Differences In Stable Isotope Composition Research Articles

Social, spatial and isotopic niche partitioning identify an estuarine community of bottlenose dolphins as a discrete management unit

Abstract Investigations of population structure across multiple niche dimensions can identify discrete management units within populations. This study examined social, spatial and isotopic niche partitioning in Indo‐Pacific bottlenose dolphins (Tursiops aduncus) across ca. 600 km2 of coastal and estuarine waters in south‐western Australia, to evaluate whether estuarine dolphins should be treated as a discrete management unit. Photo‐identification data and tissue samples were collected in 2016 and 2017 in a study area covering the Peel‐Harvey Estuary (PHE) and adjacent coastal waters. A total of 1,038 dolphin groups were encountered, and 481 individuals were identified. Tissue samples for stable isotope (δ13C and δ15N) analyses were collected from 96 individuals. Social structure and complexity analyses were conducted, full and core activity spaces were identified, and their size estimated for identified social communities. Differences in stable isotope composition among individuals and communities were examined. A socially, spatially and isotopically distinct dolphin community occurred in the PHE. The coastal waters contained four socially and spatially, but not isotopically, distinct communities as well as a substantial number of dolphins (n = 185) that were sighted infrequently and therefore were not assigned to any community. Individuals formed three levels of relationships; the majority (78%) were weak association relationships (mean half‐weight index 0.006). The estuarine community had significantly higher mean δ13C and significantly lower mean δ15N values than any of the coastal communities. There is a strong scientific basis for treating the PHE dolphin community as a discrete management unit. The estuarine and coastal communities occupied different social environments, with coastal individuals sharing space with more transient individuals. This study shows the value of integrating information from multiple niche dimensions when identifying management units, and the need to consider all encountered individuals in management planning.

Discrete Spawning Aggregations of the Loliginid Squid Doryteuthis gahi Reveal Life-History Interactions of a Dwarf Morphotype at the Center of Its Distribution Range

Heterogeneous environmental conditions along the Humboldt Current System (HCS) influence the life-history strategy of a variety of species in different ways. There is limited information on latitudinal traits of coastal cephalopods as part of the interacting species in pelagic and benthic environments. The present study used the loliginid squidDoryteuthis gahias a model organism to: (1) evaluate latitudinal traits on egg laying patterns, (2) characterize a particular spawning ground, (3) quantify the isotopic variation from different tissues, (4) evaluate potential trophic ontogenetic changes, (5) determinate trophic position, and (6) isotopically estimate the relative importance of putative preys in the squid’s assimilated diet. Results evidenced that egg-masses collected between 2014 and 2020 presented similar patterns along northern-central Chile (27–36°S), with females attaching small egg-capsules (10–50 mm length) through the year and over a variety of anthropogenic and natural substrates. At a small scale (Coquimbo; 29°S), early life history traits showed distinct patterns depending on SST, with warmer collection periods (∼18°C) evidencing larger capsules and smaller embryos, although small paralarvae were obtained over the 4-year sampling period. In this site, sampling of older ontogenetic stages supported the constant presence of small-sized squids (19–77 mm ML). Males had larger mean sizes compared to females and undetermined specimens, with a high proportion of mature stages. Observations in captivity were extended for up to 110 d, validating that small females (45–64 mm ML) spawn the small egg-capsules typically observed in the field. Differences in stable isotope composition between beaks and soft tissues of adult squids were lower for δ13C values (1.1 vs. 1.4‰, respectively) and higher for δ15N values (5.3 vs. 4.7‰, respectively). Isotopic composition through ontogeny found similar δ13C and δ15N values, suggesting that carbon sources (pelagic origin) and trophic position did not change significantly, with copepods, euphausiids and nereid polychaetes being the most important preys. This study unveils the permanent occurrence of a small reproductive morphotype ofD. gahiin shallow coastal habitats of northern-central Chile (i.e., center of the distribution range), providing the first insights for understanding the species’ potential adaptations to heterogeneous conditions in the HCS and the unexplored distribution gap between the two centers of abundance (Peru and the Falkland/Malvinas islands).

