Applying stable isotopes to examine food-web structure: an overview of analytical tools

Slow recovery in trophic structure of restored wetlands in Northeast China

Restoration measures have been widely implemented in wetland ecosystems globally to bend the curve of biodiversity loss and restore associated ecological functions. However, assessments of the effectiveness of wetland restoration have predominantly focused on the recovery of taxonomic composition, while few studies have assessed the effectiveness of these efforts from a food web perspective. Here, we incorporated stable isotope approach to investigate trophic structure in natural and restored wetlands in Northeast China. The investigated consumers, including zooplankton, macroinvertebrates, and fish, exhibited lower δ15N and higher δ13C values in restored wetlands than in natural wetlands. Natural wetlands exhibited higher trophic positions and a wider range of trophic levels compared to restored wetlands. Primary consumers in natural wetlands relied more on particulate organic matter (POM, 42.9 % ± 24.1 %), while those in restored wetlands were more dependent on substrate organic matter (SOM, 42.3 % ± 23.9 %). Compared to natural wetlands, isotopic richness was significantly lower in restored wetlands, with smaller isotopic variation (SEAs) in basal resources, aquatic invertebrates, and fish. Our findings reveal that the recovery of trophic structures in restored wetlands lags behind that of taxonomic composition. Future restoration efforts should prioritize enhancing habitat heterogeneity and resource availability to support a diverse range of trophic levels. Monitoring trophic dynamics is essential for assessing the progress of wetland restoration and should be integrated into monitoring schemes.

Testing food web theory in a large lake: The role of body size in habitat coupling in Lake Michigan

The landscape theory of food web architecture (LTFWA) describes relationships among body size, trophic position, mobility, and energy channels that serve to couple heterogenous habitats, which in turn promotes long-term system stability. However, empirical tests of the LTFWA are rare and support differs among terrestrial, freshwater, and marine systems. Further, it is unclear whether the theory applies in highly altered ecosystems dominated by introduced species such as the Laurentian Great Lakes. Here, we provide an empirical test of the LTFWA by relating body size, trophic position, and the coupling of different energy channels using stable isotope data from species throughout the Lake Michigan food web. We found that body size was positively related to trophic position, but for a given trophic position, organisms predominately supported by pelagic energy had smaller body sizes than organisms predominately supported by nearshore benthic energy. We also found a hump-shaped trophic relationship in the food web where there is a gradual increase in the coupling of pelagic and nearshore energy channels with larger body sizes as well as higher trophic positions. This highlights the important role of body size and connectivity among habitats in structuring food webs. However, important deviations from expectations are suggestive of how species introductions and other anthropogenic impacts can affect food web structure in large lakes. First, native top predators appear to be flexible couplers that may provide food web resilience, whereas introduced top predators may confer less stability when they specialize on a single energy pathway. Second, some smaller bodied prey fish and invertebrates, in addition to mobile predators, coupled energy from pelagic and nearshore energy channels, which suggests that some prey species may also be important integrators of energy pathways in the system. We conclude that patterns predicted by the LTFWA are present in the face of species introductions and other anthropogenic stressors to a degree, but time-series evaluations are needed to fully understand the mechanisms that promote stability.

The marriage between stable isotope ecology and plant metabolomics - new perspectives for metabolic flux analysis and the interpretation of ecological archives

Even though they share many thematical overlaps, plant metabolomics and stable isotope ecology have been rather separate fields mainly due to different mass spectrometry demands. New high-resolution bioanalytical mass spectrometers are now not only offering high-throughput metabolite identification but are also suitable for compound- and intramolecular position-specific isotope analysis in the natural isotope abundance range. In plant metabolomics, label-free metabolic pathway and metabolic flux analysis might become possible when applying this new technology. This is because changes in the commitment of substrates to particular metabolic pathways and the activation or deactivation of others alter enzyme-specific isotope effects. This leads to differences in intramolecular and compound-specific isotope compositions. In plant isotope ecology, position-specific isotope analysis in plant archives informed by metabolic pathway analysis could be used to reconstruct and separate environmental impacts on complex metabolic processes. A technology-driven linkage between the two disciplines could allow us to extract information on environment-metabolism interaction from plant archives such as tree rings but also within ecosystems. This would contribute to a holistic understanding of how plants react to environmental drivers, thus also providing helpful information on the trajectories of the vegetation under the conditions to come.

Effects of anticoagulants time storage on stable isotope values of crocodilians' blood tissues

Rapid coagulation of reptile blood often hinders its use in studies in remote and difficult-to-access areas, necessitating chemical preservation. Therefore, understanding the potential effects of anticoagulants on the isotopic compositions of blood is essential to avoid issues in interpreting the results for ecological studies. In this study we aimed to verify whether the storage time of the blood tissue in anticoagulants can influence its isotopic compositions of the broad-snouted caiman (Caiman latirostris), an ectothermic top predator from eastern South America. Blood samples were obtained from ten adult females of C. latirostris from a commercial breeding facility in 2015. Samples were stored in vials containing ethylenediaminetetraacetic acid (EDTA) and sodium heparin (SH) and centrifuged after 2 and 8 h to separate red blood cells and plasma. No effect of time was found on the δ13C and δ15N of whole blood, plasma, and red blood cells in contact with the two types of anticoagulants, EDTA and SH. The findings have practical implications for researchers in this field, as they suggest that anticoagulants can be used effectively for at least eight hours under refrigeration.

Trophic guilds and niche segregation among marine megafauna in the Bay of Biscay

The structure and functioning of ecosystems are largely determined by the interactions between species within a biological community. Among these interactions, species exhibiting similar vertical and spatial prey preferences can be identified, thereby belonging to the same trophic guild. Our study explored some trophic characteristics of a diverse megafaunal community (cetaceans, tunas, seabirds) in the Bay of Biscay (BoB). Using stable isotope analysis (SIA), we explored the dietary habits and niche overlap among predators. The degree of isotopic niche overlap was generally low, but with certain species exhibiting large and narrow isotopic niche areas (long-finned pilot whales and Balearic shearwaters, respectively). Our results revealed a diversity of dietary preferences leading to the identification of three distinct trophic guilds based on prey functional groups and spatial preferences: cephalopod feeders (e.g. long-finned pilot whales, Cuvier's beaked whales, striped dolphins), crustacean feeders (e.g. fin whales, albacores), and piscivores (e.g. common dolphins, harbour porpoises, bottlenose dolphins, Atlantic bluefin tunas, Balearic shearwaters). Our findings showed resource partitioning and niche differentiation among the megafaunal community, highlighting the complexity of BoB's marine ecosystem. The insights derived from this study hold important implications for ecosystem management and the implementation of conservation initiatives.

Damming of streams due to the construction of a highway in the Amazon rainforest favors individual trophic specialization in the fish (Bryconops giacopinii)

In Amazonian streams, damming caused by road construction changes the system's hydrological dynamics and biological communities. We tested whether the degree of specialization in fish (Bryconops giacopinii) individuals is higher in pristine stream environments with intact ecological conditions than in streams dammed due to the construction of a highway in the Amazon rainforest. To achieve this, stomach content data and stable isotopes (δ13C and δ15N) in tissues with varying isotopic incorporation rates (liver, muscle, and caudal fin) were used to assess the variation in consumption of different prey over time. The indices within-individual component (WIC)/total niche width (TNW) and individual specialization were employed to compare the degree of individual specialization between pristine and dammed streams. The condition factor and stomach repletion of sampled individuals were used to infer the intensity of intraspecific competition in the investigated streams. The species B. giacopinii, typically considered a trophic generalist, has been shown to be, in fact, a heterogeneous collection of specialist and generalist individuals. Contrary to our expectations, a higher degree of individual specialization was detected in streams dammed by the highway. In dammed streams, where intraspecific competition was more intense, individuals with narrower niches exhibited poorer body conditions than those with broader niches. This suggests that individuals adopting more restricted diets may have lower fitness, indicating that individual specialization may not necessarily be beneficial for individuals. Our results support the notion that intraspecific competition is an important mechanism underlying individual specialization in natural populations. Our results suggest that environmental characteristics (e.g., resource breadth and predictability) and competition for food resources interact in complex ways to determine the degree of individual specialization in natural populations.

