Isotopic Niche Research Articles

Invasive Brook Stickleback Culaea inconstans minimally alters the trophic ecology of four native fishes in Wyoming, USA

Invasive species introductions are a primary threat facing populations of native freshwater fishes. There are multiple mechanisms by which an invader can affect native species, with competition for food resources being one mechanism that can lead to declines in the distribution and abundance of native species. Invaders that are trophic generalists may cause shifts in the trophic ecology of native species and may be better suited for long-term persistence amid environmental stochasticity. Therefore, trophic studies can provide valuable information on the risk an invader poses to native species. Brook Stickleback Culaea inconstans is an invasive fish species in Wyoming whose effect on native fish assemblages is poorly understood. Our goal was to understand the potential for competitive interactions between Brook Stickleback and native fishes. We used stable isotopes of carbon (ẟ13C) and nitrogen (ẟ15N) to evaluate the feeding ecology of Brook Stickleback relative to four native fishes, and to explore whether native fish isotopic niches changed in sympatry with Brook Stickleback. We hypothesized that the isotopic niche of Brook Stickleback would be larger than that of native fishes, suggesting broader resource use. Additionally, we hypothesized that the isotopic niche of native fish populations sympatric with Brook Stickleback would contract. We did not find support for our hypotheses as the isotopic niche of Brook Stickleback was not substantially different from that of native fishes. Further, the isotopic niche of native fishes was not substantially affected by Brook Stickleback presence. As a result, we do not currently see evidence of Brook Stickleback altering the trophic ecology of native fish species. Our results provide insight to the effects of a small-bodied invasive fish species on native fishes in a previously unstudied region, and can help managers prioritize management actions to conserve native fishes.

Multi-taxa marine isoscapes provide insight into large-scale trophic dynamics in the North Pacific

Quantifying large scale trophic dynamics and interactions in marine systems remains a key challenge in ecological research and conservation, particularly in remote habitats which are difficult and costly to access. Stable isotope (SI) analysis has emerged as an effective tool for studying ecosystem interactions, especially when used to construct isotopic landscapes, or “isoscapes”. We used a combination of empirical SI data, global modeled phytoplankton isoscapes, and oceanographic data to investigate how large SI datasets might be harnessed to describe and predict patterns of regional food web dynamics and trophic structuring. We collated data from 99 published and author-contributed datasets of carbon and nitrogen SI data for open-ocean and neritic nektonic fish and invertebrates in the North Pacific Ocean to estimate mean SI values for successive trophic level groups within Longhurst biogeographic provinces. We found that these estimates reflected geographic patterns predicted by modeled phytoplankton isoscapes, suggesting that baseline SI ratios are consistently propagated up through regional food webs. This was further corroborated by significant correlations between our estimated SI values with the same oceanographic and geographic variables known to influence phytoplankton SI ratios. However, we observed variable amounts of isotopic offset between successive trophic level groups within and across provinces, indicating that the underlying drivers of trophic dynamics differ across regions. We found evidence of decreased isotopic differences between higher level trophic groups, suggesting the possibility of trophic compression and highlighting the importance of using taxa- and tissue-specific trophic discrimination factors (TDFs) when comparing or predicting consumer SI ratios. We used stable isotopic niche analyses to demonstrate that coupled use of δ13C and δ15N data increased capacity to differentiate province-specific consumer groups, particularly where adjacent provinces had similar ranges of values for a single SI. We also found that baseline isoscapes may be a viable tool for predicting broad trends of consumer SI ratios by calculating ‘predicted’ δ13C and δ15N values for successive trophic groups across provinces using modeled phytoplankton data and diet-dependent discrimination factors. These predicted values were significantly correlated with our estimates based on empirical data, with similarity of predicted δ13C values increasing for higher trophic level groups. We conclude that regional SI patterns remain consistent across diverse ecosystems. With increased spatiotemporal resolution of SI data across ocean basins and increased availability of species-specific life history traits and physiologies, multi-taxa isoscapes hold great potential for studying large scale marine trophodynamics and movement patterns of highly mobile marine species.