Isotopic discrimination between carrion and elytra clippings of lab-reared American burying beetles (Nicrophorus americanus): Implications for conservation and evaluation of feeding relationships in the wild.

Differences in stable isotope composition between an animal and its diet are quantified by experimentally derived diet-tissue discrimination factors. Appropriate discrimination factors between consumers and prey are essential for interpreting stable isotope patterns in ecological studies. While available for many taxa, these values are rarely estimated for organisms within the carrion food web. We used a controlled-diet stable isotope feeding trial to quantify isotopic diet-tissue discrimination factors of carbon (δ13 C values) and nitrogen (δ15 N values) from laboratory-reared Nicrophorus americanus raised on carrion. We used exoskeleton samples of beetle elytra (wing covers) to determine diet-tissue discrimination factors using a continuous flow isotope ratio mass spectrometer equipped with an elemental analyzer. We also measured the isotopic compositions of five species of co-occurring, wild-caught burying beetles and evaluated feeding relationships. We found differences in stable carbon discrimination between carrion sources (mammalian and avian) and lab-reared beetles, but no difference in stable nitrogen discrimination. Values for δ13 C did not differ among wild-caught burying beetle species, but values for δ15 N were significantly different for the three species with overlapping breeding seasons. Furthermore, wild-caught burying beetles within our study area do not appear to use avian carrion resources to rear their young. This study informs future interpretation of stable isotope data for insects within the carrion food web. In addition, these results provide insight into carrion resources used by co-occurring burying beetle species in situ. We also demonstrated that independent of adult food type, the larval food source has a significant impact on the isotopic signatures of adult beetles, which can be estimated using a minimally invasive elytra clipping.

Partitioning of oxygen isotopes during the aqueous solvation of nitric acid

Mixtures of water and inorganic acids show differences in isotopic composition between the vapor and liquid phases, reportedly even at an azeotrope. A recent reactive azeotropy analysis rationalized this by allowing for differences in stable isotope composition between bound solvation complexes and the bulk liquid. The objective of this study is to determine whether or not significant differences in stable isotope composition exist between bound solvation complexes involving inorganic acids and the bulk solution. Rayleigh distillation of a solution of nitric acid and water was conducted. During the distillation, it was expected that bulk water would be vaporized first while (bound) water involved with hydrating the acid would be vaporized at later times along with the acid. Samples of both distillate and residual liquid were collected over time intervals and analyzed to determine oxygen and nitrogen stable isotope composition using isotope ratio mass spectrometry. As a reference, comparative samples were collected from Rayleigh distillation of water alone. The measured oxygen fractionation factors suggest that the acid is preferentially hydrated with 18O-enriched water, compared to the unbound, or bulk, water. Preferential coordination of 18O-enriched water with the dissolved acid is consistent with heavier water forming stronger bonds relative to light water. The results, although obtained under non-equilibrum conditions, suggest that isotope-dependent partitioning between bulk solvent and dissolved complexes may prove necessary in developing accurate descriptions of isotope fractionation during vapor-liquid equilibrium.

Using Stable Isotopes to Assess the Contribution of Terrestrial and Riverine Organic Matter to Diets of Nearshore Marine Consumers in a Glacially Influenced Estuary