Bottom-up processes drive isotopic variation in the South American sea lion Otaria flavescens across a 2300 km latitudinal gradient

Spatial differences in the isotope values of widely distributed marine apex consumers may reflect geographical differences in the isotopic composition of basal resources (e.g., phytoplankton) fueling food webs (bottom-up effects) or spatial differences in the trophic ecology of the taxon of interest (top-down effects). We examined spatial variation in δ13C and δ15N values from 264 South American sea lions (SASL, Otaria flavescens) of different age classes (adults, subadults and juveniles), their putative prey consisting of pelagic and benthic coastal fishes, and particulate organic matter (POM) measured from locations situated across >2300 km of the Chilean coast (between 18°42' and 39°17' S). We used generalized least squares (GLS) models to compare the form of the relationship between δ13C and δ15N and latitude between the three functional groups. Our results show that SASL from northern, central, and southern areas were isotopically distinct, with individuals from the north having lower δ13C and higher δ15N values in comparison to individuals from the south. When the relationship for each functional group was modelled individually using GLS, results indicated that for each degree of increasing latitude δ15N decreased on average by 0.12‰ (POM), 0.15‰ (prey), and 0.14‰ (SASL), while δ13C increased by 0.06‰ (POM) and 0.05‰ in both prey and SASL. We suggest that the latitudinal differences observed in SASL δ13C and δ15N values reflect baseline isotopic variation rather than marked differences in trophic ecology of these widely distributed consumers.

Mercury concentrations and differences in isotopic niches of fish from upstream and downstream of an Amazon reservoir dam

Reservoir construction promotes many environmental impacts, including the enhancement of mercury concentrations in fish. The processes that can influence mercury concentrations in fish in Amazonian reservoirs are still little explored in depth, especially when we consider the possible particularities of the ecosystems in question. This study aims to investigate how mercury concentrations in fish could be influenced by the Tucuruí dam, considering possible changes in their feeding and trophic position according to the dam position (up or downstream). Fish were sampled upstream and downstream of the Tucuruí reservoir, and total mercury (THg) and stable isotopes of carbon and nitrogen (δ15N and δ13C) were measured in muscles. We observed three different Hg bioaccumulation patterns influenced by the dam. These differences occurred due to species trophic niche changes corroborated by the isotope analysis. Higher THg concentrations downstream compared to those upstream ones were only observed for Geophagus proximus. On the contrary, Plagioscion squamosissimus, from downstream, presented lower concentrations than upstream ones. The isotopic niche of these two species presented different changes according to the sampled site. THg biomagnification was higher upstream compared to downstream, considering that the regression slope was approximately two times higher upstream versus downstream. THg concentrations in fish were explained by the differences in their feeding habits according to their location in relation to the dam. The difference in THg biomagnification was able to reflect differences in structure of the food web chain in ecosystems under the dam's influence.

Trophic niche partitioning and intraspecific variation in food resource use in the genus Pangasianodon in a reservoir revealed by stable isotope analysis of multiple tissues

Understanding the mechanism by which non-native fish species integrate into native communities is crucial for evaluating the possibility of their establishment success. The genus Pangasianodon, comprising Pangasianodon gigas and Pangasianodon hypophthalmus, has been introduced into reservoirs, which are non-native habitats, for fishery stock enhancement. P. gigas and P. hypophthalmus often successfully establish and co-occur in several Thai reservoirs, but there is little information on differences in food resource use between the two species. To investigate the trophic niche width of P. gigas and P. hypophthalmus in a Thai reservoir, we conducted stable carbon and nitrogen ratio (δ13C and δ15N) analyses. We examined the degree of individual specialization in both species using the δ13C and δ15N values of muscle and liver tissues, which provides long- and short-term diet information. The isotopic niches did not overlap between P. gigas and P. hypophthalmus. The δ15N value of P. gigas was significantly higher than that of P. hypophthalmus, whereas the δ13C value did not significantly differ between the two species. The isotopic niche sizes were larger in P. hypophthalmus than in P. gigas. Individual specialization was observed in P. hypophthalmus but not in P. gigas, indicating that intraspecific variation in food resource use was larger in P. hypophthalmus compared to P. gigas. These findings suggest that trophic niche partitioning was one of the factors facilitating the establishment success of P. gigas and P. hypophthalmus in a reservoir, but the establishment process may differ between the two species.

Unveiling mercury levels: Trophic habits influence on bioaccumulation in two Otariid species

Mercury, a toxic metal released by various human activities, exerts environmental stress through its bioaccumulation and biomagnification, particularly in marine habitats. South American fur seals (Arctocephalus australis) and sea lions (Otaria flavescens) reproduce on the Atlantic coast of Uruguay. As top predators, they can accumulate toxic levels of mercury and are often used as sentinel species for monitoring ecosystem health. Fur seals prey on pelagic species, such as fish and squid, while sea lions consume coastal-benthic prey. We analyzed the total mercury concentration (THg) in hair and the trophic habits (δ13C and δ15N) of females from both species. The average THg concentration in adult female sea lions (30.5 ± 9.3 μg/g dry weight) was significantly higher than in fur seals (6.3 ± 2.5 μg/g dry weight). Additionally, the mean δ15N and δ13C values were significantly higher in sea lion (δ15N: 19.2 ± 0.6‰, δ13C: -13.8 ± 0.2‰) compared to fur seals (δ15N: 16.5 ± 0.5‰, δ13C: -15.5 ± 0.6‰). Our results suggest that different trophic levels and feedings areas affect the THg concentration in Uruguayan Otariids. Notably, at the intraspecific level, the THg concentration in sea lions increased with δ13C values, suggesting a link to coastal feeding habits. This indicates that coastal feeding behaviors, compared to feeding in pelagic environments, enhance mercury bioaccumulation in Otariids along the Uruguayan coast, with the discharge of freshwater from the Río de la Plata (one of the largest estuaries in South America) basin identified as a potential mercury source. THg concentrations found in female sea lion hair are the highest reported in Otariids globally. Mercury levels exceeded toxic thresholds observed in other mammals and could pose significant health risks. Our findings may explain why sea lions were particularly affected by the avian influenza outbreak in Uruguay compared to fur seals. Monitoring the declining sea lion population is crucial, making our results significant for integrated conservation and management strategies.

The path to scientifically sound biodiversity valuation in the context of the Global Biodiversity Framework

Successful implementation of the Kunming-Montreal Global Biodiversity Framework requires identifying a process for measuring and valuing changes in biodiversity that build on the recognition that economics and valuation must play a key role in "halting and reversing" biodiversity loss. Here, we discuss considerations for a practical path to valuing changes in biodiversity. Framing changes in the value of biodiversity as a summary of changes in certain natural assets enables leveraging existing approaches and international standards associated with environmental-economic accounting. We discuss why an approach that builds from individual species, evolutionary groups, or functional groups into a practical, hierarchical statistical classification system is better than the development of any one biodiversity index. We merge techniques from ecology and other natural sciences, national and environmental-economic accounting, and economics, which are all on the cusp of making measurement of the change in the value of biodiversity possible. The focus should be on scaling and integrating these approaches. The path forward appears to begin with imperfect but useful measures, grounded in robust concepts, while establishing ambition to further scale-up measurements-just like the past evolution of many other official statistical series.

Diet and between-tissue isotope comparisons reveal different foraging strategies for age and sex of a Saffron Finch (Sicalis flaveola Linnaeus, 1766) population

Measuring stable isotopes in different tissues offers the opportunity to provide insight into the foraging ecology of a species. This study aimed to assess how diet varies between yellow females, yellow males, and dull individuals of a Saffron Finch (Sicalis flaveola) population. We measured δ13C and δ15N in blood over a year, and in different feathers, to estimate seasonal consistency of resource use for each category. We conducted this study in a private farm in the Central Brazilian savannas. We sampled 195 individuals in seven field samplings between January 2017 and March 2018. The mean blood δ13C values were similar among yellow females, yellow males and dull individuals, indicating that this population of Saffron Finch predominantly accesses similar resources throughout the year, with a predominant C4 signal. Although Saffron Finch is considered a granivorous species, the mean δ15N values found indicate that both adults and juveniles also incorporate in their tissues some invertebrate. The slight isotope-tissue difference between feathers and blood is similar to the reported in previous studies and may reflect tissue-to-tissue discrimination. The isotopic space of yellow males was greater than that of yellow females and dull individuals, indicating greater dietary diversity due to greater inter-individual variation in diet. In Saffron Finch, which delays plumage maturation, competition-driven partitioning of food resources seems essential in driving carotenoid-based plumage coloration between age classes and sexes.

