our study demonstrates the critical effect of prior information on estimates of animal diets using Bayesian SIMMs, and suggests species-specific trophic discrimination factors are of secondary importance. When using stable isotope analysis to inform conservation decisions researchers should understand the limits of their data. It may be difficult to obtain useful information from SIMMs if informative priors are omitted and species-specific discrimination factors are unavailable.     

Linking intraspecific variability in trophic and functional niches along an environmental gradient

1. Intraspecific trophic variability has important ecological and evolutionary implications, and is driven by multiple interacting factors. Functional traits and environmental conditions are important in mediating the trophic niche of individuals because they determine their ability to consume certain prey, their energetic requirements, and resource availability. In this study, we aimed at investigating the interacting effects of functional traits and environmental conditions on several attributes of trophic niche in natural populations.
2. Here, we quantified intraspecific variability in the trophic niche of 12 riverine populations of European minnow (Phoxinus phoxinus) using stable isotope analyses. Functional traits (i.e. morpho-anatomical traits) and environmental conditions (i.e. upstream–downstream gradient, forest cover) were quantified to identify the determinants of (1) trophic position and resource origin, (2) trophic niche size, and (3) trophic differentiation (β-diversity) among populations.
3. We demonstrated that trophic position and resource origin covaried with functional traits related to body size and locomotion performance, and that the strength and shape of these relationships varied according to local environmental conditions. The trophic niche size also differed among populations, although no determinant was identified. Finally, trophic β-diversity was correlated to environmental differentiation among sites.
4. Overall, the determinants of intraspecific variability in trophic niche appeared highly context-dependent, and related to the interactions between functional traits and environmental conditions. Because populations are currently facing important environmental changes, understanding this context-dependency is important for predicting food web structure and ecosystem dynamics in a changing world.

     

How protein quality drives incorporation rates and trophic discrimination of carbon and nitrogen stable isotope ratios in a freshwater first-feeding fish

1. Using stable isotope ratios to explore the trophic ecology of freshwater animals requires knowledge about effects of food quality on isotopic incorporation dynamics. The aim of this experimental study was to: (1) estimate carbon and nitrogen isotopic incorporation rates and trophic discrimination factors (TDFs) of a freshwater first-feeding fish (i.e. salmonid fry) fed three diets that differed only in protein quality (animal or plant or a blend of both); (2) investigate effects of fasting and; (3) evaluate the proportion of each source assimilated when fry were fed a 50:50 animal:plant-based diet.
2. For each diet, incorporation rates of δ¹³C and δ¹⁵N values were estimated using a time or growth-dependent isotopic incorporation model. Effects of fasting on isotope ratio values were measured regularly until the death of fry. Bayesian stable-isotope mixing models were used to estimate the contribution of animal and plant material to fish fed a blend of both food types.
3. Our results show that incorporation rates were lower for fry fed a plant-based diet than for those fed an animal-based diet as growth rate decreased. Time- and growth-dependent models indicated that growth was solely responsible for isotopic incorporation in fry fed an animal-based diet, whereas catabolism increased in fry fed a plant-based diet. After lipid extraction, carbon TDFs were similar regardless of the diet, whereas nitrogen TDFs increased for fry fed a plant-based diet. Long-term fasting induced an increase of 0.63‰ in δ¹³C values of fry in 23 days, whereas δ¹⁵N values did not vary significantly. Proportions of food sources assimilated by fry fed an animal:plant-based diet were similar to those consumed when using a mixing model with the estimated TDFs, while proportions were unrealistic when using mean TDFs extrapolated from the literature.
4. The results of our study indicate that the quality of food must be considered to use an appropriate timescale to detect changes in fry diets in the field. Moreover, we recommend using different carbon and nitrogen TDFs, one for animal-derived sources and one for plant-derived sources, to increase the accuracy of mixing models.

