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1.
Rescaling the trophic structure of marine food webs   总被引:1,自引:0,他引:1  
Measures of trophic position (TP) are critical for understanding food web interactions and human‐mediated ecosystem disturbance. Nitrogen stable isotopes (δ15N) provide a powerful tool to estimate TP but are limited by a pragmatic assumption that isotope discrimination is constant (change in δ15N between predator and prey, Δ15N = 3.4‰), resulting in an additive framework that omits known Δ15N variation. Through meta‐analysis, we determine narrowing discrimination from an empirical linear relationship between experimental Δ15N and δ15N values of prey consumed. The resulting scaled Δ15N framework estimated reliable TPs of zooplanktivores to tertiary piscivores congruent with known feeding relationships that radically alters the conventional structure of marine food webs. Apex predator TP estimates were markedly higher than currently assumed by whole‐ecosystem models, indicating perceived food webs have been truncated and species‐interactions over simplified. The scaled Δ15N framework will greatly improve the accuracy of trophic estimates widely used in ecosystem‐based management.  相似文献   

2.
Stable isotopes are widely used to identify trophic interactions and to determine trophic positions of organisms in food webs. Comparative studies have provided general insights into the variation in isotopic composition between consumers and their diet (discrimination factors) in predator–prey and herbivore–plant relationships while other major components of food webs such as host–parasite interactions have been largely overlooked. In this study, we conducted a literature‐based comparative analysis using phylogenetically‐controlled mixed effects models, accounting for both parasite and host phylogenies, to investigate patterns and potential drivers in Δ13C and Δ15N discrimination factors in metazoan parasitic trophic interactions. Our analysis of 101 parasite–host pairs revealed a large range in Δ13C (–8.2 to 6.5) and Δ15N (–6.7 to 9.0) among parasite species, with no significant overall depletion or enrichment of 13C and 15N in parasites. As previously found in other trophic interactions, we identified a scaling relationship between the host isotopic value and both discrimination factors with Δ13C and Δ15N decreasing with increasing host δ13C and δ15N, respectively. Furthermore, parasite phylogenetic history explained a large fraction (>60%) of the observed variation in the Δ15N discrimination factor. Our findings suggest that the traditional isotope ecology framework (using an average Δ15N of 3.4‰) applies poorly to parasitic trophic interactions. They further indicate the need for a scaled rather than a fixed trophic discrimination factor framework along gradients of host δ15N. We also identified several conceptual and methodological issues which should to be considered in future research to help integrate parasitic interactions into a holistic isotope ecology framework across diverse trophic interactions.  相似文献   

3.
Abstract Differences between diet and tissue isotope values known as trophic shifts (Δδ13C and Δδ15N) occur during digestion and assimilation of consumed food. Consideration of trophic shifts is essential when using stable isotopes for dietary reconstruction but has received little attention for cervids. Therefore, our purpose was to determine C and N trophic shifts in tissues of captive white-tailed deer (Odocoileus virginianus) fed corn and alfalfa in known amounts over a 4-month period. Antler has also received limited consideration for use in dietary reconstruction, thus, we analyzed tissue to expose variation among locations along the main beam and between antler components. We collected antler, hair, red blood cells (RBCs), and serum at the end of the feeding trial and analyzed them to determine C (δ13C) and N (δ15N) isotope values. Trophic shifts occurred between diet and all tissues for both isotopes with mean Δδ13C = 1.19 ± 2.23% and Δδ15N = 4.93 ± 0.74%. Antler trophic shifts were greater than those in all other tissues for δ13C, whereas antler and RBCs shared similar trophic enrichment over diet but differed from hair and serum for Δδ15N. Trophic shift values were significantly related to diet in hair and serum for δ13C and antler and RBCs for Δδ15N. Isotope values for antler core and periphery plus antler locations along the main beam did not vary. Antler collagen significantly varied from whole antler for δ13C but not δ15N. Our findings provide mean trophic shift values by tissue that can be used for dietary reconstruction in the study and management of cervids.  相似文献   

