Stable Isotope Bayesian Ellipses Research Articles

Comparative analysis of trophic niche using stable isotopes provides insight into resource use of giant pandas.

Trophic niche is the result of the long-term evolution of species and can reflect the pressures they experience in habitats. The whole-genome sequencing of giant pandas (Ailuropoda melanoleuca) has indicated that populations distributed in Qinling (QIN) and non-QIN probably diverged 300 ky ago. Although many studies regarding foraging strategy, habitat preference, and niche partition have been conducted on these populations, there is still a lack of precise quantification of trophic niches. Here, we calculated and compared isotopic trophic niche widths of giant pandas from Sichuan (SC) and QIN populations by measuring carbon and nitrogen isotopes of their hairs; combined with data from sympatric mammals, we explored the relative trophic positions of giant pandas in the ecosystem, respectively. The Stable Isotope Bayesian Ellipses in R (SIBER) model results showed the trophic niche width of QIN pandas was 3.44‰2 , which was significantly bigger than those of the SC population (2.03‰2 ), with an overlapping about 1.45‰2 ; and they both occupied a unique position in the context, almost one trophic level lower than herbivores. Then, we determined the isotopic ratios of the main foods from the habitats of these pandas; the results suggested that the isotopic difference between bamboo shoots and other parts plus the various feeding selections of pandas on them accounted for pandas' trophic niche widths. We considered the higher nutrition availability and digestible food resources giving QIN pandas a wider trophic niche than pandas from SC. This conclusion provides a new insight into the resource use and trophic ecology of giant pandas and is important to develop refined management plans for the two populations.

The planktonic food web in the Gulf of Naples based on the analysis of carbon and nitrogen stable isotope ratios

AbstractPlankton play a key role in marine food webs by producing and transferring organic matter and energy to higher trophic levels. To define the trophic structure and interactions within the planktonic communities in the Gulf of Naples, we determined carbon and nitrogen stable isotope ratios in particulate organic matter (POM, <20 μm), phytoplankton (20–200 μm), and bulk (unsorted) and sorted mesozooplankton (200–2000 μm) on a weekly basis in 2019. The stable isotope values of POM and, to a lesser extent, phytoplankton reflected the short‐term (weekly) alternation between offshore and coastal waters within seasonal variability. Although the isotopic signatures of coastal and offshore sources were still detectable in bulk mesozooplankton, δ13C and δ15N of single groups remained almost unchanged throughout the year. The trophic niches of mesozooplankton groups defined using the Stable Isotope Bayesian Ellipses in R software showed a high degree of overlap among them, which was corroborated by the tendency of most groups toward omnivory, beyond their known feeding habits, as indicated by MixSIAR models. Collectively, our results highlight that the complex network of interactions within the planktonic community in the Gulf of Naples can buffer natural variability due to the specific hydrographic features of the system. This characteristic makes the trophic structure of this community an ideal model for monitoring the response of planktonic food webs to climate‐ and anthropic‐driven changes.

Isotopic niche shift in the sea urchins Echinometra lucunter and E. viridis after massive arrivals of Sargassum in the Mexican Caribbean

In the Mexican Caribbean, the decomposition of unusual amounts of Sargassum (Sargassum fluitans and S. natans) has caused changes in the benthic community. The aim of this study was to evaluate the impact of massive pelagic Sargassum arrivals on the trophic structure of sea urchins Echinometra lucunter and E. viridis, using stable isotopes of carbon (δ13C) and nitrogen (δ15N) to identify variations in relative proportions of assimilated primary producers in two reef lagoons (Xahuayxol and Xcalak) in the southern Mexican Caribbean. The survey considered two scenarios: (1) with massive Sargassum arrivals (WIMA) in 2018 and (2) without massive Sargassum arrivals (WOMA) in 2017. Results revealed a decreased in δ15N and δ13C values during WIMA in primary producers and sea urchins. Furthermore, the isotopic mixing models (MixSIAR) indicated that both E. lucunter and E. viridis preferably consumed Halimeda spp. and Thalassia testudinum. The isotopic niche width obtained from Stable Isotope Bayesian Ellipses in R (SIBER) analysis showed differences between species and localities. In Xahuayxol during WIMA the corrected standard ellipse area (SEAc) of E. lucunter increased while that of E. viridis decreased. However, the latter species had a significant increase in trophic level. In Xcalak during WIMA, for both sea urchins, the SEAc increased and overlapped while the trophic levels increased significantly. Results suggest that E. viridis in Xahuayxol was better adapted to changes in resource availability, while the niche expansion of E. lucunter in Xahuayxol and E. lucunter and E. viridis in Xcalak suggest that sea urchins exhibited a starvation stage due to a decrease in food resources.