     

Compound‐specific 15N analysis of amino acids: A tool to estimate the trophic position of tropical seabirds in the South China Sea

Compound‐specific ¹⁵N analysis of amino acids (AAs) is a powerful tool to determine the trophic position (TP) of organisms. However, it has only been used in a few studies of avian ecology because the AA patterns in the consumer‐diet nitrogen trophic discrimination factor (TDF_Glu‐Phe = ?¹⁵N_Glu??¹⁵N_Phe) were unknown in birds until recently, and tropical seabirds have never been investigated with this methodology. Here, we explore the application of this method to tropical seabirds. In this study, we recovered the fossilized bones of tropical seabirds from ornithogenic sediments on two coral islands in the Xisha Islands, South China Sea, as well as the bones and muscle of their predominant food source, flying fish (Exocoetus volitans). Compound‐specific ¹⁵N and ¹³C analyses of AAs in both seabird and fish bone collagen were conducted. The TP of flying fish was calculated based on a widely used single TDF_Glu‐Phe approach. We then calculated the TP of tropical seabirds in three different ways: (a) according to the composition of their diet; (b) based on the single TDF_Glu‐Phe approach; and (c) using a multi‐TDF_Glu‐Phe approach. The results of the multi‐TDF_Glu‐Phe approach were much closer to the results based on the composition of the seabird diet than the results of the single TDF_Glu‐Phe approach, confirming its applicability for tropical seabirds. For seabird bone samples of different ages, TP determined from the multi‐TDF_Glu‐Phe approach was most similar to that of bulk δ¹⁵N of bird collagen, with seabirds occupying higher TPs during the Little Ice Age, as previously shown. In addition, the ¹³C Suess effect was reflected in the AAs δ¹³C in our samples. This study applied a compound‐specific ¹⁵N analysis of AAs to determine the TP of tropical seabirds that has potential to extend to all tropical seabirds many of which are widely distributed and play a key role in the evolution of coral island ecosystems.     

Are you what you eat? Effects of trophic discrimination factors on estimates of food assimilation and trophic position with a new estimation method

A key factor for estimates of assimilation of resources and trophic position based on stable isotope data is the trophic discrimination factor (TDF). TDFs are assumed based on literature reviews, but may vary depending on a variety of factors, including the type of diet. We analyzed effects of alternative TDFs on estimates of assimilated resources and trophic positions for an omnivorous fish, Jenynsia multidentata, that reveals dietary variation among locations across a salinity gradient of a coastal lagoon in southern Brazil. We also compared estimates of foods ingested vs. foods assimilated. Food assimilation was estimated using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios of food sources and consumer muscle tissue and an isotopic mixing model (SIAR); consumer trophic position (TP) was estimated from consumer and production source δ¹⁵N values. Diet was estimated using an index of relative importance based on frequency of occurrence and volumetric and numeric proportions of food items from stomach contents. The effect of variation in TDF on food assimilation and TP was tested using three alternative TDFs reported in review papers. We then created a new method that used food source-specific TDFs (reported separately for herbivores and carnivores) weighted in proportion to estimated assimilation of resources according to mixing model estimates to estimate TP (hereafter TP_WAR). We found that plant material was not assimilated in a proportion similar to its importance in the diet of fish at a freshwater site, and the new method yielded best assimilation estimates. Animal material made greatest contributions to fish biomass irrespective of TDFs used in the mixing model. The new method produced TP estimates consistent with differences in estimated food assimilation along the salinity gradient. Our findings support the idea that food source-specific TDFs should be used in trophic studies of omnivores, since the method improved our ability to estimate trophic position and resource assimilation, two important ecological indicators.     

Estimating tissue‐specific discrimination factors and turnover rates of stable isotopes of nitrogen and carbon in the smallnose fanskate Sympterygia bonapartii (Rajidae)