4.
Ecologists use stable isotopes to infer diets and trophic levels of animals in food webs, yet some assumptions underlying these inferences have not been thoroughly tested. We used laboratory‐reared colonies of Solenopsis invicta Buren (Hymenoptera: Formicidae: Solenopsidini) to test the effects of metamorphosis, diet, and lipid storage on carbon and nitrogen stable isotope ratios. Effects of metamorphosis were examined in ant colonies maintained on a control diet of domestic crickets and sucrose solution. Effects of a diet shift were evaluated by adding a tuna supplement to select colonies. Effects of lipid content on stable isotopes were tested by treating worker ants with polar and non‐polar solvents. δ13C and δ15N values of larvae, pupae, and workers were measured by mass spectrometry on whole‐animal preparations. We found a significant effect of colony age on δ13C, but not δ15N; larvae, pupae, and workers collected at 75 days were slightly depleted in 13C relative to collections at 15 days (Δδ13C = ?0.27‰). Metamorphosis had a significant effect on δ15N, but not δ13C; tissues of each successive developmental stage were increasingly enriched in 15N (pupae, +0.5‰; workers, +1.4‰). Availability of tuna resulted in further shifts of about +0.6‰ in isotope ratios for all developmental stages. Removing fat with organic solvents had no effect on δ13C, but treatment with a non‐polar solvent resulted in enriched δ15N values of +0.37‰. Identifying regular patterns of isotopic enrichment as described here should improve the utility of stable isotopes in diet studies of insects. Our study suggests that researchers using 15N enrichment to assess trophic levels of an organism at different sites need to take care not to standardize with immature insect herbivores or predators at one site and mature ones at another. Similar problems may also exist when standardizing with holometabolous insects at one site and spiders or hemimetabolous insects at another site.  相似文献   

5.
Although interspecific trophic interactions plays a principal role within elasmobranch communal nurseries, little is known over variation in foraging strategies adopted by young-of-year of sympatric species. To test the hypothesis of dietary resource partitioning between batoids within a communal nursery, we investigated two cownose ray species, Rhinoptera bonasus and R. brasiliensis, which occur in heterospecific groups, a strategy predicted to increase survival and foraging success. Using two biochemical tracers, fatty acids (FA) and stable isotopes (δ15N and δ13C), the combined effects of maternal investment and the formation of heterospecific groups implying competition for, or partitioning of available food resources were investigated. Through univariate and multivariate analyses of biochemical tracers in several tissues (fin clip, muscle, liver, red blood cells; RBC) and plasma, our results revealed significant interspecific differences in tracers between the two species. Total FAs (∑saturated FA, ∑monounsaturated FA and ∑polyunsaturated FA) and trophic biomarkers (i.e., docosahexaenoic acid, arachidonic acid, oleic acid and δ15N) were the principle tracers responsible for the differences detected. These data revealed that R. brasiliensis was less enriched in physiologically important essential FAs than R. bonasus. Our findings suggest that these congeneric species differ in maternal investment strategy and moderately partition food resources over relatively fine spatial scales within a single nursery habitat to limit competition. These results provide further knowledge on the foraging strategies adopted by batoids in communal nursery areas, information that is required for improving spatial conservation and management planning.  相似文献   

6.
Arctic ecosystem dynamics are shifting in response to warming temperatures and sea ice loss. Such ecosystems may be monitored by examining the diet of upper trophic level species, which varies with prey availability. To assess interannual variation in the Beaufort Sea ecosystem, we examined spatial and temporal trends in ringed seal (Pusa hispida) δ13C and δ15N in claw growth layers grown from 1964 to 2011. Stable isotopes were correlated with climate indices, environmental conditions, seal population productivity, and geographic location. Sex and age did not influence stable isotopes. Enriched 13C was linked to cyclonic circulation regimes, seal productivity, and westward sampling locations. Higher δ15N was linked to lower sea surface temperatures, a higher percentage of pups in the subsistence harvest, and sample locations that were eastward and further from shore. From the 1960s to 2000s, ringed seal niche width expanded, suggesting a diversification of diet due to expansion of prey and/or seal space use. Overall, trends in ringed seal stable isotopes indicate changes within the Beaufort Sea ecosystem affected by water temperatures and circulation regimes. We suggest that continued monitoring of upper trophic level species will yield insights into changing ecosystem structure with climate change.  相似文献   