Compound-specific nitrogen isotope of amino acids: Toward an improved understanding of mercury trophic transfer in different habitats

Herein, we investigated the trophic transfer of mercury (Hg) through food chains in different habitats (namely aquatic, riparian, and terrestrial) through bulk stable isotope analysis of nitrogen (δ15Nbulk) and compound-specific isotope analysis of nitrogen in amino acids (δ15NAA) using bird feathers and their potential food sources from a Hg-contaminated site in southwest China. Results showed similar δ15Nphe for water birds (4.7 ± 2.6 ‰) and aquatic food sources (5.2 ± 2.1 ‰) and for land-based food sources (10.1 ± 0.4 ‰) and terrestrial birds (11.6 ± 3.0 ‰), verifying δ15Nphe as a potential discriminant indicator for different food sources. The trophic positions (TPs) of most organisms based on δ15Nbulk (TPbulk) tended to overestimate compared with those based on δ15NAA (TPAA), especially for predators (such as kingfisher: ΔTP = 1.3). Additionally, significant differences were observed in the aquatic, riparian, and terrestrial food webs between trophic magnification slope (TMS)bulk and TMSAA (p < 0.05). The trophic magnification factor (TMF)AA-multiple based on multiple-AAs in three food webs were higher than the TMFAA and TMFbulk, probably because of the greater variation of δ15Nbaseline, complex food sources or the notably different in individual organisms. Altogether, our results improve the understanding of Hg trophic transfer in aquatic, riparian, and terrestrial food webs.

Assessing the role of non-native species and artificial water bodies on the trophic and functional niche of Mediterranean freshwater fish communities

Habitat alterations and the introduction of non-native species have many ecological impacts, including the loss of biodiversity and a deterioration of ecosystem functioning. The effects of these combined stressors on the community trophic web and functional niche are, however, not completely clear. Here, we investigated how artificial ecosystems (i.e. reservoirs) and non-native species may influence the trophic and functional niche space of freshwater fish communities. To do so, we used carbon and nitrogen stable isotope and abundance data to compute a set of isotopic, trait, and functional metrics for 13 fish communities sampled from 12 distinct ecosystems in Türkiye. We show that in reservoirs, fish were more similar in their trophic niche compared to lakes, where the trophic niche was more variable, due to higher habitat complexity. However, there were no differences in the trait and functional metrics between the two ecosystem types, suggesting a higher prey diversity than assumed in reservoirs. We also found that the number of non-native species did not affect the trophic niche space, nor the trait or functional space occupied by the fish community. This indicates that non-native species tended to overlap their trophic niche with native species, while occupying empty functional niches in the recipient community functional space. Similarly, the proportion of non-native species did not affect any trophic, trait, or functional metric, suggesting that changes in community composition were not reflected in changes in the community niche space. Moreover, we found that trait richness, but not functional richness, was positively related to the isotopic niche width and diversity, indicating that a wider occupied trait niche space corresponded with a wider occupied trophic niche and lesser interspecific similarity. Our findings underscore the complexity of ecological relationships within freshwater ecosystems and highlight the need for comprehensive management strategies to mitigate the impacts of human activities and biological invasions.

Untangling the complex food webs of tropical rainforest streams

Food webs depict the tangled web of trophic interactions associated with the functioning of an ecosystem. Understanding the mechanisms providing stability to these food webs is therefore vital for conservation efforts and the management of natural systems. Here, we first characterised a tropical stream meta-food web and five individual food webs using a Bayesian Hierarchical approach unifying three sources of information (gut content analysis, literature compilation and stable isotope data). With data on population-level biomass and individually measured body mass, we applied a bioenergetic model and assessed food web stability using a Lotka-Volterra system of equations. We then assessed the resilience of the system to individual species extinctions using simulations and investigated the network patterns associated with systems with higher stability. The model resulted in a stable meta-food web with 307 links among the 61 components. At the regional scale, 70% of the total energy flow occurred through a set of 10 taxa with large variation in body masses. The remaining 30% of total energy flow relied on 48 different taxa, supporting a significant dependency on a diverse community. The meta-food web was stable against individual species extinctions, with a higher resilience in food webs harbouring omnivorous fish species able to connect multiple food web compartments via weak, non-specialised interactions. Moreover, these fish species contributed largely to the spatial variation among individual food webs, suggesting that these species could operate as mobile predators connecting different streams and stabilising variability at the regional scale. Our results outline two key mechanisms of food web stability operating in tropical streams: (i) the diversity of species and body masses buffering against random and size-dependent disturbances and (ii) high regional diversity and weak omnivorous interactions of predators buffering against local stochastic variation in species composition. These mechanisms rely on high local and regional biodiversity in tropical streams, which is known to be strongly affected by human impacts. Therefore, an urgent challenge is to understand how the ongoing systematic loss of diversity jeopardises the stability of stream food webs in human-impacted landscapes.

Rose or Red, but Still under Threat: Comparing Microplastics Ingestion between Two Sympatric Marine Crustacean Species (Aristaeomorpha foliacea and Parapenaeus longirostris)

Increasing plastic contamination poses a serious threat to marine organisms. Microplastics (MPs) ingestion can represent a risk for the organism itself and for the ultimate consumer. Through the analysis of the gastrointestinal tract, coupled with stable isotope analysis on the muscle tissue, this study provides insights into the relationship between MPs pollution and ecology in two commercial marine species caught in the Central Tyrrhenian Sea: Aristaeomorpha foliacea and Parapenaeus longirostris. Stable isotope analysis was conducted to determine the trophic position and the trophic niche width. The gastrointestinal tracts were processed, and the resultant MPs were analysed under FT-IR spectroscopy to estimate the occurrence, abundance, and typology of the ingested MPs. The trophic level of the species was similar (P. longirostris TP = 3 ± 0.10 and A. foliacea TP = 3.1 ± 0.08), with an important trophic niche overlap, where 38% and 52% of P. longirostris and A. foliacea has ingested MPs, respectively. Though species-level differences may not be evident regarding MP's abundance per individual, a high degree of dissimilarity was noted in the typologies of ingested particles. This research provides valuable insights into how MPs enter marine trophic webs, stressing that isotopic niche analysis should be combined with other methods to explain in detail the differences in MPs ingestion.

Insights on long-term ecosystem changes from stable isotopes in historical squid beaks

BACKGROUND

Assessing the historical dynamics of key food web components is crucial to understand how climate change impacts the structure of Arctic marine ecosystems. Most retrospective stable isotopic studies to date assessed potential ecosystem shifts in the Arctic using vertebrate top predators and filter-feeding invertebrates as proxies. However, due to long life histories and specific ecologies, ecosystem shifts are not always detectable when using these taxa. Moreover, there are currently no retrospective stable isotopic studies on various other ecological and taxonomic groups of Arctic biota. To test whether climate-driven shifts in marine ecosystems are reflected in the ecology of short-living mesopredators, ontogenetic changes in stable isotope signatures in chitinous hard body structures were analysed in two abundant squids (Gonatus fabricii and Todarodes sagittatus) from the low latitude Arctic and adjacent waters, collected between 1844 and 2023.

RESULTS

We detected a temporal increase in diet and habitat-use generalism (= opportunistic choice rather than specialization), trophic position and niche width in G. fabricii from the low latitude Arctic waters. These shifts in trophic ecology matched with the Atlantification of the Arctic ecosystems, which includes increased generalization of food webs and higher primary production, and the influx of boreal species from the North Atlantic as a result of climate change. The Atlantification is especially marked since the late 1990s/early 2000s. The temporal patterns we found in G. fabricii's trophic ecology were largely unreported in previous Arctic retrospective isotopic ecology studies. Accordingly, T. sagittatus that occur nowadays in the high latitude North Atlantic have a more generalist diet than in the XIXth century.

CONCLUSIONS

Our results suggest that abundant opportunistic mesopredators with short life cycles (such as squids) are good candidates for retrospective ecology studies in the marine ecosystems, and to identify ecosystem shifts driven by climate change. Enhanced generalization of Arctic food webs is reflected in increased diet generalism and niche width in squids, while increased abundance of boreal piscivorous fishes is reflected in squids' increased trophic position. These findings support opportunism and adaptability in squids, which renders them as potential winners of short-term shifts in Arctic ecosystems.

Unraveling emergent network indeterminacy in complex ecosystems: A random matrix approach

Indeterminacy of ecological networks-the unpredictability of ecosystem responses to persistent perturbations-is an emergent property of indirect effects a species has on another through interaction chains. Thus, numerous indirect pathways in large, complex ecological communities could make forecasting the long-term outcomes of environmental changes challenging. However, a comprehensive understanding of ecological structures causing indeterminacy has not yet been reached. Here, using random matrix theory (RMT), we provide mathematical criteria determining whether network indeterminacy emerges across various ecological communities. Our analytical and simulation results show that indeterminacy intricately depends on the characteristics of species interaction. Specifically, contrary to conventional wisdom, network indeterminacy is unlikely to emerge in large competitive and mutualistic communities, while it is a common feature in top-down regulated food webs. Furthermore, we found that predictable and unpredictable perturbations can coexist in the same community and that indeterminate responses to environmental changes arise more frequently in networks where predator-prey relationships predominate than competitive and mutualistic ones. These findings highlight the importance of elucidating direct species relationships and analyzing them with an RMT perspective on two fronts: It aids in 1) determining whether the network's responses to environmental changes are ultimately indeterminate and 2) identifying the types of perturbations causing less predictable outcomes in a complex ecosystem. In addition, our framework should apply to the inverse problem of network identification, i.e., determining whether observed responses to sustained perturbations can reconstruct their proximate causalities, potentially impacting other fields such as microbial and medical sciences.