This study aimed to estimate trophic discrimination factors (TDFs) and metabolic turnover rates of nitrogen and carbon stable isotopes in blood and muscle of the smallnose fanskate Sympterygia bonapartii by feeding six adult individuals, maintained in captivity, with a constant diet for 365 days. TDFs were estimated as the difference between δ¹³C or δ¹⁵N values of the food and the tissues of S. bonapartii after they had reached equilibrium with their diet. The duration of the experiment was enough to reach the equilibrium condition in blood for both elements (estimated time to reach 95% of turnover: C t95%_blood = 150 days, N t95%_blood = 290 days), whilst turnover rates could not be estimated for muscle because of variation among samples. Estimates of Δ¹³C and Δ¹⁵N values in blood and muscle using all individuals were Δ¹³C_blood = 1·7‰, Δ¹³C_muscle = 1·3‰, Δ¹⁵N_blood = 2·5‰ and Δ¹⁵N_muscle = 1·5‰, but there was evidence of differences of c.0·4‰ in the Δ¹³C values between sexes. The present values for TDFs and turnover rates constitute the first evidence for dietary switching in batoids based on long‐term controlled feeding experiments. Overall, the results showed that S. bonapartii has relatively low turnover rates and isotopic measurements would not track seasonal movements adequately. The estimated Δ¹³C values in S. bonapartii blood and muscle were similar to previous estimations for elasmobranchs and to generally accepted values in bony fishes (Δ¹³C = 1·5‰). For Δ¹⁵N, the results were similar to published reports for blood but smaller than reports for muscle and notably smaller than the typical values used to estimate trophic position (Δ¹⁵N c. 3·4‰). Thus, trophic position estimations for elasmobranchs based on typical Δ¹⁵N values could lead to underestimates of actual trophic positions. Finally, the evidence of differences in TDFs between sexes reveals a need for more targeted research.     

Amino acid δ15N underestimation of cetacean trophic positions highlights limited understanding of isotopic fractionation in higher marine consumers

Compound‐specific stable isotope analysis (CSIA) of amino acids (AAs) has been rapidly incorporated in ecological studies to resolve consumer trophic position (TP). Differential ¹⁵N fractionation of “trophic” AAs, which undergo trophic ¹⁵N enrichment, and “source” AAs, which undergo minimal trophic ¹⁵N enrichment and serve as a proxy for primary producer δ¹⁵N values, allows for internal calibration of TP. Recent studies, however, have shown the difference between source and trophic AA δ¹⁵N values in higher marine consumers is less than predicted from empirical studies of invertebrates and fish. To evaluate CSIA‐AA for estimating TP of cetaceans, we compared source and trophic AA δ¹⁵N values of multiple tissues (skin, baleen, and dentine collagen) from five species representing a range of TPs: bowhead whales, beluga whales, short‐beaked common dolphins, sperm whales, and fish‐eating (FE) and marine mammal‐eating (MME) killer whale ecotypes. TP estimates (TP_CSIA) using several empirically derived equations and trophic discrimination factors (TDFs) were 1–2.5 trophic steps lower than stomach content‐derived estimates (TP_SC) for all species. Although TP_CSIA estimates using dual TDF equations were in better agreement with TP_SC estimates, our data do not support the application of universal or currently available dual TDFs to estimate cetacean TPs. Discrepancies were not simply due to inaccurate TDFs, however, because the difference between consumer glutamic acid/glutamine (Glx) and phenylalanine (Phe) δ¹⁵N values (δ¹⁵N_Glx‐Phe) did not follow expected TP order. In contrast to pioneering studies on invertebrates and fish, our data suggest trophic ¹⁵N enrichment of Phe is not negligible and should be examined among the potential mechanisms driving “compressed” and variable δ¹⁵N_Glx‐Phe values at high TPs. We emphasize the need for controlled diet studies to understand mechanisms driving AA‐specific isotopic fractionation before widespread application of CSIA‐AA in ecological studies of cetaceans and other marine consumers.     