Trophic niche size and overlap in temperate forest land snails are affected by their lifestyle and body size

Abstract Land snails represent a large diversity of species in the forest floor. Some species prefer to stay in the leaf litter, while others like to climb up vegetation. They are considered generalist herbivores/decomposers. Although the exact trophic position of most species is often difficult to determine, a low rate of trophic niche partitioning within an assemblage is assumed. We compared isotopic niche sizes and overlaps between and within four local land snail assemblages using stable isotope Bayesian ellipses. We found significant differences in trophic niche size (expressed by corrected standard ellipse area) as a function of body size and lifestyle. We hypothesised that the larger niche size of smaller and/or arboreal snail species is due to their limited mobility and the presence of spatially structured food resources. In contrast, a broad and similar mixture of leaf litter sources results in a homogeneous diet for individuals of larger, highly mobile, epigean species, i.e., smaller niche size. We documented multiple overlaps of isotopic niches between species within an assemblage, although arboreal species exhibited slightly different niche positions, indicating specific diets. When comparing all pairwise combinations, we found that isotopic niches of species using different lifestyle did not overlap, in contrast to species using similar lifestyle. In addition, the trophic niches of opportunists with respect to lifestyle were intermediate between those of species from the outermost categories. Our results fit the general concept of variable and overlapping trophic niches in co-occurring snail species. However, we were able to demonstrate differences in species that rarely occupy the same microhabitat.

Improving stable isotope assessments of inter- and intra-species variation in coral reef fish trophic strategies.

Fish have one of the highest occurrences of individual specialization in trophic strategies among Eukaryotes. Yet, few studies characterize this variation during trophic niche analysis, limiting our understanding of aquatic food web dynamics. Stable isotope analysis (SIA) with advanced Bayesian statistics is one way to incorporate this individual trophic variation when quantifying niche size. However, studies using SIA to investigate trophodynamics have mostly focused on species‐ or guild‐level (i.e., assumed similar trophic strategy) analyses in settings where source isotopes are well‐resolved. These parameters are uncommon in an ecological context. Here, we use Stable Isotope Bayesian Ellipses in R (SIBER) to investigate cross‐guild trophodynamics of 11 reef fish species within an oceanic atoll. We compared two‐ (δ 15N and δ 13C) versus three‐dimensional (δ 15N, δ 13C, and δ 34S) reconstructions of isotopic niche space for interpreting guild‐, species‐, and individual‐level trophic strategies. Reef fish isotope compositions varied significantly among, but also within, guilds. Individuals of the same species did not cluster together based on their isotope values, suggesting within‐species specializations. Furthermore, while two‐dimensional isotopic niches helped differentiate reef fish resource use, niche overlap among species was exceptionally high. The addition of δ 34S and the generation of three‐dimensional isotopic niches were needed to further characterize their isotopic niches and better evaluate potential trophic strategies. These data suggest that δ 34S may reveal fluctuations in resource availability, which are not detectable using only δ 15N and δ 13C. We recommend that researchers include δ 34S in future aquatic food web studies.

Trophic ecology of northern gannets Morus bassanus highlights the extent of isotopic niche overlap with other apex predators within the Bay of Biscay