Plasticity in resource use explains the persistence of the largest living rodent in anthropized environments

AbstractHow organisms search for and obtain food is a determinant of their survival. It has been hypothesized that organisms facing food limitations might expand their dietary niche compared with optimal foraging conditions. Nonetheless, some organisms may reduce their dietary niche due to limiting resources. Here, we show the resource use plasticity of capybaras (Hydrochoerus hydrochaeris), the largest living rodent, in natural and anthropized environments of Brazil using stable carbon and nitrogen isotopes in hair. We assessed the resource use, size and overlap of isotopic niches, and the influence of body mass, age class, sex, and landscape composition on isotopic values among 13 capybara populations (210 individuals). We observed a wide variation in isotopic values of capybaras among and within populations in different environments. Surprisingly, diets varied from strictly C4‐based to strictly C3‐based, with most individuals feeding on C4 plants (65%). A substantial portion of individuals presented mixed diets (C3 and C4 plants), totaling 31% of the individuals, and few fed exclusively on C3 plants (4%). Isotopic niche size also varied among populations in both environments, with low overlap considering their core dietary niche. Capybara populations in natural environments had substantial incorporation of C3 resources, showing individuals with strictly C3 diets, and possibly relying on a high diversity of food items, while some populations in anthropized ones fed exclusively on C4 resources. Neither body mass, age class, sex, nor landscape composition significantly influenced isotopic values. Here, we provide evidence of the dietary plasticity of capybaras in different environments, transitioning from the consumption of C4 to C3 plants when needed, that is, from preferred to non‐preferred food items. Together with the species' behavioral plasticity, our results help explain its resilience in surviving, and even thriving, in both natural and human‐modified environments.

Dietary plasticity linked to divergent growth trajectories in a critically endangered sea turtle

Foraging habitat selection and diet quality are key factors that influence individual fitness and meta-population dynamics through effects on demographic rates. There is growing evidence that sea turtles exhibit regional differences in somatic growth linked to alternative dispersal patterns during the oceanic life stage. Yet, the role of habitat quality and diet in shaping somatic growth rates is poorly understood. Here, we evaluate whether diet variation is linked to regional growth variation in hawksbill sea turtles (Eretmochelys imbricata), which grow significantly slower in Texas, United States versus Florida, United States, through novel integrations of skeletal growth, gastrointestinal content (GI), and bulk tissue and amino acid (AA)-specific stable nitrogen (δ15N) and carbon (δ13C) isotope analyses. We also used AA δ15N ΣV values (heterotrophic bacterial re-synthesis index) and δ13C essential AA (δ13CEAA) fingerprinting to test assumptions about the energy sources fueling hawksbill food webs regionally. GI content analyses, framed within a global synthesis of hawksbill dietary plasticity, revealed that relatively fast-growing hawksbills stranded in Florida conformed with assumptions of extensive spongivory for this species. In contrast, relatively slow-growing hawksbills stranded in Texas consumed considerable amounts of non-sponge invertebrate prey and appear to forage higher in the food web as indicated by isotopic niche metrics and higher AA δ15N-based trophic position estimates internally indexed to baseline nitrogen isotope variation. However, regional differences in estimated trophic position may also be driven by unique isotope dynamics of sponge food webs. AA δ15N ΣV values and δ13CEAA fingerprinting indicated minimal bacterial re-synthesis of organic matter (ΣV < 2) and that eukaryotic microalgae were the primary energy source supporting hawksbill food webs. These findings run contrary to assumptions that hawksbill diets predominantly comprise high microbial abundance sponges expected to primarily derive energy from bacterial symbionts. Our findings suggest alternative foraging patterns could underlie regional variation in hawksbill growth rates, as divergence from typical sponge prey might correspond with increased energy expenditure and reduced foraging success or diet quality. As a result, differential dispersal patterns may infer substantial individual and population fitness costs and represent a previously unrecognized challenge to the persistence and recovery of this critically endangered species.

Increasing species richness along elevational gradients is associated with niche packing in bat assemblages.