Challenging trophic position assessments in complex ecosystems: Calculation method, choice of baseline, trophic enrichment factors, season and feeding guild do matter: A case study from Marquesas Islands coral reefs

Assessments of ecosystem functioning are a fundamental ecological challenge and an essential foundation for ecosystem-based management. Species trophic position (TP) is essential to characterize food web architecture. However, despite the intuitive nature of the concept, empirically estimating TP is a challenging task due to the complexity of trophic interaction networks. Various methods are proposed to assess TPs, including using different sources of organic matter at the base of the food web (the 'baseline'). However, it is often not clear which methodological approach and which baseline choices are the most reliable. Using an ecosystem-wide assessment of a tropical reef (Marquesas Islands, with available data for 70 coral reef invertebrate and fish species), we tested whether different commonly used TP estimation methods yield similar results and, if not, whether it is possible to identify the most reliable method. We found significant differences in TP estimates of up to 1.7 TPs for the same species, depending on the method and the baseline used. When using bulk stable isotope data, the choice of the baseline significantly impacted TP values. Indeed, while nitrogen stable isotope (δ15N) values of macroalgae led to consistent TP estimates, those using phytoplankton generated unrealistically low TP estimates. The use of a conventional enrichment factor (i.e. 3.4‰) or a 'variable' enrichment factor (i.e. according to feeding guilds) also produced clear discrepancies between TP estimates. TPs obtained with δ15N values of source amino acids (compound-specific isotope analysis) were close to those assessed with macroalgae. An opposite seasonal pattern was found, with significantly lower TPs in winter than in summer for most species, with particularly pronounced differences for lower TP species. We use the observed differences to discuss possible drivers of the diverging TP estimates and the potential ecological implications.

Individual diet variability shapes the architecture of Antarctic benthic food webs

Antarctic biodiversity is affected by seasonal sea-ice dynamics driving basal resource availability. To (1) determine the role of intraspecific dietary variability in structuring benthic food webs sustaining Antarctic biodiversity, and (2) understand how food webs and the position of topologically central species vary with sea-ice cover, single benthic individuals' diets were studied by isotopic analysis before sea-ice breakup and afterwards. Isotopic trophospecies (or Isotopic Trophic Units) were investigated and food webs reconstructed using Bayesian Mixing Models. As nodes, these webs used either ITUs regardless of their taxonomic membership (ITU-webs) or ITUs assigned to species (population-webs). Both were compared to taxonomic-webs based on taxa and their mean isotopic values. Higher resource availability after sea-ice breakup led to simpler community structure, with lower connectance and linkage density. Intra-population diet variability and compartmentalisation were crucial in determining community structure, showing population-webs to be more complex, stable and robust to biodiversity loss than taxonomic-webs. The core web, representing the minimal community 'skeleton' that expands opportunistically while maintaining web stability with changing resource availability, was also identified. Central nodes included the sea-urchin Sterechinus neumayeri and the bivalve Adamussium colbecki, whose diet is described in unprecedented detail. The core web, compartmentalisation and topologically central nodes represent crucial factors underlying Antarctica's rich benthic food web persistence.

New uses for ancient middens: bridging ecological and evolutionary perspectives

Rodent middens provide a fine-scale spatiotemporal record of plant and animal communities over the late Quaternary. In the Americas, middens have offered insight into biotic responses to past environmental changes and historical factors influencing the distribution and diversity of species. However, few studies have used middens to investigate genetic or ecosystem level responses. Integrating midden studies with neoecology and experimental evolution can help address these gaps and test mechanisms underlying eco-evolutionary patterns across biological and spatiotemporal scales. Fully realizing the potential of middens to answer cross-cutting ecological and evolutionary questions and inform conservation goals in the Anthropocene will require a collaborative research community to exploit existing midden archives and mount new campaigns to leverage midden records globally.

Amino acid-specific isotopes reveal changing five-dimensional niche segregation in Pacific seabirds over 50 years

Hutchison's niche theory suggests that coexisting competing species occupy non-overlapping hypervolumes, which are theoretical spaces encompassing more than three dimensions, within an n-dimensional space. The analysis of multiple stable isotopes can be used to test these ideas where each isotope can be considered a dimension of niche space. These hypervolumes may change over time in response to variation in behaviour or habitat, within or among species, consequently changing the niche space itself. Here, we use isotopic values of carbon and nitrogen of ten amino acids, as well as sulphur isotopic values, to produce multi-isotope models to examine niche segregation among an assemblage of five coexisting seabird species (ancient murrelet Synthliboramphus antiquus, double-crested cormorant Phalacrocorax auritus, Leach's storm-petrel Oceanodrama leucorhoa, rhinoceros auklet Cerorhinca monocerata, pelagic cormorant Phalacrocorax pelagicus) that inhabit coastal British Columbia. When only one or two isotope dimensions were considered, the five species overlapped considerably, but segregation increased in more dimensions, but often in complex ways. Thus, each of the five species occupied their own isotopic hypervolume (niche), but that became apparent only when factoring the increased information from sulphur and amino acid specific isotope values, rather than just relying on proxies of δ15N and δ13C alone. For cormorants, there was reduction of niche size for both species consistent with a decline in their dominant prey, Pacific herring Clupea pallasii, from 1970 to 2006. Consistent with niche theory, cormorant species showed segregation across time, with the double-crested demonstrating a marked change in diet in response to prey shifts in a higher dimensional space. In brief, incorporating multiple isotopes (sulfur, PC1 of δ15N [baselines], PC2 of δ15N [trophic position], PC1 and PC2 of δ13C) metrics allowed us to infer changes and differences in food web topology that were not apparent from classic carbon-nitrogen biplots.

Mercury bioaccumulation and its relationship with trophic biomarkers in a Mediterranean elasmobranch mesopredator

Human activity has led to increased concentrations of mercury (Hg) in the world's oceans. Mercury can bioaccumulate and biomagnify in animal tissues via trophic transfer, thus, becoming most pronounced in larger and older predators. Here, we measured Hg concentrations and their relationship with stable isotopes-based proxies of trophic level (δ13C and δ15N values) in multiple tissues of Mustelus spp. from the Mediterranean Sea. We found higher Hg concentrations in muscle than in liver and fin tissues. The relationship between Hg concentrations and δ15N values in muscle suggested repeated foraging for low trophic level and Hg-poor prey, and biomagnification of Hg at higher trophic levels. Seasonal variations in δ13C values could indicate shifts in primary production sources and/or in local prey availability. The HBVSe index suggested no risk to human health, however the safe meal limit recommendations are 4.5 and 2.2 portions per month for adults and children, respectively.

Feeding behavior of yellowfin tuna around two insular regions of the western Atlantic Ocean

The yellowfin tuna is a very abundant tropical tuna species in the western equatorial Atlantic Ocean and an important fishery resource for the Brazilian tuna fleet. In this study we performed stable isotope analysis to better understand the spatial trophodynamics and dietary changes in yellowfin tuna around two insular marine protected areas in Brazil. A total of 65 yellowfin tuna specimens measuring between 47 and 138 cm LT (total length) were sampled around the archipelagos of Fernando de Noronha (FNA; n = 34) and Saint Peter and Saint Paul (SPSPA; n = 31) between July 2018 and September 2019. Bayesian mixing models and generalized additive models were used to investigate the contributions of four different prey items (zooplankton, cephalopods, fish larvae, and flying fish) to yellowfin tuna diet in each area and their potential changes in relation to predator growth. The four prey items were found to have different overall contributions between the two studied areas, with zooplankton being the most important prey in FNA, whereas flying fish was the most relevant prey to the species' diet in SPSPA. Significant changes in the species diet by size were also found, with fish smaller than 90 cm (TL) having a more generalist diet and larger animals relying more on consuming larger and more nutritious prey (i.e., flying fish). Our results suggest that these two marine protected areas play an important role in ocean dynamics, providing important and different foraging grounds for the development of this predator species.