Understanding the linkages within complex and evolving marine food webs is essential to comprehend marine ecosystem structure and dynamics. Carbon (δ13C) and nitrogen (δ15N) stable isotope signatures are recognized to be powerful descriptors of the trophic ecology and trophic relationships within marine communities. Apex predators such as seabirds can influence the structure of communities by predating in lower trophic levels. They also convey information both over a range of spatial and temporal scales due to their high mobility and longevity, respectively. For this reason, here, we studied the trophic ecology of northern gannets and the extent of niche overlap with other apex predators within the Bay of Biscay (North-East Atlantic), a key feeding area for numerous predatory species. Mixing models indicated that northern gannets fed primarily on European anchovy Engraulis encrasicolus and on a mixed group composed of European sardine Sardina pilchardus, European sprat Sprattus sprattus, juvenile hake Merluccius merluccius, Atlantic mackerel Scomber scombrus and Atlantic horse mackerel Trachurus trachurus. Within the apex predator community, the northern gannets trophic niche overlapped the most with common guillemots Uria aalge (15.58%, based on Stable Isotope Bayesian Ellipses (SIBER)) and overlapped to a lesser extent with cetacean species occupying shelf habitats (i.e., the harbour porpoise Phocoena phocoena (9.99%) and the short-beaked common dolphin Delphinus delphis (10.37%)). Overall, the overlap found was moderate indicating trophic similarities and dissimilarities between these species. Information on trophic interactions between co-existing species is necessary for holistic environmental management, particularly in areas where several megafauna species share their foraging grounds with fisheries activity.

Variability of amphidromous organism isotopic niches in three Guadeloupe rivers affected by damming and water catchment

Native fauna of the tropical volcanic part of Guadeloupe is amphidromous: juveniles born in rivers but that grow in the sea need to migrate upstream to colonise their adult habitat in rivers. This migration is affected by any human-made obstacles placed in their way. Moreover, on volcanic tropical islands, streams are the main source of water catchment for the human population. This deeply affects river hydrology and characteristics. Both damming and water catchment potentially affect community diversity and species demography, but they may also alter the trophic ecology of the river fauna. Using stable isotopes and the stable isotope Bayesian ellipses approach in R (SIBER), this study aimed to assess the isotopic niche variability of riverine fauna of three persistent small rivers of Basse-Terre Island (Guadeloupe) affected by damming and water catchment. Using electrofishing, decapods and fishes (gobies) of three rivers were sampled upstream and downstream of dams. Our results demonstrated that the variability of the isotopic niches was extremely high between rivers but varied less between stations of the same river. Our results revealed complex and river-specific effects and a pattern merged with natural variability. Our two hypotheses (i.e., increase of resources upstream of dams and differential responses of trophic guilds to damming and water catchment) were only weakly supported and never in an unambiguous manner. Our study showed that it is necessary to consider the ‘noise’ generated by natural variability to observe and understand changes in the trophic ecology of associated fauna in relation to damming and water catchment.

Resource Utilization of Three Copepods Along Salinity Gradient in the Seomjin River Estuary, Revealed by Stable Isotope Analysis

Estuaries are one of the most productive regions in ecosystems and serve as important nursery and recruitment habitats for various living resources. However, information on the trophic dynamics of estuarine copepods along the salinity gradient is not well known. Here, we investigated resource partitioning among three copepods, Pseudodiaptomus japonicus, Sinocalanus tenellus, and Tortanus dextrilobatus in the Seomjin River estuary of the Korea using stable carbon and nitrogen isotope analyses. Among the three copepods, S. tenellus showed the lowest δ13C values, while the highest δ13C values were observed in T. dextrilobatus. Stable Isotope Analysis in R (SIAR) showed that S. tenellus fed mainly on terrestrial particulate organic matter (POM), while the contribution of marine POM was relatively high in the diet of T. dextilobatus. In contrast, P. japonicus showed relatively high trophic flexibility between terrestrial and marine POM. Thus, the Stable Isotope Bayesian Ellipses in R (SIBER) showed no dietary niche overlap among the three copepods. Therefore, we suggest that the coexistence of S. tenellus, P. japonicus, and T. dextrilobatus in the Seomjin River estuary can be closely associated with species-specific differences in resource utilization along the salinity gradient.

Sargassumblooms in the Caribbean alter the trophic structure of the sea urchinDiadema antillarum

