Isotopic Niche Research Articles

From narrow and overlapped to wide and segregated: The isotopic niche of a tropical cetacean community

Species in sympatry can coexist due to some degree of niche partitioning. Four cetacean species, Guiana (Sotalia guianensis), franciscana (Pontoporia blainvillei), Atlantic spotted (Stenella frontalis), and rough-toothed dolphins (Steno bredanensis), frequently occur in Ilha Grande Bay (RJ), with three of them presenting residence pattern. This tropical bay is considered a biodiversity hotspot and preserved by marine protected areas, though located in a highly developed region of the Southwestern Atlantic Ocean. To verify niche partitioning and segregation in these species and to explore the trophic structure of this cetacean community, their isotopic niches were investigated through carbon (δ13C) and nitrogen (δ15N) stable isotopes in the muscle of specimens stranded in the region between 2012 and 2022. The rough-toothed dolphin had the largest niche area and high segregation. On the other hand, there was a high niche overlap between the species that have site fidelity patterns in the bay. The Atlantic spotted dolphin had the highest values for almost all the Layman metrics, a large niche area, and trophic diversity; the Guiana dolphin explored a diverse variety of resources, and the franciscana dolphin presented the narrowest and most overlapped niche and the lowest niche diversification. This is the first study to access the trophic ecology of this cetacean community. Our results indicate that the region is an important foraging area for three of the studied species, with a high diversity and availability of resources that might enable the high niche overlap observed for resident species. The findings for the threatened and small franciscana population in this recently discovered habitat bring concern. Therefore, the area deserves conservation attention to prevent anthropogenic stressors from interfering in the relationship balance between these sympatric predators.

A melting-pot for Pterodroma petrels on Rapa Nui: ecological divergence and reproductive isolation in a contact zone

Speciation is thought to depend on general ecological segregation rather than on strict allopatry in species with planetary-scale foraging ranges, such as Pterodroma petrels. Separation in both breeding islands and foraging grounds are generally considered to be the conditions for prezygotic isolation. However, it is unclear how both can be maintained in a context of ongoing distributional range shifts due to global change, with range contractions and expansions caused by changing land use in breeding territories and oceanographic conditions impacting productivity areas. In this context, we expect that wherever the fragile balance of allopatric segregation breaks down, gene flow may be re-established across the borders of what is currently considered separated species. In these “melting pot” areas, fine-scale ecological differentiation is likely the only force opposing panmixia in otherwise highly similar organisms. In this study, we focus on six Polynesian Pterodroma species currently found on Motu Nui, a protected islet offshore of Rapa Nui (Easter Island), four of which have likely extended their breeding range to that island recently. To understand the fine-scale interaction between these closely-related species in their new contact zone, we used an integrative approach combining ecological and genetic methods, including morphometric, bioacoustics, stable isotope niche reconstruction, breeding site characterisation, phenology traits, and both nuclear and mitochondrial DNA markers. We found that significant amounts of gene flow occur across what is currently considered species boundaries, sometimes questioning the relevance of the accepted species classification. Interestingly, our results indicate that multivariate ecological distance between individuals in a group of closely related species (P. alba, P. atrata, P. heraldica, and P. neglecta) is a relevant predictor of gene flow intensity, while more distant species such as P. ultima and P. nigripennis seem to maintain reproductive isolation. Thus, the case of Pterodroma petrels supports the idea that incomplete allopatric speciation processes may be halted or even reversed on a secondary contact zone. Beyond their direct taxonomical relevance, our results underline the importance of considering fine ecological structures for biodiversity mapping and conservation policies.

Habitat coupling dynamics of mobile consumers along a freshwater and marine resource gradient in a sub-Arctic estuarine system

Food webs consist of numerous connections between consumers and resources that can couple adjacent ecosystems together by the movement of nutrients, prey, and consumers leading to habitat coupling. Habitat coupling and isotopic niche dynamics among mobile consumers at the individual and population-level has been examined extensively in freshwater systems but has received little attention in sub-Arctic estuaries, which act as transition zones between freshwater and marine habitats and resources. The objective of this study was to quantify the diet composition between freshwater-and marine-derived resources and the isotopic niche size of mobile consumers (13 fishes and 2 seal species) within the lower Churchill River, Manitoba, Canada using stable isotopes (δ13C, δ15N and δ34S) in a Bayesian framework. Cisco, lake whitefish, and northern pike represented the habitat couplers in this system and also exhibited the greatest amount of individual variability since most (75%, 56%, 65% of individuals for cisco, lake whitefish, and pike respectively) consumed a mix of both freshwater-and marine-derived resources. The largest isotopic niche sizes were found for lake whitefish and northern pike, whereas the smallest isotopic niche sizes were found for both harbour and ringed seals. The isotopic niche of lake whitefish overlapped with the most species (5 in total), which supports their broader use of both freshwater- and marine-derived resources. Habitat couplers in this system exhibited more variability in their foraging strategy than the other consumers, which aligns with their known life histories and migratory or resident movement strategies. Future changes in the relative availability of freshwater and marine-derived resources due to climate and anthropogenic stressors could pose consequences to these habitat coupling species and overall trophic dynamics of sub-Arctic estuarine systems.