Ecological interruption on food web dynamics by eutrophic water discharge from the world's longest dike at Saemangeum, Yellow Sea

The man-made sea dike has disrupted the natural link between riverine and marine ecosystems and caused eutrophication within the aquatic ecosystem. The eutrophic water discharge has also raised concerns. As a representative tidal flat with the longest dike in the world, Saemangeum has experienced the problem of eutrophication. To elucidate the discharge water effects on the benthic food web dynamics, a four-year round sampling was conducted in/outside of the Saemangeum sea dike. Stable isotope analysis was applied to benthos (a total of 54 species) and their potential diets. Water discharge tripled in period II (2021-2022) compared to the period I (2019-2020). However, there were no significant impact changes in food web structure between the two periods due to improved lake water quality in period II. A positive correlation of nutrient concentration between the inner and outer areas of the dike revealed a direct effect of the water discharge on the outer tidal flat. The water discharge altered the spatial environmental conditions and the food web structure of the outer tidal flat. High TN concentrations stimulated the biomass of microphytobenthos (MPB) near the water gates, which in turn increased MPB consumption by benthos, demonstrating the in/direct impacts of water discharge on the food web. Furthermore, filter feeders exhibited a more sensitive response to spatial organic matter distribution compared to deposit feeders in diet utilization. Overall, our novel findings on food web dynamics in a representative tidal flat with artificial structures emphasize the necessity of continuous monitoring to ensure the sustainability of coastal ecosystems.

Interpopulation trophic niches and ontogenetic shifts of a mangrove fish predator

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 δ13 C and δ15 N 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.

Hurricane disturbance drives trophic changes in neotropical mountain stream food webs

Food webs are complex ecological networks that reveal species interactions and energy flow in ecosystems. Prevailing ecological knowledge on forested streams suggests that their food webs are based on allochthonous carbon, driven by a constant supply of organic matter from adjacent vegetation and limited primary production due to low light conditions. Extreme climatic disturbances can disrupt these natural ecosystem dynamics by altering resource availability, which leads to changes in food web structure and functioning. Here, we quantify the response of stream food webs to two major hurricanes (Irma and María, Category 5 and 4, respectively) that struck Puerto Rico in September 2017. Within two tropical forested streams (first and second order), we collected ecosystem and food web data 6 months prior to the hurricanes and 2, 9, and 18 months afterward. We assessed the structural (e.g., canopy) and hydrological (e.g., discharge) characteristics of the ecosystem and monitored changes in basal resources (i.e., algae, biofilm, and leaf litter), consumers (e.g., aquatic invertebrates, riparian consumers), and applied Layman's community-wide metrics using the isotopic composition of 13 C and 15 N. Continuous stream discharge measurements indicated that the hurricanes did not cause an extreme hydrological event. However, the sixfold increase in canopy openness and associated changes in litter input appeared to trigger an increase in primary production. These food webs were primarily based on terrestrially derived carbon before the hurricanes, but most taxa (including Atya and Xiphocaris shrimp, the consumers with highest biomass) shifted their food source to autochthonous carbon within 2 months of the hurricanes. We also found evidence that the hurricanes dramatically altered the structure of the food web, resulting in shorter (i.e., smaller food-chain length), narrower (i.e., lower diversity of carbon sources) food webs, as well as increased trophic species packing. This study demonstrates how hurricane disturbance can alter stream food webs, changing the trophic base from allochthonous to autochthonous resources via changes in the physical environment (i.e., canopy defoliation). As hurricanes become more frequent and severe due to climate change, our findings greatly contribute to our understanding of the mechanisms that maintain forested stream trophic interactions amidst global change.

Assessment of the impact of dams on aquatic food webs using stable isotopes: Current progress and future challenges

Dams have disrupted natural river systems worldwide and although population and community level effects on aquatic biota have been well documented, food web responses remain poorly understood and difficult to characterize. The application of stable isotope analysis (SIA) provides a means to assess the effect of dams on food webs. Here we review the effect of dams on aquatic food webs using SIA, aiming to detect knowledge gaps in the field of dam impacts on aquatic food webs and propose a conceptual framework to help formulate hypotheses about dam impacts on food webs guided by food web theory. Dams can affect aquatic food webs via two pathways: a bottom-up pathway with altered basal food sources and their transfer to consumers through changes in flow, nutrients, temperature and sediment, and a top-down pathway with consumer species composition altered mainly through habitat fragmentation and related physiochemical changes. Taking these mechanisms into consideration, the impact of dams on food web attributes derived from SIA was evaluated. These studies generally apply mixing models to determine how dams alter the dominant carbon sources supporting food webs, use δ15N to examine how dams alter food-chain length, or use Layman metrics of isotope variability to assess niche changes for invertebrate and fish assemblages. Most studies compare the patterns of SIA metrics spatially (e.g. upstream vs reservoir vs downstream of dams; regulated vs unregulated rivers) and temporally (before vs after dam construction), without explicit hypotheses and/or links to theoretical concepts of food webs. We propose several steps to make SIA studies of dam impacts more rigorous and enhance their potential for producing novel insights. Future studies should quantify the shape and strength of the effect of dams on SIA-measured food web response, be conducted at larger temporal and spatial scales (particularly along the river longitudinal continuum and the lateral connected ecosystems (e.g., floodplains)), and consider effects of dams on food web resilience and tipping points.

The fate of nitrogen in the Zarin-Gol River receiving trout farm effluent

This study investigated the Zarrin-Gol River ecosystem in Iran to trace organic matter in the food web and evaluate the impact of aquaculture farm effluent using stable isotopes of nitrogen (δ15N) and carbon (δ13C). Using a previously-developed model (Islam 2005), we estimated that a trout farm in the vicinity released 1.4 tons of nitrogen into the river. This was comparable to an estimated total nutrient load of 2.1 tons of nitrogen for the six-month fish-rearing period based on a web-based constituent load estimator (LOADEST). A model estimate of river nitrogen concentration at the time of minimum river discharge (100 L/s) was 2.74 mg/L. Despite relatively high nitrogen loading from the farm, isotope data showed typical food web structure. Several biological groups had elevated δ13C or δ15N values, but there was limited evidence for the entry of organic matter from the trout farm into the food web, with sites above and below trout farms having inconsistent patterns in 15N enrichment. By coupling nitrogen load modeling with stable isotope analysis we showed that stable isotopes might not be effective tracers of organic matter into food webs, depending on surrounding land use and other point sources of nutrients. The Zarrin-Gol River ecosystem, like other basins with high human population density, remains vulnerable to eutrophication in part due to trout farm effluent.

Climate change-informed dietary modeling in Pacific cod: Experimentally-derived effects of temperature and dietary quality on carbon and nitrogen stable isotope trophic discrimination factors

Stable isotope analysis is a powerful tool for dietary modeling and trophic ecology research. A crucial piece of information for isotopic dietary modeling is the accurate estimation of trophic discrimination factors (TDFs), or the isotopic offset between a consumer's tissue and its diet. In order to parameterize stable isotope dietary models for future climate scenarios, we investigated the effect of water temperature and dietary protein and lipid content on TDFs in juvenile Pacific cod (Gadus macrocephalus). Pacific cod are a commercially and ecologically important species, with stock numbers in the northeast Pacific recently having dropped by more than 70%. We tested four water temperatures (6, 8, 10, and 12°C) and two dietary regimens (low and high lipid content), representing a range of potential ocean temperature and prey quality scenarios, in order to determine carbon and nitrogen TDFs in juvenile Pacific cod. Additionally, we assessed dietary intake and proximate composition of the experimental fish in order to estimate consumption, assimilation, and retention of dietary nutrients. The results of this study suggest that dietary protein catabolism is a primary driver of nitrogen TDF variability in juvenile Pacific cod. Across all temperature treatments from 6 to 12°C, fish reared on the lower quality, lower lipid content diet had higher nitrogen TDFs. The mean TDFs for fish raised on the higher lipid, lower protein diet were +3.40 ‰ for nitrogen (Δ15N) and +0.36 ‰ for lipid-corrected carbon (Δ LC 13C). The mean TDFs for fish raised on the lower lipid, higher protein diet were +4.09 ‰ for nitrogen (Δ15N) and 0.00 ‰ for lipid-corrected carbon (Δ LC 13C). Lipid-corrected carbon isotope data showed that, regardless of temperature, fish consuming the lower lipid diet had essentially no trophic discrimination between diet and bulk tissues. We found no ecologically meaningful differences in TDFs due to water temperature across the 6°experimental range. The results of this experiment demonstrate that dietary quality, and more specifically the use of dietary protein for energetic needs, is a primary driver of trophic discrimination factors. The TDFs determined in this study can be applied to understanding trophic ecology in Pacific cod and closely related species under rapidly changing prey availability and ocean temperature conditions.