The arrival of large masses of drifting Sargassum since 2011 has caused changes in the natural dynamics of Caribbean coastal ecosystems. In the summer of 2015, unprecedented and massive mats of S. fluitans and S. natans have been observed throughout the Mexican Caribbean including exceptional accumulations ashore. This study uses stable isotopes to assess the impact of Sargassum blooms on the trophic dynamics of the Diadema antillarum sea urchin, a keystone herbivore on many Caribbean reefs. Bayesian models were used to estimate the variations in the relative proportions of carbon and nitrogen of assimilated algal resources. At three lagoon reef sites, the niche breadth of D. antillarum was analysed and compared under massive influx of drifting Sargassum spp. vs. no influx of Sargassum blooms. The effects of the leachates generated by the decomposition of Sargassum led to hypoxic conditions on these reefs and reduced the taxonomic diversity of macroalgal food sources available to D. antillarum. Our trophic data support the hypothesis that processes of assimilation of carbon and nitrogen were modified under Sargassum effect. Isotopic signatures of macroalgae associated with the reef sites exhibited significantly lower values of δ15N altering the natural herbivory of D. antillarum. The Stable Isotopes Analysis in R (SIAR) indicated that, under the influence of Sargassum blooms, certain algal resources (Dictyota, Halimeda and Udotea) were more assimilated due to a reduction in available algal resources. Despite being an abundant available resource, pelagic Sargassum was a negligible contributor to sea urchin diet. The Stable Isotope Bayesian Ellipses in R (SIBER) analysis displayed differences between sites, and suggests a reduction in trophic niche breadth, particularly in a protected reef lagoon. Our findings reveal that Sargassum blooms caused changes in trophic characteristics of D. antillarum with a negative impact by hypoxic conditions. These dynamics, coupled with the increase in organic matter in an oligotrophic system could lead to reduce coral reef ecosystem function.

A historical perspective of niche differentiation between two top predators in the Uruguayan coastal area

Context As top predators, marine mammals play a key role consuming in different trophic levels and the trophic niche characterization may help to understand how species utilize and share resources . On the coast of the Río de la Plata and the South-west Atlantic, the South American sea lion (Otaria flavescens) and the franciscana dolphin (Pontoporia blainvillei) are two important predators. Aims The present study investigated potential trophic overlap of both species by measuring stable carbon (δ13C) and nitrogen (δ15N) isotopes over two periods: historical (1959–79) and recent (2002–15) on the Uruguayan coast. Methods Bone samples of P. blainvillei and O. flavescens were used to determine the isotopic niche using the Stable Isotope Bayesian Ellipses in R (SIBER) analysis. Key results The isotopic niche did not overlap between species in any period. δ15N was higher in O. flavescens in both periods (20.29‰±0.73 in the historical and 19.95‰±1.0 in the recent period), indicating that it feeds at a higher trophic level than P. blainvillei. The δ13C was also significantly higher in O. flavescens than in P. blainvillei during the two periods (O. flavescens: –11.43±0.6‰ historic, –12.72±0.4‰ recent, and P. blainvillei: –12.69±1.1‰ historic, –13.84±1.3‰ recent). The isotopic niche areas of P. blainvillei in recent and historic periods confirmed they forage in 2 distinct environments, marine and estuarine, with low isotopic overlap. This overlap was higher in the recent period. Conclusions and Implications O. flavescens and both P. blainvillei groups were segregated in both periods, with a higher overlap in the recent. These species appear to reduce competition by using different resources in the same coastal habitat. O. flavescens preferentially feeds on benthic fish and showed wider trophic amplitude in both periods, whereas P. blainvillei has a more coastal–pelagic diet and included a greater variability of resources in its diet. The differences between species trophic niches can still be detected after both marine mammals species abundance has declined and after the development of fisheries.

Stable Isotope Analysis Enhances Our Understanding of Diamondback Terrapin (Malaclemys terrapin) Foraging Ecology

Dietary studies on generalist predators may provide valuable information on spatial or temporal changes in the structure of ecological communities. We initiated this study to provide baseline data and determine the utility of stable isotope analysis (SIA) to evaluate the foraging strategies of an opportunistic reptilian predator, the diamondback terrapin (Malaclemys terrapin), which specializes in salt marshes and mangrove estuaries along the Atlantic and Gulf coasts. We evaluated stable carbon (δ13C) and nitrogen (δ15N) isotope values of multiple tissues from terrapins inhabiting mainland and island mangrove habitats in south Florida and potential food sources to examine spatial and temporal variations in terrapin resource use. We fit linear regression models to determine the best predictors of isotopic values for both terrapins and their prey, and Stable Isotope Bayesian Ellipses in R (SIBER) analysis to examine terrapin isotopic niche space and overlap between groups. We identified differences in terrapin isotopic δ13C and δ15N values among all sites. Blood and scute tissues revealed different isotopic compositions and niche overlap between sites, suggesting diets or foraging locations may change over time, and amount of variation is site specific. Niche overlap between size classes was larger for blood (short term) versus scute (long term), suggesting greater variability in food habits or resource isotopes over the long term versus short term. These results demonstrate the usefulness of SIA in examining the spatial and temporal variability in diamondback terrapin resource use within estuary systems and further define their niche within these dynamic food webs.