The change in species richness along elevational gradients is a well-known pattern in nature. Niche theory predicts that increasing species richness in assemblages can either lead to denser packing of niche space ('niche packing') or an expansion into its novel regions ('niche expansion'). Traditionally, these scenarios have been studied using functional traits but stable isotopes provide advantages such as identifying the degree of resource specialisation, or niche partitioning among functionally similar species. In this study, we evaluate the relevance of niche packing versus niche expansion by investigating stable carbon and nitrogen isotopic niche width and overlap among 23 bat species from six functional groups across a 1500 m elevational gradient in the Himalaya. Our results suggest that an increase in species richness in the low elevation is accompanied by small niche width with high overlap, whereas the high elevation assemblage shows large niche width with low overlap among functional group members. At the functional group level, edge-space foraging, trawling, and active gleaning bats have the highest niche width while passive gleaning bats that are only found in high elevations are isotopic specialists showing low overlap with other groups. Edge and open-space foraging bats showed idiosyncratic changes in niche width across elevations. We also find that the niches of rhinolophid bats overlap with edge-space and open-space foraging bats despite their unique functional traits. These results support the idea that at low elevations high species richness is associated with niche packing while at high elevations strong niche partitioning prevails in dynamic and resource-poor environments. We conclude that although high elevation animal assemblages are often 'functionally underdispersed', that is show homogenous functional traits, our approach based on stable isotopes demonstrates niche partitioning among such functionally similar species.

The lateral transport of zooplankton explains trophic and taxonomic similarities over the zonal gradient of central Chile

The highly productive upwelling zone in the Southeast Pacific provides inputs of C and N to the oceanic deep-water system via lateral transport, although the quality of this organic matter, the organisms being transported, and the controlling mechanisms are unclear. Here, we assessed whether the taxonomic and trophic structure of zooplankton over more oceanic offshore waters differ from that of zooplankton located in the upwelling zone, along with the characterization of the oceanographic processes and variables associated with the lateral transport. For this, epipelagic, mesopelagic, and bathypelagic zooplankton along with oceanographic variables and particulate organic matter (POM) were analyzed at the upwelling zone and adjacent oceanic stations off Central Chile (27–33°S), some of which were located over the Atacama trench, during September 2016. The community structure of size-fractioned zooplankton (<200 μm, 200–500 μm, 500–1000 μm, 1000–2000 μm, and >2000 μm size classes) was assessed by an automated method, along with the analysis of the isotopic niche computed from their δ 15 N and δ 13 C. The isotope signatures of POM indicated that diatoms contributed mostly to production of new organic C, being the main food source for small-sized zooplankton in coastal waters. The zooplankton community structure differed between the upwelling and oceanic areas, but their isotopic signature showed a large overlap (78%). Satellite-derived geostrophic flow and the depth of the mixed layer appeared as the main factors explaining the homogeneity in the isotopic and biogeochemical signatures of zooplankton between the upwelling zone and offshore waters. Cross-shelf advection is thus suggested as a key process promoting zooplankton export to the deep-water ecosystem, including the ultra-deep Atacama Trench. • The organic matter produced in the upwelling zone can be transported over the Atacama trench by strong offshore advection. • Isotopic niches of zooplankton from the Atacama Trench and the upwelling zone exhibit large overlapping (78%). • The upwelling systems acts as a C subsidiary for the offshore deep-sea ecosystem including the Atacama Trench.

Geographical, temporal, and individual-based differences in the trophic ecology of female Cape fur seals.