Trophic response to ecological conditions of habitats: Evidence from trophic variability of freshwater fish

1. To adapt to ecological and environmental conditions, species can change their ecological niche (e.g., interactions among species) and function (e.g., prey‐predation, diet competition, and habitat segregation) at the species and guild levels. Stable isotope analysis of bulk carbon and nitrogen of organisms has conventionally been used to evaluate such adaptabilities in the scenopoetic and bionomic views as the isotopic niche width.
2. Compound‐specific stable isotope analysis (CSIA) of nitrogen within amino acids provides trophic information without any disruption of scenopoetic views in the isotope ratios, unlike conventional bulk isotope analysis provides both information and therefore frequently hinders its usefulness for trophic information.
3. We performed CSIA of amino acids to understand the trophic variability of the pike gudgeon Pseudogobio esocinus and largemouth bass Micropterus salmoides as representative specialist and generalist fish species, respectively, from 16 ecologically variable habitats in the four major rivers of Korea.
4. There was little variation (1σ) in the trophic position (TP) among habitats for P. esocinus (± 0.2); however, there was considerably large variation for M. salmoides (± 0.6). The TP of M. salmoides was negatively correlated with the benthic invertebrate indices of the habitats, whereas the TP of P. esocinus showed no significant correlation with any indices. Thus, these two representative fish species have different trophic responses to ecological conditions, which is related to known differences in the trophic niche between specialists (i.e., small niche width) and generalists (i.e., large niche width).
5. Over the past four decades, the conventional bulk isotope analysis has not been capable of deconvoluting “scenopoetic” and “bionomic” information. However, in the present study, we demonstrated that the CSIA of amino acids could isolate trophic niches from the traditional ecological niche composed of trophic and habitat information and evaluated how biological and ecological indices influence the trophic response of specialists and generalists.

     

The importance of quantifying inherent variability when interpreting stable isotope field data

Stable isotope data are often used to assess diet, trophic level, trophic niche width and the extent of omnivory. Notwithstanding ongoing discussions about the value of these approaches, variations in isotopic signatures among individuals depend on inherent variability as well as differences in feeding habitats. Remarkably, the relative contributions of diet variation and inherent variability to differences in δ¹⁵N and δ¹³C among individuals have not been quantified for the same species at the same life history stages, and inherent variability has been ignored or assumed. We quantified inherent variability in δ¹³C and δ¹⁵N among individuals of a marine fish (the European sea bass, Dicentrarchus labrax) reared in a controlled environment on a diet of constant isotopic composition and compared it with variability in δ¹³C and δ¹⁵N among individuals from wild bass populations. The analysis showed that inherent variability among reared individuals on a controlled diet was equivalent to a large proportion of the observed variability among wild individuals and, therefore, that inherent variability should be measured to establish baseline variability in wild populations before any assumptions are made about the influence of diet. Given that inherent variability is known to be dependent on species, life history stage and the environment, our results show that it should be quantified on a case-by-case basis if diet studies are intended to provide absolute assessments of dietary habits.     

Isotopic marking of natural enemies fed on C4 honey for habitat management studies in agroecosystems

1. To improve biological control and habitat management, how pest predators spread from natural habitats to crops must be understood. We studied whether intrinsic differences in stable-isotopic ratios of C and N from an artificial C₄ (sugar-cane) or C₃ (muti-flower honey) diet could help mark and track predators that feed on them.
2. Two aphid predators solely feeding on sugar resources as adults were used: the green lacewing Chrysoperla carnea s.l., and the parasitic wasp Aphidius colemani. δ¹³C and δ1⁵N values from wild individuals helped distinguish them from marked ones and determine the habitat resources they used.
3. Green lacewings fed on C₄ showed significantly higher δ¹³C values than those on C3 and wild individuals. However, parasitoid values were unaffected, with no mark acquired. Logistic regression was fitted to assess the probability of green lacewings having either diet with a probability of 0.93. Marks were acquired after 5 days and were detectable 20 days after switching diets with a probability of 0.67. Similar δ¹⁵N values for both wild populations indicated both predators had similar prey, but different vegetal resources (different δ¹³C values).
4. Sugar-cane honey is a natural and reliable marker for tracking lacewing populations in the field, but not for parasitoids.