Variability in trophic level and habitat use in response to environmental forcing: isotopic niche dynamics of breeding seabirds in the southeastern Bering Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 593:247-260 (2018) - DOI: https://doi.org/10.3354/meps12471 Variability in trophic level and habitat use in response to environmental forcing: isotopic niche dynamics of breeding seabirds in the southeastern Bering Sea Alexis P. Will*, Alexander S. Kitaysky Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775, USA *Corresponding author: awill4@alaska.edu ABSTRACT: Climate-driven changes in the marine environment may affect inter- and intraspecific resource partitioning by marine organisms. When and how resources are partitioned may depend on access to diverse foraging habitats. Here we examined the variability in the isotopic niche of breeding seabirds with respect to trophic level and habitat use in years with cold and warm sea temperatures in the Bering Sea. Between 1999 and 2015 (n = 12 yr), we collected blood from black-legged kittiwakes Rissa tridactyla, and common Uria aalge and thick-billed murres U. lomvia breeding on St. George and St. Paul Islands on the southeastern Bering Sea continental shelf. We examined isotopic niche dynamics at the group and species levels. Stable isotope values of blood tissues corroborated published observations of seabird distributions in the region. All 3 species increased foraging on shelf-based prey during warm oceanographic conditions, in contrast to a higher reliance on oceanic-based prey during cold conditions. Under warm ocean conditions, the isotopic niche of the seabird group with access to only shelf habitat (St. Paul Island) contracted, whereas the isotopic niche of the seabird group with access to shelf, slope, and basin habitats (St. George Island) expanded. These group-level responses were associated with increased food availability. We conclude that habitat heterogeneity in the vicinity of breeding colonies may mediate how predators partition food resources in response to changes in climate-driven food availability. KEY WORDS: Black-legged kittiwake · Murre · Stable Isotope Bayesian Ellipses in R · SIBER · Layman’s metrics · Stable isotope analysis · Habitat heterogeneity · Resource partitioning Full text in pdf format Supplementary material PreviousNextCite this article as: Will AP, Kitaysky AS (2018) Variability in trophic level and habitat use in response to environmental forcing: isotopic niche dynamics of breeding seabirds in the southeastern Bering Sea. Mar Ecol Prog Ser 593:247-260. https://doi.org/10.3354/meps12471 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 593. Online publication date: April 12, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Characterization of food web structure of the upper continental slope of the Celtic Sea highlighting the trophic ecology of five deep-sea fishes

Summary In marine ecosystems, the study of trophic relationships has extensively benefited from the development of stable isotope analyses (SIA) as dietary tracers. SIA are particularly useful in elucidating the structure of deep sea food webs given the constraints involved in obtaining gut-content data from deep trawling. We used carbon and nitrogen stable isotope analyses and Stable Isotope Bayesian Ellipses in R (SIBER) and Stable Isotope Analysis in R (SIAR) routines, to determine the trophic ecology of five deep-sea fishes from the upper continental slope of the Celtic Sea. SIA made it possible to deduce some general tendencies in food-web structure and species trophic interactions and confirmed diet determined by gut-content analysis for the same species, in other ecoregions. More specifically, mixing models revealed that the deep sea species considered are omnivorous and are able to feed on all the sampled taxa. Based on isotopic ratio, no clear differences in fish diet could be detected from one species to another except for rabbit fish, which has benthic affinities. Three species, blackbelly rosefish, greater forkbeard and softhead grenadier showed overlapping isotopic niches. This study is the first attempt to describe the trophic ecology of deep sea species on the Celtic Sea upper continental slope. In the context of the development of ecosystem integrated modeling approaches for managing fisheries in the Celtic sea, and considering the vulnerability of deep-water species, improving the knowledge on the trophic ecology of these local species is of importance in order to allow their sustainable exploitation.