7.
8.
Stable isotope composition of organisms from different trophic groups collected from a semi‐isolated wetland pool in the Ross River estuary, northern Australia, was analysed to determine if there was a consistent relationship between δ13C, δ15N and trophic level that could be used to assign trophic positions. A strong linear negative relationship between δ13C and δ15N was detected for the three trophic levels considered (primary producers, primary consumers and secondary consumers). This relationship was consistent among trophic levels, differing only in height, that is, on δ15N values, which indicate trophic positions. A difference of 3.6–3.8‰ between trophic levels was present, suggesting a δ15N fractionation of approximately 3.7‰, a value slightly higher than the commonly assumed δ15N fractionation of approximately 3.4‰. The relationship between δ13C and δ15N was similar for invertebrate and fish primary consumers, indicating similar δ15N trophic fractionation for both groups, meaning trophic positions and trophic length could be reliably calculated based on either invertebrates or fish.  相似文献   

9.
Living in close association with other organisms has proven to be a widespread and successful strategy in nature. Some communities are completely driven by symbiotic associations and therefore, intimate relationships among the partners can be expected. Here, we analyzed in‐depth the food web of a particularly rich community of arthropods found in strict association with European red wood ants (Formica rufa group). We studied the trophic links between different ant‐associated myrmecophiles and food sources associated with the host ant, but also tested predator–prey links among myrmecophiles themselves. Our approach combined direct feeding tests and stable carbon and nitrogen isotope analyses for a large number of myrmecophiles. The results of the direct feeding tests reveal a complex food web. Most myrmecophiles were found to parasitize on ant brood. Moreover, we encountered multiple trophic predator– prey links among the myrmecophiles. The results of the stable isotope analyses complement these findings and indicate the existence of multiple trophic levels and trophic isotopic niche compartmentalization. δ15N values were strongly correlated with the trophic levels based on the direct tests, reflecting that δ15N values of myrmecophiles increased with higher trophic levels. This strong correlation underlines the strength of stable isotopes as a powerful tool to assess trophic levels. In addition, the stable isotope data suggest that most species only facultatively prey on ant brood. The presence of numerous trophic interactions among symbionts clearly contrasts with the traditional view of social insects nests as offering an enemy‐free space for symbionts. Interestingly, the ant host can indirectly benefit from these interactions because brood predators are also preyed upon by other myrmecophiles. Overall, this study provides unique insights into the complex interactions in a small symbiont microcosm system and suggests that the interactions between host and symbiont might be mediated by other symbionts in the same community.  相似文献   

10.
Longnose gar Lepisosteus osseus were collected from May 2012 to July 2013 in the Charleston Harbor and Winyah Bay estuaries (SC, U.S.A.). This study examined trends in stomach fullness, described major prey components and their importance in the diet of L. osseus, compared stomach content‐based trophic level estimates with the stable‐isotope‐based proxy: δ15N and tested for the occurrence of an ontogenetic diet shift using stomach content analysis and stable C and N isotopes (δ13C and δ15N). Dominant prey families were Clupeidae, Sciaenidae, Penaeidae, Fundulidae and Mugilidae, with the highest consumption rates in autumn. Trophic levels calculated using stomach contents did not correspond to δ15N (P > 0·05). Stomach contents and stable‐isotope signatures indicate ontogenetic prey composition shifts from low trophic level benthic prey (fundulids) to higher trophic level pelagic prey (clupeids) as the fish grow between 400 and 600 mm in standard length. Due to their biomass, abundance and top predator status, L. osseus play a significant ecological role in the estuarine community composition, although this effect has often been overlooked by past researchers and should be considered in future estuarine community studies.  相似文献   