Terrestrial and marine ecosystems in Southeast Alaska are linked by the flow of freshwater from precipitation and glacial runoff, which transports nutrients and organic matter (OM) downstream to estuaries. We examined the contribution of terrestrial-riverine and marine OM to diets of fishes (N = 257, four species) and invertebrates (N = 90, six species) collected from glacially influenced estuaries in Southeast Alaska using multiple stable isotopes (δ13C, δ15N, and δ34S). Multivariate analysis of similarity (ANOSIM) was used to quantify variation in stable isotope composition of consumers across 6 months and three sites with watersheds that differed in their glacier and forest composition. Fishes showed weak differences (ANOSIM R = 0.141) in stable isotope composition among sampling months, moderate differences (ANOSIM R = 0.375) among sites, and strong differences (ANOSIM R = 0.583) among species. Invertebrates showed moderate differences (ANOSIM R = 0.352) in stable isotope composition among sampling months and strong differences among sites (ANOSIM R = 0.710) and species (ANOSIM R = 0.858). We found the greatest differences in stable isotope composition between the two estuary sites with watersheds containing the highest and lowest glacial coverage, indicating that the contribution of allochthonous OM to consumer diets varies across watershed types. Invertebrates collected from the site with the lowest glacial coverage in the watershed were more depleted in δ13C and δ34S, indicating higher use of terrestrial-riverine OM, than those at sites with higher watershed glacial coverage. High variation in stable isotope composition among species, months, and sites underscores the complexity of estuary food web responses to future glacier loss.

Half-sibling loblolly pine clones exhibited intraspecific variation, a G × E interaction, and differences in stable isotope composition in response to soil moisture availability

We examined the response of three loblolly pine half-sibling clones (hereafter referred to as Clone 1, 2, and 3) to soil moisture availability in a greenhouse study. Three soil moisture treatments were imposed. Two were constant soil moisture treatments at either high (−0.3MPa) or low (−1.5MPa) soil water potential. The third was an alternating wet-dry treatment that alternated between one week at high (−0.3MPa) and three weeks at low (−1.5MPa) soil water potential, which simulated several natural wetting-drying cycles. At intervals throughout the experiment we measured biomass accumulation (via sequential harvesting), change in height, pre-dawn needle water potential, afternoon needle water potential, dark respiration, and morning and afternoon gas exchange (photosynthesis, stomatal conductance, intercellular CO2, and transpiration). Carbon allocation to root and shoot biomass, stomatal density, and carbon isotope composition were measured at the end of the experiment.All clones accumulated more biomass in the high soil moisture treatment than in the low soil moisture treatment, but there was a genetic by environment (G×E) interaction: Clone 1 accumulated more biomass than the other clones in the low soil moisture treatment and less than the others in the high soil moisture treatment. Stable carbon isotope composition, which we interpreted as a time-integrated index of water use efficiency, was significantly higher (indicating greater water use efficiency) in Clone 2, compared to Clones 1 and 3 across all soil moisture treatments except in the wet-dry treatment, where Clone 1 and 3 were not significantly different. A strong negative correlation was found between stable carbon isotope composition and biomass accumulation. Gas exchange parameters and needle water potential were higher in the high soil moisture treatment compared to the other treatments across all clones. Morning measurements of gas exchange were higher in Clones 2 and 3 than in Clone 1 across all treatments. Stomatal density varied significantly among clones but not among treatments. These results demonstrated that closely related clones differed in their growth and physiological responses to moisture availability, suggesting that clonal variation could be used to breed for increased drought resistance or to identify drought resistant genotypes for selective deployment.

Plant family identity distinguishes patterns of carbon and nitrogen stable isotope abundance and nitrogen concentration in mycoheterotrophic plants associated with ectomycorrhizal fungi.

Mycoheterotrophy entails plants meeting all or a portion of their carbon (C) demands via symbiotic interactions with root-inhabiting mycorrhizal fungi. Ecophysiological traits of mycoheterotrophs, such as their C stable isotope abundances, strongly correlate with the degree of species' dependency on fungal C gains relative to C gains via photosynthesis. Less explored is the relationship between plant evolutionary history and mycoheterotrophic plant ecophysiology. We hypothesized that the C and nitrogen (N) stable isotope compositions, and N concentrations of fully and partially mycoheterotrophic species differentiate them from autotrophs, and that plant family identity would be an additional and significant explanatory factor for differences in these traits among species. We focused on mycoheterotrophic species that associate with ectomycorrhizal fungi from plant families Ericaceae and Orchidaceae. Published and unpublished data were compiled on the N concentrations, C and N stable isotope abundances (δ(13)C and δ(15)N) of fully (n = 18) and partially (n = 22) mycoheterotrophic species from each plant family as well as corresponding autotrophic reference species (n = 156). These data were used to calculate site-independent C and N stable isotope enrichment factors (ε). Then we tested for differences in N concentration, (13)C and (15)N enrichment among plant families and trophic strategies. We found that in addition to differentiating partially and fully mycoheterotrophic species from each other and from autotrophs, C and N stable isotope enrichment also differentiates plant species based on familial identity. Differences in N concentrations clustered at the plant family level rather than the degree of dependency on mycoheterotrophy. We posit that differences in stable isotope composition and N concentrations are related to plant family-specific physiological interactions with fungi and their environments.