Feeding habits of juvenile yellowfin tuna (Thunnus albacares) in Ecuadorian waters assessed from stomach content and stable isotope analysis

Size-related shifts in feeding habits of juvenile yellowfin tuna (YFT), Thunnus albacares (Bonnaterre, 1788), in Ecuadorian waters were assessed using stomach content analysis (SCA) (n=339) and stable isotope analysis (n=83). In all size classes, fishes were the predominant prey group in the diet (Alimentary Index, %AI=96.86) followed by cephalopods (%AI=3.29) and crustaceans (%AI=0.03). PERMANOVA analysis revealed significant variations in diet with body size: i) Class I YFT (≤50cm in fork length, FL) fed mainly on laternfishes, family Myctophidae (%AI=37.34), and jumbo squid, Dosidicus gigas (%AI=36.06), Class II YFT (50–60cm, FL) consumed bullet and frigate tunas, Auxis spp., unidentifiable fish and jumbo squid (%AI=47.75, %AI=25.06 and%AI=21.67, respectively), and Class III YFT (≥60cm, FL) preyed almost exclusively on bullet and frigate tunas (%AI=90.85). Mean±SD muscle isotope values were 12.14±1.95‰ for δ15N and −17.42±0.27 for δ13C‰; mean (±SD) liver values were 11.30‰ for δ15N and −17.42±0.27‰ for δ13C. Overall, liver and muscle samples showed significant size-related differences in δ15N and δ13C. Stable isotope Bayesian ellipses, SEAc, did not show trophic overlap among size classes. A significant positive linear correlation was observed between δ15N and δ13C measured in liver and muscle tissues with body size, suggesting an increase of prey size as tuna grew. These results indicate that the observed size-related diet shifts are not only related to prey type, but also to prey size.

Prey availability and feeding ecology of juvenile Chinook (Oncorhynchus tshawytscha) and coho (O. kisutch) salmon in the northern California Current ecosystem, based on stomach content and stable isotope analyses

To understand the factors that regulate early marine survival of juvenile Chinook (Oncorhynchus tshawytscha) and coho (Oncorhynchus kisutch) salmon, it is necessary to characterize their prey availability, selectivity, and dietary niche widths. Currently, no sampling protocol exists that directly measures the salmonid prey field and quantifies dietary niche width via stable isotope analysis (SIA). The main goals of this study were to quantify the dietary niche widths of juvenile salmon and to compare environmental prey community compositions to juvenile salmon diets. Juvenile Chinook and coho salmon and their prey were collected in the northern California Current ecosystem via herring and Nordic trawls during 9–15 June 2010. The catch per unit effort of the salmonid prey field was compared to juvenile salmon stomach contents, and a Manly–Chesson index was used to quantify prey selectivity. Salmon and their prey were measured for δ13C and δ15N values and salmon were analyzed via Stable Isotope Bayesian Ellipses in R. Results indicated that there was no significant difference between juvenile Chinook and coho salmon diets and that they principally consumed rockfish juveniles (Sebastes spp.) and Cancer megalopae. Juvenile salmon consumed somewhat different prey items compared to both herring and Nordic nets and these nets sampled different assemblages of similar prey communities. Though juvenile salmon were in isotopic disequilibrium with their marine prey, both species occupied similar isotopic niche widths. These results demonstrate that SIA provides complementary information to stomach content analysis.

Diet and consumption rate of Atlantic bluefin tuna (Thunnus thynnus) in the Strait of Gibraltar

The diet of Atlantic bluefin tuna (ABFT), Thunnus thynnus, caught in the Strait of Gibraltar area (SoG) from late summer through fall was assessed by stomach content (SCA) and stable isotope (SIA) analyses. Our results show that the SoG provides suitable food resources for sub-adult and adult ABFT. The SCA did not show inter-annual differences, and indicated that crustaceans were the most important prey group (alimentary index, AI=37.47 and AI=47.22 for 2012 and 2013, respectively), followed by fishes (AI=28.71 and AI=25.62, respectively) and cephalopods (AI=1.55 and AI=3.03). Diet compositions inferred from mixing models based on muscle and liver isotopic data showed that squid are an important food resource for the ABFT living in the SoG. Feeding patterns were analysed by the graphical method of Amundsen, which suggested that the ABFT exhibits a varying degree of specialisation on different prey. Thus, while most prey species were unimportant and rare, Sergia robusta can be considered as the dominant prey. The trophic niche width, estimated from SCA as the antilogarithm of Shannon’s entropy, as well as from stable isotope Bayesian ellipses, indicated that the ABFT sampled in 2012 consumed a more diverse diet than those sampled in 2013. Inter-annual differences in daily meal and daily ration were apparent; whereas the estimated daily food intake was 918.91gday−1 in 2012 and 1924.95gday−1 in 2013, the daily ration showed values of 2.52±1.24% body mass (BM) day−1 and 5.84±1.06% BM day−1. Such substantial differences are probably due to differences in water temperatures between the sampling dates of the two years.