11.
1. Use of the natural ratios of carbon and nitrogen stable isotopes as tracers of trophic interactions has some clear advantages over alternative methods for food web analyses, yet is limited to situations where organic materials of interest have adequate isotopic separation between potential sources. This constrains the use of natural abundance stable isotope approaches to a subset of ecosystems with biogeochemical conditions favourable to source separation. 2. Recent studies suggest that stable hydrogen isotopes (δD) could provide a robust tracer to distinguish contributions of aquatic and terrestrial production in food webs, but variation in δD of consumers and their organic food sources are poorly known. To explore the utility of the stable hydrogen isotope approach, we examined variation in δD in stream food webs in a forested catchment where variation in δ13C has been described previously. 3. Although algal δD varied by taxa and, to a small degree, between sites, we found consistent and clear separation (by an average of 67‰) from terrestrial carbon sources. Environmental conditions known to affect algal δ13C, such as water velocity and stream productivity did not greatly influence algal δD, and there was no evidence of seasonal variation. In contrast, algal δ13C was strongly affected by environmental factors both within and across sites, was seasonally variable at all sites, and partially overlapped with terrestrial δ13C in all streams with catchment areas larger than 10 km2. 4. While knowledge of isotopic exchange with water and trophic fractionation of δD for aquatic consumers is limited, consistent source separation in streams suggests that δD may provide a complementary food web tracer to δ13C in aquatic food webs. Lack of significant seasonal or spatial variation in δD is a distinct advantage over δ13C for applications in many aquatic ecosystems.  相似文献   

12.
1. Stable isotopes of nitrogen are useful for quantifying the trophic structure of food webs, but only if the variation in trophic enrichment (ΔN), which is the difference in δ15N between a consumer and its food, is small relative to the value of ΔN itself. 2. We examined the sources of variation in zooplankton ΔN by measuring the trophic enrichment (ΔN) of seven species of freshwater cladocerans, and by testing for an effect of age and temperature on the ΔN of Daphnia pulicaria. 3. We found that ΔN was similar among Cladocera and was not correlated with body size. Overall, the ΔN for D. pulicaria was 1.4‰ (SE = 0.69, n = 57), as was expected for the detritus diet that we used in our experiments. We found no effect of temperature (15–25 °C) on ΔN, but found that ΔN of D. pulicaria increased with increasing age (10–30 days). 4. We developed a new method to test for trophic‐level variation in a group of consumers that explicitly accounts for the uncertainty in ΔN. Using this approach, we confirmed that natural assemblages of zooplankton feed at several trophic levels in lake food webs.  相似文献   

13.
Arctic food webs are being impacted by borealisation and environmental change. To quantify the impact of these multiple forcings, it is crucial to accurately determine the temporal change in key ecosystem metrics, such as trophic position of top predators. Here, we measured stable nitrogen isotopes (δ15N) in amino acids in harp seal teeth from across the North Atlantic spanning a period of 60 years to robustly assess multi-decadal trends in harp seal trophic position, accounting for changes in δ15N at the base of the food web. We reveal long-term variations in trophic position of harp seals which are likely to reflect fluctuations in prey availability, specifically fish- or invertebrate-dominated diets. We show that the temporal trends in harp seal trophic position differ between the Northwest Atlantic, Greenland Sea and Barents Sea, suggesting divergent changes in each local ecosystem. Our results provide invaluable data for population dynamic and ecotoxicology studies.  相似文献   