Information on resource use and trophic dynamics of marine predators is important for understanding their role in ecosystem functioning and predicting population-level responses to environmental change. Where separate populations experience different local environmental conditions, geographic variability in their foraging ecology is often expected. Within populations, individuals also vary in morphology, physiology, and experience, resulting in specialization in resource use. In this context, isotopic compositions of incrementally grown tissues such as keratinous hairs offer a valuable opportunity to study long-term variation in resource and habitat use. We investigated the trophic ecology of female Cape fur seals (Arctocephalus pusillus pusillus) using carbon and nitrogen isotopic compositions of serially sampled whiskers collected at four breeding sites along the coast of South Africa. Drawing on over 900 isotopic measurements, we assessed geographic variability in isotopic niche width between colonies and the degree of individual specialization. We found slight, but clear geographic differences in isotopic ratios and isotopic niche widths, seemingly related to ecological setting, with niche widths being proportional to the area of available shelf and shelf-slope habitat surrounding the colony. We further identified periodic oscillations in isotopic ratios, which likely reflect temporal patterns in foraging distribution and prey type, linked to shifts in the availability of prey resources and their interaction with constraints on individual females throughout their breeding cycle. Finally, individual specialization indices revealed that each of the study populations contain specialist individuals that utilize only a small subset of the total population niche width. The degree of individual specialization was, however, not consistent across colonies and may reflect an interactive influence between density-dependent effects and habitat heterogeneity. Overall, this study provides important information on the trophic ecology of Cape fur seals breeding in South Africa and highlights the need to consider geographic and individual variability when assessing the foraging ecology of marine predators.

Isotopic niche partitioning between an invasive fish and two native mesopredators in the Colombian Caribbean

The ecological interactions between invasive species and members of the invaded community determine the impacts of biological invasions. Severe detrimental impacts of the invasive lionfish (Pterois volitans) in the Western-Atlantic have been well documented in northern regions of the invaded area. Yet, the fundamental nature of lionfish interactions with native predators remains unclear and understudied in the Southern Caribbean. Here, we use a series of quantitative tools applied on bulk muscle tissue stable isotope data (δ13C and δ15N), to compare inter and intraspecific isotopic niche metrics, similarity in the use of food resources, and the proportional contribution of two primary carbon sources to the diet of lionfish and two common ecologically similar mesopredators (Cephalopholis cruentata and Lutjanus mahogoni) in Taganga Bay (Colombian Caribbean). Limited overlap in the Bayesian standard ellipses of native mesopredators and lionfish suggested a substantial amount of partitioning in the use of food resources and, therefore, little apparent ecological impacts on native mesopredators by lionfish, in terms of strict competition for the use of food resources. Native mesopredators showed low trophic diversity, and a high degree of trophic redundancy (for C. cruentata). In contrast, lionfish had broad trophic diversity and high among-individual isotopic variability. This was consistent with Bayesian mixing models that revealed differential patterns in the importance of primary carbon sources in the diet of lionfish compared to native mesopredators. These results imply that different resource-consumer dynamics exist between lionfish and native mesopredators, despite fulfilling similar functional roles in the food web structure of Taganga bay, as indicated by similar Bayesian estimated trophic positions. Thus, our study highlights the importance of species-specific trophic characteristics in delineating invasive-native ecological interactions and constitutes the first study to examine trophic interactions among native mesopredators and the invasive lionfish in the Southern Caribbean.

Isotopic Space of the House Mouse in the Gradient of Anthropogenic Habitats

The house mouse (Mus musculus) is a most extensively distributed omnivorous rodent species, usually living in close association with humans. Its diet includes various vegetable matter, insects and any available human food. For the first time, we assessed the dietary niche of this species by the isotopic (δ15N and δ13C) compositions of animal hair samples in the gradient of habitats, ranging from natural to fully commensal. The main factors explaining the differences in the isotopic niche of the mice, being the proxy of their diet, were the season and the source of available food. Influence of the habitat was weak, while gender, age, body mass and body condition had no influence on the diet differences. We found that M. musculus dietary niches overlap between different habitats if mice have access to human food. Niches diverge when mice forage outdoors on natural food compared to farms where livestock feed is available. Compared to omnivorous bank vole (Clethrionomys glareolus) living synoptically, M. musculus has much wider dietary niche and consumes more foods of animal origin. Variability of the diet increases the ecological plasticity of this strongly commensal species and, together with behavioural and reproductive adaptability, allows irresistibly occupy various environments.