Biological invasions alter the structure of a tropical freshwater food web

Biological invasions are expected to alter food web structure, but there are limited empirical data directly comparing invaded versus uninvaded food webs, particularly in species-rich, tropical systems. We characterize for the first time the food web of Lake Gatun-a diverse and highly invaded tropical freshwater lake within the Panama Canal. We used stable isotope analysis to reconstruct the trophic structure of the fish community of Lake Gatun and to compare it to that of a minimally invaded reference lake, Lake Bayano. We found significant differences between the trophic structures of these two Neotropical lakes, notably that Lake Gatun's fish community was characterized by a longer food chain, greater isotopic diversity, a broader range of trophic positions and body sizes, and shifts in the isotopic positions of several native taxa relative to Lake Bayano. The degree of isotopic overlap between native and non-native trophic guilds in Lake Gatun was variable, with herbivores exhibiting the lowest (20%-29%) overlap and carnivores the greatest (81%-100%). Overall, our results provide some of the first empirical evidence for the ways in which multiple introduced and native species may partition isotopic space in a species-rich tropical freshwater food web.

Spatial and temporal variations in trophic structure of fish assemblages in the Yellow Sea revealed by C and N stable isotopes

In this study, we assessed spatial and temporal variations in the trophic structure of fish assemblages in the Yellow Sea during spring and summer 2022 and compared their isotopic niches between the Provisional Measure Zone (PMZ) and Korea's west areas (non-PMZ) within the Yellow Sea. Spatial and temporal differences in the diversity and dominant species of fish assemblages were found between the PMZ and non-PMZ areas between the seasons. The mean δ13C values of fish assemblages were relatively higher in the non-PMZ areas than in the PMZ areas. In contrast, no significant differences were found in the mean δ15N values between the areas. Generally, the isotopic niche indices were relatively narrow in the PMZ areas compared to those in the non-PMZ areas. Overall, these spatial differences between the PMZ and non-PMZ areas suggest different trophic diversity of fish assemblages, resulting from site-specific variations in environmental conditions and community composition.

Reconstructing Pleistocene Australian herbivore megafauna diet using calcium and strontium isotopes

Isotopes in fossil tooth enamel provide robust tools for reconstructing food webs, which have been understudied in Australian megafauna. To delineate the isotopic composition of primary consumers and understand dietary behaviour at the base of the food web, we investigate calcium (Ca) and strontium (Sr) isotope compositions of Pleistocene marsupial herbivores from Wellington Caves and Bingara (New South Wales, Australia). Sr isotopes suggest small home ranges across giant and smaller marsupial herbivores. Ca isotopes in Pleistocene marsupial herbivores cover the same range as those in modern wombats and placental herbivores. Early forming teeth are depleted in heavy Ca isotopes compared to late-forming teeth of a given individual, suggesting a weaning signal. Distinct Ca compositions between taxa can be interpreted as dietary niches. Some niches conform to previous dietary reconstructions of taxa, while others provide new insights into niche differentiation across Australian herbivores. Combined with the small roaming ranges suggested by Sr isotopes, the Ca isotope niche diversity suggests rich ecosystems, supporting a diversity of taxa with various diets in a small area.

Trophic Transfer of Halogenated Organic Pollutants in a Wetland Food Web: Insights from Compound-Specific Nitrogen Isotope of Amino Acids and Food Source Analysis

A trophic position (TP) model (TPmix model) that simultaneously considered trophic discrimination factor and βGlu/Phe variations was developed in this study and was first applied to investigate the trophic transfer of halogenated organic pollutants (HOPs) in wetland food webs. The TPmix model characterized the structure of the wetland food web more accurately and significantly improved the reliability of TMF compared to the TPbulk, TPAAs, and TPsimmr models, which were calculated based on the methods of stable nitrogen isotope analysis of bulk, traditional AAs-N-CSIA, and weighted βGlu/Phe, respectively. Food source analysis revealed three interlocking food webs (kingfisher, crab, and frogs) in this wetland. The highest HOP biomagnification capacities (TMFmix) were found in the kingfisher food web (0.24-82.0), followed by the frog (0.08-34.0) and crab (0.56-11.7) food webs. The parabolic trends of TMFmix across combinations of log KOW in the frog food web were distinct from those of aquatic food webs (kingfisher and crab), which may be related to differences in food web composition and HOP bioaccumulation behaviors between aquatic and terrestrial organisms. This study provides a new tool to accurately study the trophic transfer of contaminants in wetlands and terrestrial food webs with diverse species and complex feeding relationships.

Water-Level Fluctuation Control of the Trophic Structure of a Yangtze River Oxbow

Seasonal water-level fluctuations can profoundly impact nutrient dynamics in aquatic ecosystems, influencing trophic structures and overall ecosystem functions. The Tian-e-Zhou Oxbow of the Yangtze River is China's first ex situ reserve and the world's first successful case of ex situ conservation for cetaceans. In order to better protect the Yangtze finless porpoise, the effects of water-level fluctuations on the trophic structure in this oxbow cannot be ignored. Therefore, we employed stable isotope analysis to investigate the changes in the trophic position, trophic niche, and contribution of basal food sources to fish during the wet and dry seasons of 2021-2022. The research results indicate that based on stable isotope analysis of the trophic levels of different dietary fish species, fish trophic levels during the wet season were generally higher than those during the dry season, but the difference was not significant (p > 0.05). Fish communities in the Tian-e-Zhou Oxbow exhibited broader trophic niche space and lower trophic redundancy during the wet season (p < 0.05), indicating a more complex and stable food web structure. In both the wet and dry seasons, fish in the oxbow primarily relied on endogenous carbon sources, but there were significant differences in the way they were utilized between the two seasons (p < 0.05). In light of the changes in the trophic structure of the fish during the wet and dry seasons, and to ensure the stable development of the Yangtze finless porpoise population, we recommend strengthening the connectivity between the Tian-e-Zhou Oxbow and the Yangtze River.

Plant invasion causes alterations in Darwin's finch feeding patterns in Galápagos cloud forests

Invasive species pose a major threat to forest biodiversity, particularly on islands such as the Galapágos. Here, invasive plants are threatening the remnants of the unique cloud forest and its iconic Darwin's finches. We posit that food web disturbances caused by invasive Rubus niveus (blackberry), have contributed to the rapid decline of the insectivourous green warbler finch (Certhidae olivacea). We compared the birds' dietary changes in long-term management, short-term management and unmanaged areas. We measured C:N ratios, and δ15N‑nitrogen and δ13C‑carbon values in both consumer tissues (bird-blood) and food sources (arthropods), as indicators of resource use change, and collected mass abundance, and arthropod diversity data. We characterised the birds' diets using isotope mixing models. The results revealed that finches in (blackberry-invaded) unmanaged areas foraged more on abundant, yet lower quality, arthropods present in the invaded understory. This suggests that blackberry encroachment leads to a decrease in food source quality with physiological consequences for green warbler finch chicks. Results also implied that blackberry control has a short-term impact on food source quantity, which led to a decrease in chick recruitment that we observed in our previous studies; despite this, in the long-term, these managed systems show signs of recovery within three years of restoration.

A melting pot of Roman dogs north of the Alps with high phenotypic and genetic diversity and similar diets

Several dog skeletons were excavated at the Roman town of Augusta Raurica and at the military camp of Vindonissa, located in the northern Alpine region of Switzerland (Germania Superior). The relationships between them and the people, the nature of their lives, and the circumstances of their deaths are unclear. In order to gain insight into this dog population, we collected 31 dogs deposited almost simultaneously in two wells (second half of the third century CE), three dogs from burial contexts (70-200 CE and third to fifth century CE) at Augusta Raurica, and two dogs from burial contexts at Vindonissa (ca. first century CE). We detected a mixed population of young and adult dogs including small, medium and large sized individuals. Three small dogs had conspicuous phenotypes: abnormally short legs, and one with a brachycephalic skull. Stable isotope analysis of a subset of the dogs showed that their diets were omnivorous with a substantial input of animal proteins and little variation, except one with a particularly low δ15N value, indicating a diet low in animal proteins. Partial mitochondrial DNA sequences from 25 dogs revealed eight haplotypes within canine haplogroup A (11 dogs; 44%; 5 haplotypes), C (8 dogs; 32%; 1 haplotype), D (4 dogs, 16%; 1 haplotype) and B (2 dogs, 8%; 1 haplotype). Based on shotgun sequencing, four Roman mitogenomes were assembled, representing sub-haplogroups A1b3, A1b2 and C2. No canine pathogens were identified, weakening the assumption of infectious disease as a cause for dog disposal. The genetic and morphological diversity observed in dogs of Augusta Raurica and Vindonissa is similar to modern dog diversity.