     

Host range determination in a novel outbreak pest of sugarcane,Cacosceles newmannii (Coleoptera: Cerambycidae,Prioninae), inferred from stable isotopes

1. An outbreak of Cacosceles newmannii (Coleoptera: Cerambycidae) was detected for the first time on sugarcane (Saccharum spp.) in 2015 in KwaZulu-Natal, South Africa. Although primary host plants of this native species remain unknown, these are central to testing hypotheses concerning the outbreak.
2. We hypothesized that this species has undergone a host plant shift (i.e. a feeding association with a novel host plant).
3. We compared δ¹³C and δ¹⁵N ratios of adult beetles retrieved from South African museum collections, collected between 1891 and 2016 (n = 23; ‘pre-outbreak’), with samples from infested fields in 2017 (n = 9, ‘post-outbreak’) and in 2019 (n = 23, ‘post-outbreak’), as well as diverse, plausible host plants (n = 42 samples across 10 species) from infested fields and surrounding patches of indigenous and commercial forest vegetation. We used Bayesian isotope mixing models to infer the relative contribution of the different plants to the diet of C. newmannii.
4. Pre-outbreak, C₃ plants contributed strongly to the larval diet, whereas post-outbreak, C₄ plants were the largest component of their diet. There was some indication of C₄ plants contributing to their diet pre-outbreak.
5. Our results suggest that the outbreak of this polyphagous beetle was not a dramatic host shift but rather a rapid increase in the proportion of C₄ plants already in their diet.
6. We concluded that plants from the families Fabaceae and Poaceae are the most likely host plants of this species. Nevertheless, the drivers of this rapid outbreak on sugarcane remain poorly determined and should be the focus of future research.

     

Behavioural plasticity and trophic niche shift: How wintering geese respond to habitat alteration

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Beyond ecological opportunity: Prey diversity rather than abundance shapes predator niche variation

1. Ecological opportunity (i.e. the diversity of available resources) has a pivotal role in shaping niche variation and trophic specialisation of animals. However, ecological opportunity can be described with regard to both diversity and abundance of resources. The degree to which these two components contribute to niche variation remains unexplored.
2. To address this, we used an extensive dataset on fish diet and benthic invertebrate diversity and density from 73 sampling events in three Norwegian rivers in order to explore realised trophic niches and the response of dietary niche variation along gradients of resource diversity (potential trophic niches), resource density (as a proxy of resource abundance) and fish density (as a proxy of inter‐ and intra‐specific competition) in a freshwater top predator (the brown trout, Salmo trutta L.).
3. Linear models indicated that individual and population niche variation increased with increasing ecological opportunity in terms of prey diversity. However, no simple cause‐and‐effect associations between niche indices and prey abundance were found. Our multiple regression analyses indicated that the abundance of certain resources (e.g. Chironomidae) can interact with prey diversity to determine individual and population realised trophic niches. Niche variation (within‐individual component and inter‐individual diet variation) decreased with increasing inter‐ and intra‐specific competition.
4. This study extends prevailing trophic ecology theory by identifying diversity, rather than density, of available prey resources as a primary driver of niche variation in fish of temperate riverine systems with no extensive resource limitation. The study also shows that ecological opportunity may mask the direction of the effect (compression or expansion) of competition on niche variation when food resources are diverse.
5. Our study provides novel empirical insight to the driving forces behind niche variation and reveals that diversity, rather than density, of available prey resources may be a primary driver of niche variation in freshwater fish. Our study supports the view that a broader potential trophic niche promotes broader realised trophic niche variation by individuals, which leads to individual niche diversification by opening access to alternatives resources, resulting in a concomitant rise in the realised trophic niche width of the population.

     

The role of the southern water vole Arvicola sapidus in the diet of predators: a review

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Food web pathways for fish communities in small tropical streams