Anthropogenic nutrient loading affects both individual species and the trophic structure of river fish communities

Although the concept of trophic interactions has been used for a long time, there are still considerable gaps in our understanding of the effect of various environmental factors on trophic interactions within river fish assemblages. Carbon (δ13C) and nitrogen (δ15N) stable isotope ratios of 20 species of fish belonging to both eurytopic and rheophilic ecological groups from a large temperate rivers were used to evaluate overall trophic niche use and trophic position of species, and to find out how environmental variability associated with nutrient loading affects individual and community-wide aspects of trophic structure. The study was carried out at 11 sites along the European rivers Vltava and Elbe, representing a continuous gradient of pollution and habitat degradation. Corrected Standard Ellipse Area (SEAc) was significantly larger for the group of eurytopic ecological species than for rheophilic species. Despite narrower isotopic niche space, rheophilic fish species occupied a higher trophic position, suggesting that these species use resources more enriched in 15N. Of the 11 environmental variables tested, nutrients had a significant effect on trophic niche area of species (SEAc), indicating that eutrophication is of critical importance for fish assemblages. Isotopic niche area of species was found to be positively influenced by total phosphorus, and negatively affected by concentrations of nitrate (N-NO3−) and ammonia (N-NH4+). A negative association between oxygen demand and a measure of trophic diversity - mean distance to centroid (CD)- and a measure of density and clustering of species - mean nearest neighbor distance (MNND)- were found, indicating that the oxygen demand is a key factor influencing community trophic structure. An observed pattern where nutrient loading influenced both individual species and trophic structure of the fish assemblage provides strong support for an anthropogenic influence on riverine food webs.

Evaluating food metrics of lanternfishes in waters of the southeastern Pacific Ocean

Using carbon and nitrogen stable isotope values, we analyzed the trophic position (TP) and the isotopic niche width of lanternfishes from three different areas in the Southern Pacific Ocean. Fishes from Perú had slightly higher δ13C values compared with fish from Chilean areas. In contrast, δ15N values increased with latitude (North to South). Myctophids TP differed between the three study areas (highest in Central Chile, lowest in Peru). Peruvian fishes had a smaller isotopic niche than the lanternfishes of those from Chile.

Cascade effects of seafloor litter on benthic ecosystems in the northern Yellow Sea

The accumulation of seafloor litter changes the habitats of benthic organisms and thus affects their spatial distribution. Previous studies suggested that the accumulation of seafloor litter might contribute to the bloom of Metridium senile fimbriatum, given that seafloor litter could serve as “vectors” for M. senile dispersal and provide with “natural habitat” for their settlement. As a common fouler and invader, M. senile bloom will inevitably affect the benthic ecosystems. The cascading effects of seafloor litter on benthic ecosystems can be explored by studying the interactions between this litter-associated anemone and other co-occurred species. Therefore, this study uses stable isotope analysis to study the food composition of M. senile attached to the seafloor litter and the niche relationship with co-occurred species. First, by comparing the stable isotope ratios of the pedal disc and tentacle of M. senile, it was found that the δ15N of the foot disc is more stable than that of the tentacle, so the pedal disc was chosen as the sampling tissue. The mean δ13C value of M. senile was -19.34 ± 0.86‰, which ranged from -22.30‰ to -18.17‰. For δ15N, the mean value was 12.02 ± 0.95‰, which ranged from 9.22‰ to 13.31‰. Then, we quantified the contribution of the potential food resources of M. senile using the SIMMR model. M. senile has the highest feeding proportion to crustaceans, followed by juvenile fish, zooplankton and other macrobenthos. Meanwhile, M. senile has high uncertainty in the feeding proportion of crustaceans and juvenile fish. In addition, the trophic niche overlap between different functional groups was calculated by the SIBER model, and the results showed that the isotopic niche of M. senile was overlapped with different functional groups, ranging from 34.29 to 35.25%. Therefore, this study suggests that M. senile, as litter-associated carnivorous anemone, interacts with other components in the ecosystems through predation and competition, which also reveals the cascade effect of seafloor litter on the benthic ecosystems of the northern Yellow Sea.