Differences in stable isotope compositions of freshwater snails from surface sediments of two Polish shallow lakes

The study reports and discusses the differences in δ13C and δ18O values of shells between several species of freshwater snails. Shells were derived from sediment samples collected from depths of 0.5, 1, 2 and 3 m along transects in two shallow eutrophic lakes located in mid-western Poland. Mean δ13C values of the shells ranged between −7.5 and −3.8‰ in Lake Jarosławieckie and between −8.1 and −5.2‰ in Lake Rosnowskie Duże, whereas mean δ18O values ranged between −2.2 and −0.2‰ and between −2.2 and 0.4‰ respectively in the studied lakes. A similar order of species in terms of shell isotope values, from least to most 13C and 18O-depleted was observed in both lakes and seems to indicate constancy of the factors controlling the stable isotope compositions of snail shells. We postulate that the nearly 4‰ difference in the mean carbon stable isotope values between the species was primarily controlled by the amount of metabolic carbon incorporated into the shells and the δ13C values of the snail food. Different growth cessation temperatures and microhabitats of the species studied result in temporally and spatially varied DIC δ13C values, water δ18O values and water temperature of shell precipitation, and may thus differentiate the δ13C and δ18O values of shells. The range of δ13C and δ18O values of individual shells from a sediment sample (mean 2.35 and 2.15‰, respectively) is interpreted as reflecting an intraspecific variability of isotope compositions in shells from a population and changes of the ambient conditions during the accumulation of the sediment layer. The species-specificity and intraspecific variability in C and O isotopic compositions of shells allow concluding that in palaeolimnological studies, stable isotope analyses should be performed on a set of mono-specific shells representing mean isotope compositions of the species for the interval studied rather than single shells or multispecific bulk shell material.

Stable Isotope Signatures Reveal Cryptic Differences in Diet of Sociable Weavers

Foraging habits of animals are a critical component of their ecology; however, estimates of feeding that rely solely on observation may not detect all of the factors that affect foraging decisions. Importantly, few studies have examined the influence of sociality on dietary regimes in avian species. The stable isotope ratios of carbon (δ13Q and nitrogen (δ15N) were used to assess the foraging habits of Sociable Weavers ( Philetairus socius) at a field site in Namibia. Sociable Weavers were sampled from three different nests, at two different time points over the course of a year. We found significant differences in stable isotope composition between nests, and subsequently compared the isotopic profile to the isotopic profile of the preferred food item. Isotopic structuring between nests suggests that sociality may constrain foraging decisions and could be a common feature of species of animals where foraging groups are temporally stable. We also found that larger nests are more isotopically similar to the preferred foraging source, and thus larger populations may be supported by nearby foraging resources.

Determination of δ13C and δ15N and trophic fractionation in jellyfish: implications for food web ecology

Application of stable isotope analysis (SIA) in jellyfish allows definition of trophic patterns not detectable using gut content analysis alone, but analytical protocols require standardization to avoid bias in interpreting isotopic data. We determined δ13C and δ15N in Aurelia sp. from the northern Gulf of Mexico (30°00′N, 89°00′W–30°24′N, 88°00′W) to define differences in stable isotope composition between body parts and whole body, the effect of lipid extraction on δ13C in tissues, and fractionation values from medusa to prey. The isotopic composition of bell and whole Aurelia sp. was not different. The increase in δ13C values after lipid removal suggested a correction is needed. To aid future analyses, we derived a correction equation from empirical data for jellyfish samples. Laboratory feeding experiments indicated medusae increased +4 ‰ in δ13C and +0.1 ‰ in δ15N compared to their diet. These results suggest protocols commonly applied for other species may be inaccurate to define Aurelia sp. trophic ecology. Because Aurelia spp. are commonly found in marine ecosystems, accurately defining their trophic role by use of SIA has implications for understanding marine food webs worldwide.