The histology and growth rate of Nile crocodile (Crocodylus niloticus) claws

The histology and growth of reptilian and crocodilian claws (ungues) have been extensively studied; however, Nile crocodile (Crocodylus niloticus) claws have not received adequate attention. Furthermore, age estimations for reptilian claws remain unexplored, despite Nile crocodile claws being used in long-term dietary reconstruction studies, assuming certain age-related patterns. In this study, we investigate the histology and growth patterns of Nile crocodile claws, aiming to infer axes for sampling cornified material for radiocarbon dating and establish age estimations for crocodilian claws. Our findings reveal that Nile crocodile claws exhibit growth patterns similar to other reptilians, presenting as modified scutes/scales with an age profile along the sagittal plane. This profile starts at the basal germ matrix and progressively expands in thickness and age dorsoventrally towards the apex or "tip." Consequently, the oldest corneous material is concentrated at the most dorsal point of the claw's apex. To validate previous dietary reconstruction assumptions, we conducted radiocarbon dating on this region of the claw, which supported the idea that retained corneous material in the claws is typically relatively young (5-10 years old) due to abrasion. Our study contributes insights into the histology and growth dynamics of Nile crocodile claws, shedding light on their use in dietary reconstruction studies and emphasizing the significance of considering age-related assumptions in such investigations.

Effects of artificial reef and fishing moratorium on trophic structure of biological community in the Pearl River Estuary marine ranch based on stable isotopes

Deployment of artificial reefs (ARs) has become popular technique to create new hard-bottom habitats, increase biodiversity and richness for fisheries. We compared the faunal community structure and food web structure associated with before and after fishing moratorium between ARs and non-ARs in Wanshan Island, Pearl River Estuary using stable isotope techniques. Community composition showed higher differences between ARs and non-ARs. The range of δ13C and δ15N of different functional groups can distinguish the pelagic and benthic trophic pathways of the food web in reef-or-not area before and after fishing moratorium. The isotopic niches of entire faunal, as well as individual functional groups, overlapped less between ARs and non-ARs in Wanshan Island, which makes the isotopic functional indices non-equivalent. The total convex hull area (TA) of ARs was larger than that of non-ARs, indicating that nutrient pathways of ARs were more diverse. Overall, however, these results suggest that trophic structure was convergence between ARs and non-ARs, and differences before and after fishing moratorium, possibly due to seasonal differences. Finally, it was shown that the construction of ARs had a weak effect on the restoration of fishery resources in this area, which might be related to lack of further management, or even similar community composition to non-ARs areas.

Mercury accumulation, biomagnification, and relationships to δ13C, δ15N and δ34S of fishes and marine mammals in a coastal Arctic marine food web

Combining mercury and stable isotope data sets of consumers facilitates the quantification of whether contaminant variation in predators is due to diet, habitat use and/or environmental factors. We investigated inter-species variation in total Hg (THg) concentrations, trophic magnification slope between δ15N and THg, and relationships of THg with δ13C and δ34S in 15 fish and four marine mammal species (249 individuals in total) in coastal Arctic waters. Median THg concentration in muscle varied between species ranging from 0.08 ± 0.04 μg g-1 dw in capelin to 3.10 ± 0.80 μg g-1 dw in beluga whales. Both δ15N (r2 = 0.26) and δ34S (r2 = 0.19) best explained variation in log-THg across consumers. Higher THg concentrations occurred in higher trophic level species that consumed more pelagic-associated prey than consumers that rely on the benthic microbial-based food web. Our study illustrates the importance of using a multi-isotopic approach that includes δ34S when investigating trophic Hg dynamics in coastal marine systems.

Trophic niche influences ingestion of micro- and mesoplastics in pelagic and demersal fish from the Western Mediterranean Sea

Plastic pollution has been extensively documented in the marine food web, but targeted studies focusing on the relationship between microplastic ingestion and fish trophic niches are still limited. In this study we investigated the frequency of occurrence and the abundance of micro- and mesoplastics (MMPs) in eight fish species with different feeding habits from the western Mediterranean Sea. Stable isotope analysis (δ13C and δ15N) was used to describe the trophic niche and its metrics for each species. A total of 139 plastic items were found in 98 out of the 396 fish analysed (25%). The bogue revealed the highest occurrence with 37% of individuals with MMPs in their gastrointestinal tract, followed by the European sardine (35%). We highlighted how some of the assessed trophic niche metrics seem to influence MMPs occurrence. Fish species with a wider isotopic niche and higher trophic diversity were more probable to ingest plastic particles in pelagic, benthopelagic and demersal habitats. Additionally, fish trophic habits, habitat and body condition influenced the abundance of ingested MMPs. A higher number of MMPs per individual was found in zooplanktivorous than in benthivore and piscivorous species. Similarly, our results show a higher plastic particles ingestion per individual in benthopelagic and pelagic species than in demersal species, which also resulted in lower body condition. Altogether, these results suggest that feeding habits and trophic niche descriptors can play a significant role in the ingestion of plastic particles in fish species.

Fatty acid isotopic composition in Atlantic pollock is not influenced by environmentally relevant dietary fat concentrations

The application of fatty acid (FA) isotopic analysis has great potential in elucidating food web structure, but it has not experienced the same wide-spread use as amino acid isotopic analyses. The failure to adopt FA isotopic methods is almost certainly linked to a lack of reliable information on trophic fractionation of FA, particularly in higher predators. In this work, we attempt to address this shortfall, through comparison of FA δ¹³C values in captive Atlantic pollock (Pollachius virens) liver and their known diets. Since catabolism is likely the main cause of fractionation and it may vary with dietary fat content, we investigated the impact of dietary fat concentration on isotopic discrimination in FA. We fed Atlantic pollock three formulated diets with similar FA isotopic compositions but different fat concentrations (5-9% of diet), representative of the range found in natural prey, for 20 weeks. At the conclusion of the study, δ¹³C values of liver FA were very similar to the FA within the corresponding diets, with most discrimination factors < 1. For all FA except 22:6n-3, dietary fat had no effect on discrimination factors. Only for 22:6n-3 did fish fed the highest fat diet have lower δ¹³C values than the diet consumed. Thus, these FA-specific discrimination factors can be applied to evaluate diets in marine fish consuming natural diets and will serve as additional and valuable biomarkers in fish feeding ecology.

Coastal upwelling influences population structure of dusky grouper Epinephelus marginatus: An integrative approach based on otolith chemistry and muscle stable isotopes

The dusky grouper (Epinephelus marginatus) is an overfished and threatened fish species with coastal distribution. In the Southwestern Atlantic, it occurs across a broad region influenced by two major oceanographic features: the Cabo Frio (23°S) and the Cabo Santa Marta (28°S) upwelling systems. Along the Brazilian coast, the species may present continuous or discrete populations, depending on the methodological approach used. In this study we combined otolith chemistry and muscle stable isotope analyses to examine the population structure of dusky groupers and its association with the two upwelling systems. Fish were collected in shallow coastal waters of the Southwest Atlantic Ocean, covering the southeastern and southern Brazilian coasts, among Macaé (22°S), Santos (24°S), Florianópolis (27°S), and in Rio Grande (32°S). The results show three statistically well-separated population groups along the region. We named these population groups as North (north of Cabo Frio); Center (between upwelling regions); and South (south of the Cabo Santa Marta system). Our findings allow to suggest that the upwelling systems may influence the distribution of E. marginatus stocks along the Brazilian south-western coast, even though a causal effect may not be attributed at this point. Overall, this combined approach, leveraging information from distinct natural tags, and reflecting variability of water chemistry and food webs with latitude, allowed us to enhance our understanding on how major upwelling systems influence the structuring of fish populations along the southwestern Atlantic Ocean.

Stable isotopes and foraging behaviors support the role of antipredator benefits in driving the association between two marine fishes

Research from terrestrial communities shows that diminished predation risk is a principal driver of heterospecific grouping behavior, with foraging ecology predicting the roles that species play in groups, as more vulnerable foragers preferentially join more vigilant ones from whom they can benefit. Meanwhile, field studies examining the adaptive significance of heterospecific shoaling among marine fish have focused disproportionately on feeding advantages such as scrounging or prey-flushing. Juvenile bonefish (Albula vulpes) occur almost exclusively among mojarras (Eucinostomus spp.) and even elect to join them over conspecifics, suggesting they benefit from doing so. We evaluated the roles of risk-related and food-related factors in motivating this pattern of affiliation, estimating: (1) the relative levels of risk associated with each species' search and prey capture activities, via behavioral vulnerability traits discerned from in situ video of heterospecific shoals, and (2) resource use redundancy, using stable isotopes (δ¹³C, δ¹⁵N, and δ³⁴S) to quantify niche overlap. Across four distinct metrics, bonefish behaviors implied a markedly greater level of risk than those of mojarras, typified by higher activity levels and a reduced capacity for overt vigilance; consistent with expectations if their association conformed to patterns of joining observed in terrestrial habitats. Resource use overlap inferred from stable isotopes was low, indicating that the two species partitioned resources and making it unlikely that bonefish derived substantive food-related benefits. Collectively, these findings suggest that the attraction of juvenile bonefish to mojarras is motivated primarily by antipredator advantages, which may include the exploitation of risk-related social cues.