Ontogenetic shifts in feeding habits of orangemouth weakfish (Cynoscion xanthulus): From estuarine benthic feeder to marine nektivore top predator

AbstractOrangemouth weakfish is a highly important commercial and game fish species in the Tropical Eastern Pacific. However, studies assessing changes in its trophic ecology relevant to size, habitat (estuarine or marine), sex, and season are non‐existent. We assessed its feeding habits and trophic position (TP) according to the aforementioned factors. Its dietary breadth was composed of 33 types of prey. The feeding habits and isotopic values (δ15N and δ13C) registered differences between sizes and habitat, indicating ontogenetic changes in habitat as well as TP, as small organisms (TP 3.1) inhabited the estuarine area, feeding on benthic and demersal organisms, whilst large individuals (TP 4.1) were found in the marine environment, preying on nektonic fauna. No differences were found according to sex or season. The isotopic niche showed that the different sizes of this species are generalist consumers with a clear separation between them and between habitats, as estuarine juvenile and preadult organisms have a larger isotopic niche than adult marine organisms, being an opportunist tertiary estuarine consumers during their early stages, and a top marine predator during the adult phase.

Variation in isotopic niche partitioning between adult roseate and common terns in the Northwest Atlantic

Co-occurring species with similar resource requirements often partition ecological niches at different spatial and temporal scales. In the Northwest Atlantic (NWA), federally endangered roseate terns Sterna dougallii nest almost exclusively in coastal island colonies alongside common terns S. hirundo. Roseate terns are prey specialists compared to common terns, which are opportunistic generalists; however, the 2 species forage on similar resources during the breeding season. The degree to which these species overlap in their adult foraging ecologies is not well understood. We compared the isotopic niches of nesting adult roseate and common terns by analyzing stable carbon (δ13C) and nitrogen (δ15N) isotopes in eggshell membrane tissues collected in 2018 and 2019 from 10 colonies that span their NWA breeding range. Our aim was to characterize interspecific patterns in δ13C and δ15N values, isotopic niche breadth, and isotope niche overlap. We additionally examined interannual and subregional differences between ‘cold-water’ colonies in the Gulf of Maine and ‘warm-water’ colonies in Southern New England and Long Island Sound. At the range-wide scale, there was a high degree of overlap in the overall isotopic niches of the 2 species; however, more variable patterns were observed at the colony scale, ranging from nearly complete overlap to complete separation. The isotopic niches of roseate terns were generally narrower than those of common terns, consistent with their respective specialist/generalist tendencies. While the influence of isotopic baselines limits our interpretation of interannual and subregional differences, isotopic niche breadths and overlap suggest consistency of relative foraging ecologies across these scales.

Estimating trophic guilds based on isotopic niches of demersal fishes, squids and crustaceans in the East China Sea

Context Identifying trophic guilds, i.e. species groups having similar trophic niches, is a first step in effective stock and fisheries management with consideration of multiple species interactions or ecosystems. Aims We evaluated isotopic niches by using stable isotope values (δ13C and δ15N) for 53 species, including commercially important demersal fishes, squids and crustaceans, from the continental shelf to the slope of the East China Sea (ECS), to segregate these species into guilds. Methods We inferred the isotopic niche space of each species and community metrics for guilds using Bayesian statistics. Key results Values of δ13C and δ15N showed different isotopic niches among species, reflecting a range of trophic positions and baseline food sources. The 53 species were segregated into 12 guilds on the basis of isotopic niche overlaps. Niche size and evenness differed among guilds; half of the guilds had smaller and less diverse niches than did the others. Conclusions The guilds identified for ECS demersal biota reflected similarities of taxonomy, behaviour, habitat and feeding type; niche size and evenness within each guild might have been influenced by the degree of species interaction. Implications These results provide important ecological information for considering effective multi-species management.