Intrinsic and Synthetic Stable Isotope Marking of Tsetse Flies

The sterile insect technique has been successfully used to eliminate tsetse populations in a number of programs. Program monitoring in the field relies on the ability to accurately differentiate released sterile insects from wild insects so that estimates can be made of the ratio of sterile males to wild males. Typically, released flies are marked with a dye, which is not always reliable. The difference in isotopic signatures between wild and factory-reared populations could be a reliable and intrinsic secondary marker to complement existing marking methods. Isotopic signatures are natural differences in stable isotope composition of organisms due to discrimination against the heavier isotopes during some biological processes. As the isotopic signature of an organism is mainly dependent on what it eats; by feeding factory-reared flies isotopically different diets to those of the wild population it is possible to intrinsically mark the flies. To test this approach unlabeled samples of Glossina pallidipes (Austen) (Diptera: Glossinidae) from a mass rearing facility and wild populations were analyzed to determine whether there were any natural differences in signatures that could be used as markers. In addition experiments were conducted in which the blood diet was supplemented with isotopically enriched compounds and the persistence of the marker in the offspring determined. There were distinct natural isotopic differences between factory reared and wild tsetse populations that could be reliably used as population markers. It was also possible to rear artificially isotopically labeled flies using simple technology and these flies were clearly distinguishable from wild populations with greater than 95% certainty after 85 days of “release”. These techniques could be readily adopted for use in SIT programs as complimentary marking techniques.

Evaluating the need for acid treatment prior to δ13C and δ15N analysis of freshwater fish scales: effects of varying scale mineral content, lake productivity and CO2 concentration

In order to evaluate the need for using scale acidification to remove carbonates prior to stable isotope analysis, we compared acidified and non-acidified scales of six freshwater fish species (perch, roach, rudd, pike, tench and bream) with contrasting mineral content in their scales. Fish samples were taken from six lakes with variable trophic conditions, ranging from oligotrophic to hypertrophic, and differing in CO2 concentrations. The scale mineral content of the six species studied ranged between 31.8 and 61.3% dry weight (DW) in tench and perch, respectively. The elemental composition was characterised by high amounts of phosphorus, varying from 4.5 to 9.1% DW. The mineral fraction was dominated by apatite (range 24.4–49.2% DW), carbonates constituted a very small proportion of the total carbon content (average ± SD: 5.5 ± 1.7%). The average effect of acidification was very small for all species (average ± SD: 0.181 ± 0.122 and −0.208 ± 0.243 for carbon and nitrogen, respectively), albeit significant for five out of the six species (excepting tench that had the lowest mineral content). Linear regression slopes between acidified and untreated scales did not differ significantly from one for almost all the species and isotopes. The effects of acidification on the two isotopes were correlated with the relative carbonate content as well as with the CO2 concentration for carbon and total phosphorus for nitrogen. We conclude that the need for scale acidification depends on the different species and on the system studied, although in most cases the acidification effect will be biologically irrelevant. However, dual analysis of acidified and untreated scales may provide useful information on differences in stable isotope composition of dissolved inorganic carbon and on phytoplankton carbon fractionation generated by varying levels of CO2 availability.