Trophic ecology of sea urchins in coral-rocky reef systems, Ecuador

Sea urchins are important grazers and influence reef development in the Eastern Tropical Pacific (ETP). Diadema mexicanum and Eucidaris thouarsii are the most important sea urchins on the Ecuadorian coastal reefs. This study provided a trophic scenario for these two species of echinoids in the coral-rocky reef bottoms of the Ecuadorian coast, using stable isotopes. We evaluated the relative proportion of algal resources assimilated, and trophic niche of the two sea urchins in the most southern coral-rocky reefs of the ETP in two sites with different disturbance level. Bayesian models were used to estimate the contribution of algal sources, niche breadth, and trophic overlap between the two species. The sea urchins behaved as opportunistic feeders, although they showed differential resource assimilation. Eucidaris thouarsii is the dominant species in disturbed environments; likewise, their niche amplitude was broader than that of D. mexicanum when conditions were not optimal. However, there was no niche overlap between the species. The Stable Isotope Analysis in R (SIAR) indicated that both sea urchins shared limiting resources in the disturbed area, mainly Dictyota spp. (contributions of up to 85% for D. mexicanum and up to 75% for E. thouarsii). The Stable Isotope Bayesian Ellipses in R (SIBER) analysis results indicated less interspecific competition in the undisturbed site. Our results suggested a trophic niche partitioning between sympatric sea urchin species in coastal areas of the ETP, but the limitation of resources could lead to trophic overlap and stronger habitat degradation.

Latitudinal changes in the structure of marine food webs in the Southwestern Atlantic Ocean

Food chain length is known to increase with primary productivity, species richness and environmental stability. Primary productivity and species richness decrease poleward at mid and high latitudes in the Southwestern Atlantic, and hence food chain length of coastal ecosystems is expected to decrease poleward, unless seasonal changes in water temperature destabilise food webs at mid latitude. We used stable isotopes of carbon and nitrogen and SIBER (Stable Isotope Bayesian Ellipses in R) to determine the food chain length and other patterns of interconnections (i.e. topology) of coastal food webs from 3 contrasting regions: the temperate estuarine zone associated with the Rio de la Plata plume, the tidal zone off northern and central Patagonia, and the cold estuarine zone off southern Patagonia. Results indicate that food chain length decreases and trophic redundancy increases as latitude increases in all of the compartments of the food web considered. This is in agreement with results emerging from reconstructions of marine food web length during the late Holocene in the same regions, thus suggesting that they are not artefacts caused by the intense exploitation of top predators in the region since European settlement. Furthermore, differences in primary productivity do not fully explain the observed patterns. Instead, we argue that the latitudinal reduction of species diversity reported for the Southwestern Atlantic may be the primary reason for the observed pattern. The reduction in the diversity of forage fishes and squids at high latitude is proposed to result into a wasp-waist ecosystem, characterised by a high trophic redundancy at high trophic levels.

Comparing isotopic niche widths among and within communities: SIBER - Stable Isotope Bayesian Ellipses in R

1. The use of stable isotope data to infer characteristics of community structure and niche width of community members has become increasingly common. Although these developments have provided ecologists with new perspectives, their full impact has been hampered by an inability to statistically compare individual communities using descriptive metrics. 2. We solve these issues by reformulating the metrics in a Bayesian framework. This reformulation takes account of uncertainty in the sampled data and naturally incorporates error arising from the sampling process, propagating it through to the derived metrics. 3. Furthermore, we develop novel multivariate ellipse-based metrics as an alternative to the currently employed Convex Hull methods when applied to single community members. We show that unlike Convex Hulls, the ellipses are unbiased with respect to sample size, and their estimation via Bayesian inference allows robust comparison to be made among data sets comprising different sample sizes. 4. These new metrics, which we call SIBER (Stable Isotope Bayesian Ellipses in R), open up more avenues for direct comparison of isotopic niches across communities. The computational code to calculate the new metrics is implemented in the free-to-download package Stable Isotope Analysis for the R statistical environment.