Intraguild niche partitioning in granivorous birds from the late past

Niche partitioning is a widespread ecological strategy within trophic guilds, ensuring the coexistence of sympatric species by reducing interspecific competition. Stable isotope analysis of carbon and nitrogen and isotopic niche metrics (width and overlap) are used as a proxy to investigate niche partitioning among species of a guild. In our study, we investigated if niche partitioning was an ecological mechanism contributing to the coexistence of granivorous birds from the same geographic region along time, employing an isotopic approach. We sampled and isotopically analyzed (δ13C and δ15N values) wing feathers from 58 specimens of granivorous birds collected between 1900 and 1966 in southeastern Brazil. We grouped birds according to the main habitat type used by them (forest: Cyanoloxia brissonii, Leptotila rufaxilla, and Leptotila verreauxi; and grassland: Columbina squammata and Sicalis flaveola). We used the Isotopic Richness (IRic) metric to measure the isotopic niche width and the Isotopic Similarity (ISim) and Isotopic Nestedness (INes) metrics to measure the niche overlap between groups and species. The results of low isotopic niche overlap suggest that common granivorous birds had a clear niche partitioning in southeastern Brazil in the late past, especially through foraging in distinct habitat types (forest and grassland; ISim ​= ​0.1, INes ​= ​0.2). Niche overlaps in almost all species-by-species comparisons were low (ISim ≤0.3, INes ≤0.4), except between C. brissonii and L. verreauxi (ISim ​= ​0.6, INes ​= ​1.0). These results suggest that these birds, belonging to the same trophic guild, coexisted through niche and resource partitioning. Despite being considered a plastic trophic guild, the isotopic niche partitioning among granivorous birds indicates that each of these species uses a set of specific resources and habitats. This raises an ecological concern about the homogenization of landscapes across the Neotropics, oversimplifying food resources and habitats to granivorous birds.

Trophic partitioning among seasonally resident predators in a temperate estuary

Many migratory coastal shark species enter temperate estuaries seasonally, tracking suitable environmental conditions (i.e. temperature) and food resources. Understanding the functional role that migratory sharks play in coastal food webs depends on information regarding the distinctness among predators’ trophic ecology. We examined the trophic ecology of 4 species of coastal sharks (Atlantic sharpnose, n = 22; bonnethead, n = 29; blacktip, n = 8; blacknose, n = 28) that migrate into temperate coastal ecosystems of the northwest Atlantic Ocean during the summer. We quantified their trophic relationships using bulk stable isotope ratios (δ13C, δ15N, and δ34S) measured in 2 tissue types representing long-term (red blood cells; 8-10 mo) and short-term (blood plasma; 10-13 wk) metabolic turnover. The primary carbon sources for the 4 shark species, determined via stable isotope mixing models, were consistently phytoplankton (mean ± SD: 65.8 ± 4.8%) and microphytobenthos (18.6 ± 5.4%), regardless of tissue type. However, interspecific isotopic niche overlap between any 2 species ranged widely (0-92%). The niche space occupied exclusively by one species also varied considerably but was lowest for blacktips (&lt;20%) and highest for bonnetheads (&gt;90%). Isotopic niche size estimated using short-term tissues, was substantially larger for 3 of the 4 species (Atlantic sharpnose, 41% increase; blacknose, 140%; blacktips, 159%) than long-term tissues. These data suggest species-specific trophic redundancy and increasing overlap in the summer months, indicated by larger isotopic niches during estuarine residency. As coastal habitats become increasingly altered, understanding the functional diversity of predators is vital for the prediction and assessment of resulting ecosystem-level change.

Diet and food assimilation of the Plata pompano Trachinotus marginatus Cuvier, 1832 in a subtropical sandy beach inferred by stomach content and stable isotope analyzes