Differences in stable isotope composition within and among zooplanktivorous Utaka cichlid populations from Lake Malawi

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Routine Isotope Marking for the Mediterranean Fruit Fly (Diptera: Tephritidae)

A simple method of marking Mediterranean fruit fly Ceratitis capitata (Wiedemann) using stable isotopes is described. This species is economically important and is a target species of many successful area-wide integrated pest management (AW-IPM) programs using the sterile insect technique (SIT). Program monitoring in the field relies on being able to accurately differentiate released sterile insects from wild insects so that estimates can be made of the ratio of sterile males to wild males. Typically, released flies are marked with fluorescent dust, which is not always reliable. The difference in isotopic signatures between wild and factory-reared populations could be a reliable and intrinsic secondary marker to complement existing marking methods. Isotopic signatures are natural differences in stable isotope composition of organisms caused by discrimination against the heavier isotopes during some biological processes. The isotopic signature of an organism is mainly dependent on what it eats; by feeding factory-reared flies isotopically different diets to those of the wild population, it is possible to intrinsically mark the flies. The majority of fruit fly species feed on C3 plants in the wild, which have a carbon isotope signature of around -28 per thousand. However, almost all mass-rearing facilities use cane sugar in the larval and adult diet, which is a C4 sugar source (with a signal of around -11 per thousand), and this could provide an easy signature to differentiate released flies from wild flies. To test this approach, samples of flies from several mass-rearing facilities and wild populations were analyzed. It was clearly shown that using C4 sugar in the larval-rearing diet was an effective and economic way of intrinsically labeling Mediterranean fruit flies, and it was possible to distinguish mass-reared from wild populations with > 95% confidence. The C4 marker was detectable and distinguishable from wild populations up to 12 d after "release." This technique could be adopted for use in any other SIT program with similar rearing protocols to Mediterranean fruit fly.

δ 13C and δ 18O Trends Across Overstory Environments in Whole Foliage and Cellulose of Three Pinus Species

Stable isotope ratios of carbon (delta(13)C) and oxygen (delta(18)O) are increasingly used to investigate environmental influences on plant physiology. Cellulose is often isolated for isotopic studies, but some authors have questioned the value of this process. We studied trends in delta(13)C and delta(18)O of whole foliage and holocellulose from seedlings of three Pinus species across three overstory environments to evaluate the benefits of holocellulose extraction in the context of a traditional ecological experiment. Both tissue types showed increasing delta(13)C from closed-canopy controls to thinned plots to 0.3 ha canopy gaps, and no change in delta(18)O between overstory environments. delta(13)C of P. resinosa and P. strobus was greater than delta(13)C of P. banksiana in whole foliage and holocellulose samples, and there were no differences in delta(18)O associated with species in either tissue type. Our results suggest whole foliage and holocellulose provide similar information about isotopic trends across broad environmental gradients and between species, but holocellulose may be better suited for studying differences in stable isotope composition between multiple species across several treatments.

Evaluating patterns of fog water deposition and isotopic composition on the California Channel Islands

Fog deposition is an important water source for endemic conifer species during the annual summer drought along the California coast (and in other coastal and montane areas). We present a new design for a passive fog collector that is useful both for characterizing fog regimes (timing and quantity of deposition) and for collecting fog water for subsequent isotopic analysis. The new collector both mimics vegetation collection efficiency and minimizes isotopic fractionation under a range of fog conditions. Low construction cost and collector durability allow widely distributed installation and greater insight into spatially heterogeneous fog patterns. We installed 21 fog collectors throughout a stand of Bishop pines (Pinus muricata D. Don) on Santa Cruz Island. In general, there was greater fog deposition with increasing elevation and decreasing frequency farther inland. Within these broad patterns, there was large spatial and temporal variability in fog deposition. Monthly samples of fog and rain waters reveal differences in stable isotope composition (δ18O and δD) large enough to serve as tracers of different water sources moving through the ecosystem.