14.
The ratio of 15N/14N (δ15N) from consumer and prey tissue is commonly used in ecological studies to determine trophic level, food web structure, and mean trophic level in aquatic ecosystems. There is a predictable positive relationship between the δ15N values in tissue and trophic level, caused by the bioaccumulation of 15N in tissues of consumers with each step up the food chain. Reconstructing trophic structure or food chain length over time may provide resource managers with insights about ecosystem biodiversity and resilience. Yet, in many marine systems the absence of baseline information before anthropogenic disturbances makes comparative studies addressing ecosystem responses extremely difficult. Here we attempt to retrospectively reconstruct trophic position in four species of fish from the upper Gulf of California, Mexico before perturbations such as overfishing or the damming of the Colorado River. We first validated if otolith δ15N approximates the δ15N observed in fish tissue. We then used the δ15N encapsulated in ancient fish otoliths that are between 1,000 and 5,500 years old to define the food web structure. Our results suggested that δ15N in otoliths has slightly more positive δ15N than soft tissue. The δ15N values from ancient otoliths appropriately defined the fishes’ relative trophic position. We found significant differences in δ15N between functional groups, apex predator versus intermediate predators. Juveniles and adult fishes displayed trophic separation between functional groups. Our findings advocate the application of δ15N analysis of prehistoric otoliths for establishing pre-disturbance ecological benchmarks.  相似文献   

15.
Ecologists routinely set out to estimate the trophic position of individuals, populations, and species composing food webs, and nitrogen stable isotopes (δ15N) are a widely used proxy for trophic position. Although δ15N values are often sampled at the level of individuals, estimates and confidence intervals are frequently sought for aggregations of individuals. If individual δ15N values are correlated as an artifact of sampling design (e.g., clustering of samples in space or time) or due to intrinsic groupings (e.g., life history stages, social groups, taxonomy), such estimates may be biased and exhibit overly optimistic confidence intervals. However, these issues can be accommodated using hierarchical modeling methods. Here, we demonstrate how hierarchical models offer an additional quantitative tool for investigating δ15N variability and we explicitly evaluate how δ15N varies with body size at successively higher levels of taxonomic aggregation in a diverse fish assemblage. The models take advantage of all available data, better account for uncertainty in parameters estimates, may improve inferences on coefficients corresponding to groups with small to moderate sample sizes, and partition variation across model levels, which provides convenient summaries of the ‘importance’ of each level in terms of unexplained heterogeneity in the data. These methods can easily be applied to diet-based studies of trophic position. Although hierarchical models are well-understood and established tools, their benefits have yet to be fully reaped by stable isotope and food web ecologists. We suggest that hierarchical models can provide a robust framework for conceptualizing and statistically modeling trophic position at multiple levels of aggregation.  相似文献   

16.
Fish trophic niches reflect important ecological interactions and provide insight into the structure of mangrove food webs. Few studies have been conducted in mangrove fish predators to investigate interpopulation trophic niches and ontogenetic shifts. Using stable isotope analysis and two complementary approaches, the authors investigated trophic niche patterns within and between two ontogenetic groups (juveniles and sub-adults) of a generalist predator (Acentrogobius viridipunctatus) in four mangroves with heterogeneous environmental conditions (e.g., tidal regimes, salinity fluctuations and mangrove tree community). The authors hypothesized that the trophic niche between populations would vary regionally and trophic position would increase consistently from juvenile to sub-adult stages. The results revealed that both δ13C and δ15N values varied greatly across populations and between ontogenetic groups, and complex spatio-ontogenetic variations were expressed by Layman's metrics. They also found some niche separation in space, which is most likely related to resource availability in spatially diverse ecosystems. In addition, trophic niche position increased consistently from juveniles to sub-adults, indicating ontogenetic feeding shifts. The isotopic plasticity index and Fulton's condition index also showed significant spatial-ontogenetic variation, which is consistent with optimal foraging theory. The findings highlight that trophic plasticity has a high adaptive value for mangrove fish predators in dynamic ecosystems.  相似文献   