1. It is often assumed that invertebrate consumers in small tropical streams are dependent on allochthonous sources, although recent studies indicate that algae can form the base of food webs in tropical streams. Fish in tropical streams can feed across several trophic levels and the origin and path of energy and nutrient flow is uncertain for many species.
2. We collected fish, insects, periphyton, and leaf litter from 20 streams across four Atlantic Forest catchments. We analysed stomach contents of fish to define trophic guild and fish dietary trophic position. We also analysed stable isotopes of carbon and nitrogen of fish and their resources to identify the main basal resources of the food web and to estimate trophic positions and identify the path of energy flow.
3. We found that autochthonous sources were the primary resource base for fish communities. Trophic positions estimated from diet and isotopes were similar and correlated for insectivore and algivore–insectivore fish, but not for algivore–detritivore or omnivore fish. Using path analysis, fish classified as algivore–detritivores appear to have derived their biomass through a diet of primary consumer insects and periphytic algae and thus, are more likely to play a trophic role as algivore–insectivores in these streams. However, omnivores probably derived much of their biomass from aquatic insects.
4. Our findings support other studies of tropical systems in which the main basal resource is autochthonous, even in small streams. We also show that the assignment to a specific trophic guild for some fish species, based on gut contents, does not reflect what they assimilate into their bodies. In some species, food sources that are uncommon can make a disproportionately important contribution to their biomass.
5. This study affirms the important role of inconspicuous algal resources in aquatic food webs, even in small forested streams, and demonstrates the effectiveness of taking a combined approach of diet analysis, isotopic tracing, and modelling to resolve food web pathways where the level of omnivory is high.

     

Nutritional ecology provides insights into competitive interactions between closely related Martes species

1. Closely related predator species often share several prey items, making it hard to differentiate the effects on their feeding habits of variation in food availability and of competition. We hypothesised that we could overcome this obstacle by quantifying and comparing nutritional niches.
2. We reviewed dietary studies that assessed the relative bulk of each food item, as either per cent biomass or per cent mean volume, in the diet of two closely related species, pine marten Martes martes and stone marten Martes foina, and calculated the nutrient profiles (intakes of protein, lipids and carbohydrates) of each diet.
3. Both martens’ diets were tightly clustered (mean values: 47% of energy from protein, 39% from lipid, and 14% from carbohydrate). In allopatry, the nutritional niches of the two species did not differ, but in sympatry, the stone marten ate more carbohydrates and less protein than the pine marten. In allopatry, the protein intake of the stone marten remained high (45–52%) in very different habitats, from cultivated lowland to Alpine forests.
4. Our data suggest that stone marten frugivory may, at least partially, be the result of interspecific competition. By analysing dietary data in the framework of nutritional ecology, we could compare the feeding requirements of pine martens and stone martens more effectively than by using classical estimates of trophic niche overlap at the food item level. This approach may help to shed light on the trophic relationships of other competing species.

     

Quantitative evaluation of marine protein contribution in ancient diets based on nitrogen isotope ratios of individual amino acids in bone collagen: An investigation at the Kitakogane Jomon site

Nitrogen stable isotopes analysis of individual bone collagen amino acids was applied to archeological samples as a new tool for assessing the composition of ancient human diets and calibrating radiocarbon dates. We used this technique to investigate human and faunal samples from the Kitakogane shell midden in Hokkaido, Japan (5,300–6,000 cal BP). Using compound‐specific nitrogen isotope analysis of individual amino acids, we aimed to estimate i) the quantitative contribution of marine and terrestrial protein to the human diet, and ii) the mean trophic level (TL) from which dietary protein was derived from marine ecosystems. Data were interpreted with reference to the amino acid trophic level (TL_AA) model, which uses empirical amino acid δ¹⁵N from modern marine fauna to construct mathematical equations that predict the trophic position of organisms. The TL_AA model produced realistic TL estimates for the Kitakogane marine animals. However, this model was not appropriate for the interpretation of human amino acid δ¹⁵N, as dietary protein is derived from both marine and terrestrial environments. Hence, we developed a series of relevant equations that considered the consumption of dietary resources from both ecosystems. Using these equations, the mean percentage of marine protein in the Kitakogane human diet was estimated to be 74%. Although this study is one of the first systematic investigations of amino acid δ¹⁵N in archeological bone collagen, we believe that this technique is extremely useful for TL reconstruction, palaeodietary interpretation, and the correction of marine reservoir effects for radiocarbon dating. Am J Phys Anthropol 143:31–40, 2010. © 2010 Wiley‐Liss, Inc.