Stable Isotope Variations in Extraterrestrial Materials

The materials of the planets and the small bodies of the solar system contain a rich record of stable isotope variations in the light elements. As in terrestrial isotope geochemistry, this record reflects physical and chemical processes involving isotopic mixing among different reservoirs as well as fractionations arising in chemical reactions. The processes that influence the isotopic records of extraterrestrial materials range widely in environmental conditions from very high-energy events such as formation of refractory inclusions and chondrules by evaporation, condensation and melting in the solar nebula to lower temperature fluid-rock interactions in asteroids and planets. In addition, however, stable isotope cosmochemistry must consider issues that are beyond the scope of isotope geochemistry. For example, in the terrestrial sphere one may assume the existence of an isotopic reservoir that was originally homogenized during planet formation and the actual isotope compositions of the bulk earth do not need to be known in order to study the differences in stable isotope compositions that have been generated subsequently by various geochemical processes. This assumption of homogenization cannot be made for extraterrestrial samples, and in fact stable isotopes in meteorites preserve some of the most dramatic evidence for the incomplete nature of the mixing of distinct presolar materials during formation of the solar system. Such ‘isotopic anomalies’ are present in the isotopic distributions of H, C, N, and O on all spatial scales—from microscopic zoning in certain meteoritic minerals to the bulk compositions of asteroids and planets. Thus, some of the isotopic heterogeneities of ‘primitive’ solar system materials represent vestiges of primordial differences that could not be fully erased during the processing of presolar materials. In other cases, isotopic heterogeneities reflect the preservation of unique clues to processes occurring during formation of the solar system and planetary accretion, including early ‘geologic activity’ on planetesimals …

Stable isotopes of carbon and nitrogen in the study of avian and mammalian trophic ecology

Differential fractionation of stable isotopes of carbon during photosynthesis causes C4 plants and C3 plants to have distinct carbon-isotope signatures. In addition, marine C3 plants have stable-isotope ratios of carbon that are intermediate between C4 and terrestrial C3 plants. The direct incorporation of the carbon-isotope ratio (13C/12C) of plants into consumers' tissues makes this ratio useful in studies of animal ecology. The heavy isotope of nitrogen (15N) is preferentially incorporated into the tissues of the consumer from the diet, which results in a systematic enrichment in nitrogen-isotope ratio (15N/14N) with each trophic level. Consequently, stable isotopes of nitrogen have been used primarily to assess position in food chains. The literature pertaining to the use of stable isotopes of carbon and nitrogen in animal trophic ecology was reviewed. Data from 102 studies that reported stable-isotope ratios of carbon and (or) nitrogen of wild birds and (or) mammals were compiled and analyzed relative to diet, latitude, body size, and habitat moisture. These analyses supported the predicted relationships among trophic groups. Carbon-isotope ratios differed among species that relied on C3, C4, and marine food chains. Likewise, nitrogen-isotope ratios were enriched in terrestrial carnivorous mammals relative to terrestrial herbivorous mammals. Also, marine carnivores that ate vertebrates had nitrogen-isotope ratios that were enriched over the ratios of those that ate invertebrates. Data from the literature also indicated that (i) the carbon-isotope ratio of carnivore bone collagen was inversely related to latitude, which was likely the result of an inverse relationship between the proportion of carbon in the food chain that was fixed by C4 plants and latitude; (ii) seabirds and marine mammals from northern oceans had higher nitrogen-isotope ratios than those from southern oceans; (iii) the nitrogen-isotope ratios of terrestrial mammals that used xeric habitats were higher than the ratios of those that used mesic habitats, indicating that water stress can have important effects on the nitrogen-isotope ratio; (iv) there was no relationship between body mass and nitrogen-isotope ratio for either bone collagen or muscle of carnivores; and (v) there was linear covariation between stable-isotope ratios of carbon and nitrogen in marine food chains (but not in terrestrial C3 or C4 food chains), which is likely a product of increases in carbon-isotope ratio with trophic level in marine food chains. Differences in stable-isotope composition among trophic groups were detected despite variation attributable to geographic location, climate, and analytical techniques, indicating that these effects are large and pervasive. Consequently, as knowledge of the distribution of stable isotopes of carbon and nitrogen increases, they will probably become an increasingly important tool in the study of avian and mammalian trophic ecology.