Abstract Investigating trophic linkages sustaining consumers is crucial to understanding their functional ecological role in communities and ecosystems. In this work, we combined stomach content (SCA) and stable isotope (SIA) analyses to investigate the trophic ecology of the Plata pompano Trachinotus marginatus during a critical phase of its life cycle along a subtropical sandy beach. This species is a conspicuous component of the southwestern Atlantic coast and commonly targeted by coastal fisheries. The diet was described using SCA, whereas the relative importance of food sources sustaining juveniles, as well their food niche structure and trophic position (TP), were evaluated using SIA. Juveniles consumed mainly crustaceans like the hippid crab Emerita brasiliensis, although other invertebrates (annelids, insects, molluscs) and fishes (including cannibalism) were also recorded. Although microcrustaceans dominated the diet, SIA showed that more palatable preys without carapaces or shells, like fishes and annelids, were the most assimilated preys in the muscle tissue of juvenile T. marginatus. There were marked changes in their isotopic niches (SEAc) and TP across ontogeny. SEAc ranged from 1.41‰2 for smaller individuals (0–40 TL mm) to 0.3‰2 for larger individuals (&gt;80 TL mm). TP ranged from 3.1 (95% CI 2.7–3.6) for smaller to 4.5 (95% CI 3.9–5.1) for larger individuals. SIA suggest that juvenile T. marginatus derived most of their primary nutrients from a planktonic food web and, to a lesser extent, from a benthic pathway. Future studies are needed to better understand its functional role in food web of surf-zone ecosystems.

Trophic ecology of mesopelagic fishes in the northern and southern Benguela Upwelling Systems revealed through stable isotope patterns

Mesopelagic fishes are important components of marine food webs as trophic links between primary consumers and higher trophic levels, but their trophic ecology is poorly understood. We used stable isotopes (δ13C and δ15N) to compare the trophic position, trophic enrichment factor, and isotopic niche among feeding guilds of mesopelagic fishes (including myctophids, stomiids) during different seasons in the northern and southern Benguela Upwelling Systems. These subsystems are influenced by differing water masses with various physical and biological properties, which can lead to changes in the marine food web. We aimed to determine if and how the trophic ecology of mesopelagic fishes differs between seasons and subsystems. When separating fishes into guilds based on dietary preferences (zooplanktivores vs. piscivores) and migration patterns (migrating vs. non-migrating), the trophic position was highest during winter in the south for non-migrating piscivores (4.5), but the trophic positions of piscivores (3.2-4.4) were not always higher than those of zooplanktivores (3.1-4.5). This may indicate different isotopic enrichment between species or a very generalist diet from zooplanktivores such as myctophids and their prey. Trophic enrichment factors for δ15N among guilds ranged from 3.8 to 7.5‰ between the baseline and zooplanktivores and from -0.8 to 1.6‰ between zooplanktivores and piscivores. Previously, mesopelagic fishes have been incorporated into food web models in the Benguela Upwelling Systems, but only as a single entity. Elucidating the complex seasonal and regional trophodynamics of various mesopelagic feeding guilds may contribute to more accurate assessments of trophic transfer efficiencies and higher-resolution models in these systems.

Similar breeding performance despite inter-annual differences in diet composition of seabirds inhabiting a tropical environment

Amidst unprecedented anthropogenic pressure on marine ecosystems, seabirds can serve as sentinels for shifts in marine prey availability. We examined the diet and foraging ecology of 2 sympatric Procellariiformes, the Cape Verde shearwater Calonectris edwardsii and Bulwer’s petrel Bulweria bulwerii, during their breeding period in Cabo Verde, West Africa. By analysing fatty acids (FAs), δ15N and δ13C values, and using GPS loggers, we quantitatively assessed the diet of both adults and chicks, estimated the isotopic niche of adult birds, and identified their main foraging regions. Our findings revealed annual variations in the diet of both adults and chicks, supported by quantitative prey estimates and FA signatures. Adult Cape Verde shearwaters consistently displayed a diet enriched in commercial fishes (&gt;40%), with varying proportions of squid or non-commercial fishes across different years. Bulwer’s petrel displayed greater diet variability, consuming higher proportions of squid (54%), mesopelagic fishes (81%), and commercial fishes (29%) in 2017, 2018, and 2019, respectively. Chicks of both species exhibited varying diets across years. However, chick growth and fledging parameters remained fairly consistent among years. The isotopic niche, GPS tracking, and oceanographic conditions within foraging regions provided partial support for the annual variations observed in the diet of adults, especially for Cape Verde shearwaters. However, the diet differences for Bulwer’s petrels were less straightforward, likely due to the equitable distribution of their main prey. Notably, both species exhibited resilience in coping with unfavourable foraging conditions, enabling adult breeders to exploit a wide variety of prey resources without compromising breeding outcomes.