17.
A proper assessment of the foraging habits of the Guadalupe fur seal (GFS; Arctocephalus townsendi) is a priority to better understand its recovery, in which the potential for intraspecific competition for prey and space resources is expected to lead to segregation. This study aimed to determine the foraging habits of different sex and age classes. A total of 146 GFS fur samples was collected at Guadalupe Island, Mexico (2014–2020) for stable isotopes (δ13C and δ15N) analysis. Isotopic areas were created (SIBER package in R). Significant isotopic differences were observed between classes. Male (3.6‰2) and female (3.0‰2) juveniles had the largest isotopic areas due to a greater foraging dispersion. Adult females showed the lowest mean δ15N value (16.1‰ ± 0.5‰) due to foraging trips that are mostly performed towards high latitudes. Except for pups, adult and subadult males presented the highest mean δ15N (17.4‰ ± 0.4‰) and δ13C values (−17.0‰ ± 0.8‰) due to a possible higher trophic level and coastal foraging habits, whereas pups presented the highest mean δ15N value (17.6‰ ± 0.3‰) because of lactation, which reflects their mothers δ15N signal plus their own enrichment. Our findings suggest a segregation explained by differences in life history, energy requirements, and a possible strategy to avoid competition.  相似文献   

18.
Pteropods are a group of small marine gastropods that are highly sensitive to multiple stressors associated with climate change. Their trophic ecology is not well studied, with most research having focused primarily on the effects of ocean acidification on their fragile, aragonite shells. Stable isotopes analysis coupled with isotope‐based Bayesian niche metrics is useful for characterizing the trophic structure of biological assemblages. These approaches have not been implemented for pteropod assemblages. We used isotope‐based Bayesian niche metrics to investigate the trophic relationships of three co‐occurring pteropod species, with distinct feeding behaviors, sampled from the Southern Kerguelen Plateau area in the Indian Sector of the Southern Ocean—a biologically and economically important but poorly studied region. Two of these species were gymnosomes (shell‐less pteropods), which are traditionally regarded as specialist predators on other pteropods, and the third species was a thecosome (shelled pteropod), which are typically generalist omnivores. For each species, we aimed to understand (a) variability and overlap among isotopic niches; and (b) whether there was a relationship between body size and trophic position. Observed isotopic niche areas were broadest for gymnosomes, especially Clione limacina antarctica, whose observed isotopic niche area was wider than expected on both δ13C and δ15N value axes. We also found that trophic position significantly increased with increasing body length for Spongiobranchaea australis. We found no indication of a dietary shift toward increased trophic position with increasing body size for Clio pyramidata f. sulcata. Trophic positions ranged from 2.8 to 3.5, revealing an assemblage composed of both primary and secondary consumer behaviors. This study provides a comprehensive comparative analysis on trophodynamics in Southern Ocean pteropod species, and supports previous studies using gut content, fatty acid and stable isotope analyses. Combined, our results illustrate differences in intraspecific trophic behavior that may be attributed to differential feeding strategies at species level.  相似文献   

19.
Southern African forests are naturally fragmented yet hold a disproportionately high number of bird species. Carbon and nitrogen stable isotopes were measured in feathers from birds captured at Woodbush (n = 27 species), a large afromontane forest in the eastern escarpment of Limpopo province, South Africa. The δ13C signatures of a range of forest plants were measured to categorise the food base. Most plants sampled, including two of five grass species, had δ13C signatures typical of a C3 photosynthetic pathway (?29.5 ± 1.9‰). Three grass species had a C4 signature (?12.0 ± 0.6‰). Most bird species had δ13C values representing a predominantly C3‐based diet (?24.8‰ to ?20.7‰). δ15N values were as expected, with higher levels of enrichment associated with a greater proportion of dietary animal matter. The cohesive isotopic niche defining most species (n = 22), where the ranges for δ13C and δ15N were 2.4‰ and 3.4‰, respectively, highlight the difficulties in understanding diets of birds in a predominantly C3‐based ecosystem using carbon and nitrogen stable isotopes. However, variation in isotopic values between and within species provides insight into possible niche width and the use of resources by different birds within a forest environment.  相似文献   

20.
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 [(RGRlast)]. 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 RGRlast, 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.  相似文献   

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