Assessment of aquatic food web and trophic niche as a measurement of recovery function in restored mangroves in the Southern Gulf of Mexico

Mangroves are coastal wetlands with high biodiversity and productivity, with great interaction with coastal environments. In the face of worldwide mangrove loss, restoration projects attempt to recover ecosystem composition and functioning over time. Our objective was to examine and compare the food webs in mangrove areas with different restoration times and in a reference mangrove in Términos Lagoon, Mexico. We estimated the trophic structure, identified the carbon resources that maintain aquatic consumers through the analysis of stable isotopes, and compared the trophic niche of the restored mangroves with the reference mangrove. We analyzed environmental variables, trophic structure, and contributions of resources during three seasons: rainy, dry, and "nortes". Environmental changes and food structure changed in response to regional seasons. Bayesian mixing models indicated that food webs varied seasonally as a response to the primary productivity developed at Términos Lagoon. As expected, the assimilation of C₃ plants in the reference mangrove was highest, as a primary ("nortes" season) and secondary resource (dry and rainy seasons). The restored mangroves depended mainly on allochthonous resources (seagrass, epiphytes, and phytoplankton). The assimilation of these resources highlighted the importance of connectivity and the input of sources of carbon from nearby coastal environments. Trophic niche analysis showed that the area with longer restoration time was more similar to the reference mangrove, which is evidence of the importance and efficacy of the restoration process, as well as the restoration of the ecosystem function over time.

Colonial waterbirds provide persistent subsidies to swamp forests along an estuarine island food chain

Birds are excellent vectors of allochthonous matter and energy due to their high mobility, with more intense flow when waterbirds congregate in breeding colonies, feeding in surrounding aquatic and terrestrial areas, and promoting nutritional pulses to nutrient-poor environments. In southern Brazil, a swamp forest on an estuarine island is used by waterbirds for breeding, providing an opportunity to investigate the potential effects of transport of matter between nutrient-rich environments. Soil, plants, invertebrates, and blood from terrestrial birds were collected and stable isotopes compared to similar organisms in a control site without heronries. Values of δ¹⁵N and δ¹³C from waterbirds were higher in the colony in comparison to the control site (spatial effect). The enrichment of ¹⁵N and ¹³C provided during the active colony period persisted after the breeding period, especially for δ¹⁵N, which was higher in all compartments (temporal effect). Moreover, the enrichment of ¹⁵N occurred along the entire trophic chain (vertical effect) in the colony environment, including different guilds of invertebrates and land birds. The enrichment in ¹³C seems to lose strength and was mostly explained by factors such as trophic guild rather than site, especially in birds. Bayesian mixture models with terrestrial vs. estuarine endpoints demonstrated that all organisms from both colony and control environments had assimilated estuarine matter. Finally, detritivorous invertebrates showed greater assimilation when compared to other guilds. This study demonstrates that adjacent nutrient-rich environments, such as palustrine forests and estuaries, are nutritionally enriched in several dimensions from nearby autochthonous subsidies that are maintained throughout the year.

Using Stable Isotopes to Determine Natal Origin and Feeding Habits of the Army Cutworm Moth, Euxoa auxiliaris (Lepidoptera: Noctuidae)

The army cutworm, Euxoa auxiliaris (Grote), is a migratory noctuid that is both an agricultural pest and an important late-season food source for grizzly bears, Ursus arctos horribilis (Linnaeus, Carnivora: Ursidae), within the Greater Yellowstone Ecosystem. Beyond the confirmation of the moths' seasonal, elevational migration in the mid-1900s, little else has been documented about their migratory patterns. To address this missing ecological component, we examined (1) migratory routes during their spring and fall migratory periods throughout their natal range, the Great Plains, and (2) natal origin at two of their summering ranges using stable hydrogen (δ2H) analyses of wings from samples collected within the areas of interest. Stable carbon (δ13C) and stable nitrogen (δ15N) analyses of wings were used to evaluate larval feeding habits of the migrants and agricultural intensity of natal origin sites, respectively. Results suggest that, rather than migrating exclusively east to west, army cutworm moths are also migrating north to south during their spring migration. Moths did not exhibit natal origin site fidelity when returning to the Great Plains. Migrants collected from the Absaroka Range had the highest probability of natal origin in Alberta, British Columbia, Saskatchewan, the most southern region of the Northwest Territories, and second highest probability of origin in Montana, Wyoming, and Idaho. Migrants collected in the Lewis Range had the highest probability of origin in the same provinces of Canada. Results suggest that migrants of the Absaroka Range fed exclusively on C3 plants as larvae and rarely fed in heavily fertilized agroecosystems.

Ecological Drivers of Mercury Bioaccumulation in Fish of a Subarctic Watercourse

Mercury (Hg) is a serious concern for aquatic ecosystems because it may biomagnify to harmful concentrations within food webs and consequently end up in humans that eat fish. However, the trophic transfer of mercury through the aquatic food web may be impacted by several factors related to network complexity and the ecology of the species present. The present study addresses the interplay between trophic ecology and mercury contamination in the fish communities of two lakes in a pollution-impacted subarctic watercourse, exploring the role of both horizontal (feeding habitat) and vertical (trophic position) food web characteristics as drivers for the Hg contamination in fish. The lakes are located in the upper and lower parts of the watercourse, with the lower site located closer to, and downstream from, the main pollution source. The lakes have complex fish communities dominated by coregonids (polymorphic whitefish and invasive vendace) and several piscivorous species. Analyses of habitat use, stomach contents, and stable isotope signatures (δ¹⁵ N, δ¹³ C) revealed similar food web structures in the two lakes except for a few differences chiefly related to ecological effects of the invasive vendace. The piscivores had higher Hg concentrations than invertebrate-feeding fish. Concentrations increased with size and age for the piscivores and vendace, whereas habitat differences were of minor importance. Most fish species showed significant differences in Hg concentrations between the lakes, the highest values typically found in the downstream site where the biomagnification rate also was higher. Mercury levels in piscivorous fish included concentrations that exceed health authorization limits, with possible negative implications for fishing and human consumption. Our findings accentuate the importance of acquiring detailed knowledge of the drivers that can magnify Hg concentrations in fish and how these may vary within and among aquatic systems, to provide a scientific basis for adequate management strategies. Environ Toxicol Chem 2023;42:873-887. © 2023 SETAC.

Food web dynamics in the mangrove ecosystem of the Pearl River Estuary surrounded by megacities

Global recognition that mangroves support coastal ecosystem services has increased; however, studies on trophic dynamics in mangrove ecosystems remain limited. We seasonally analysed the δ¹³C and δ¹⁵N of 34 consumers and 5 diets to elucidate the food web dynamics in the Pearl River Estuary (PRE). Fish had a large niche space during the monsoon summer, reflecting increased trophic roles. In contrast, the small niche space of benthos over seasons reflected consistent trophic positions. Consumers mainly utilized plant-derived organic matters in the dry season and particulate organic matters in the wet season. The present study with literature reviews revealed characteristics of the PRE food web with the depleted δ¹³C and enriched δ¹⁵N, indicating a high contribution of mangrove-derived organic carbon and sewage input, particularly in the wet season. Overall, this study confirmed the seasonal and spatial trophic dynamics in mangrove forests surrounding megacities for future sustainable mangrove ecosystem management.

Living in human-modified landscapes narrows the dietary niche of a specialised mammalian scavenger

Anthropogenic impacts on carnivores can be complex, posing numerous threats to many species, yet also benefits to those able to exploit certain resources. This balancing act is particularly precarious for those adapters that exploit dietary resources provided by humans, but still require other resources only available in native habitat. Here we measure the dietary niche of one such species, the Tasmanian devil (Sarcophilus harrisii), a specialised mammalian scavenger, across an anthropogenic habitat gradient stretching from cleared pasture to undisturbed rainforest. Populations inhabiting areas of greater disturbance showed restricted dietary niches, suggesting that all individuals fed on similar food items, even within regenerated native forest. Populations in undisturbed rainforest habitats had comparatively broad diets and showed evidence of niche partitioning by body size, which may reduce intraspecific competition. Despite the potential benefits of reliable access to high-quality food items in anthropogenically-modified habitats, the constrained niches we observed may be harmful, indicating altered behaviours and potentially increasing the rate of fights between individuals over food. This is of particular concern for a species at risk of extinction due to a deadly cancer primarily transmitted through aggressive interactions. The lack of diversity in devil diets within regenerated native forest compared to those in old-growth rainforest also indicates the conservation value of the latter for both the devil and the species which they consume.