Correction: A catastrophic coal mine spill in the Athabasca River watershed induces isotopic niche shifts in stream biota including an endangered rainbow trout ecotype

Correction: A catastrophic coal mine spill in the Athabasca River watershed induces isotopic niche shifts in stream biota including an endangered rainbow trout ecotype

Loss of sea ice and intermittent winds alter distributions and diet resources of young forage fish in the Chukchi sea

Loss of sea ice alters habitat for organisms that function as energetic links in Arctic food webs. To address the impacts of such changes on lower trophic levels we focus on forage fish during the early life stages (polar cod: Boreogadus saida and saffron cod: Eleginus gracilis) and copepods (Calanus spp. and Pseudocalanus spp.) in the Chukchi Sea with differing reliance on sea ice environments. We assess distributions throughout years of varying ice extent (2010–2013, 2015, 2017–2018) and diet resources during unprecedented warm years (2017, 2018) using stable isotope analyses (bulk δ13C and δ15N and δ13C compound-specific isotope analysis of amino acids). Calanus spp. and polar cod were found at relatively higher latitudes in closer proximity to recent sea ice, whereas saffron cod were rare and Pseudocalanus spp. were ubiquitous. Polar cod juveniles, but not larvae, were more common in open water, suggesting that low ice and northward ocean currents interact to influence juvenile distributions. Low summer sea ice also coincided with northward expansion of warm Pacific-origin water across the shelf. Stable isotope δ13C values reflected this latitudinal variation in water masses as well as inshore-offshore gradients due to convergence of ocean currents. When summer sea ice was absent during 2017, juvenile polar cod were dispersed across the shelf, abundances were low for both copepod taxa, isotopic niche was reduced for polar cod, and carbon sources for some larval polar cod reflected boreal-associated phytoplankton. Distributions and isotopic patterns suggest that under low ice conditions, along-shelf wind reversals and weak currents can lead to dominance of Pacific-origin water, mixing of organisms from various source locations, and reductions in sea ice-influenced environments. Northward expansion of subarctic water masses and organisms across the Chukchi shelf alters basal resources for lower trophic levels and modifies pelagic habitats to more uniformly resemble lower latitude marine ecosystems.

Hydroclimatic influence on the trophic ecology of Atlantic goliath grouper juveniles (Epinephelus itajara) in a tropical estuary using non-lethal methodologies

The Atlantic goliath grouper Epinephelus itajara is the largest species of groupers in the Atlantic Ocean, reaching over 2 m total length and 400 kg. It has an ontogenetic migration, with adults using nearshore and offshore marine areas and juveniles inhabiting estuarine/mangrove areas. Despite of its high ecological relevance and classification as a threatened species for several decades in the IUCN red lists, critical phases of its life cycle, like juveniles inhabiting estuaries, are still poorly investigated. In this study, we evaluated if hydroclimatic changes may affect the trophic ecology of juveniles in a tropical estuary. We described their diet composition, isotopic niche area, food assimilation, and trophic position of juveniles across seasons using two non-lethal techniques: stomach content analysis using a stomach flushing procedure and stable isotope analysis using a dermal biopsy. A total of 87 individuals with TL ranging from 38 to 331 mm had their stomachs analyzed, of which 31 had food content. We identified 11 prey items, shrimp being the most abundant prey group (IRI% = 88.4), followed by fish (6.6), blue crab (2.8), and crab (2.2). A total of 93 specimens had their muscle's carbon (δ13C) and nitrogen (δ15N) stable isotope ratios analyzed (38–332 mm TL). Average δ13C values showed statistically significant changes across seasons, but no significant changes were observed for δ15N. As initially predicted, isotopic niche size of juveniles changed consistently along the entire hydroclimatic cycle, reaching its peak at the end of the wet season. We also observed changes in the proportion of prey assimilation by juveniles between seasons. Shrimp (92.1%) was the most assimilated prey group during the late dry season, whereas tidal crabs (36.7%), blue crab (27.8%), and shrimp (25.1%) were the most important in the late wet season. Average trophic position of juveniles remained similar across wet (3.32 ± 0.10) and dry (3.23 ± 0.11) seasons. These findings provide important information to help decision makers to promote more efficient management and conservation legislations to protect early life stages of the Atlantic goliath grouper in tropical estuaries.

Differential feeding habits of the shallow-water hydrothermal vent crab Xenograpsus testudinatus correlate with their resident vent types at a scale of meters

Abstract. The shallow-water hydrothermal vents (SVs) located off Kueishan (KS) Island, Taiwan, are one of the world's most intensively studied vent systems. It has long been known that white vents (WVs) and yellow vents (YVs) differ in the color and composition of the vent plumes. The endemic vent crabs (Xenograpsus testudinatus) are abundant in both vent types, and ovigerous females migrate to the vent periphery with a distance of 100–200 m to release their offspring. However, most research on the vent crabs was associated with WV or unspecified vent areas. To increase our knowledge of the crabs dwelling in other vent types, we compared the feeding habits of the vent crabs living in WV and YV with 2 sampling months. Specifically, we examined the benthic community of WV and YV, the isotopic niche width, and protein expression patterns of the crabs from the two vents at a distance of 100 m and sampled in July and August 2010. The coverage of sessile organisms and low-mobility fauna in WV was more abundant than in YV, based on the survey in August 2010. The δ13C and δ15N values of crabs differed spatially and temporally (multivariate analysis of variance test; p<0.05). The niche width of the vent crabs from YV-August (0.88 ‰2) narrowed substantially compared to the rest, i.e., YV-July (2.94 ‰2), WV-July (2.88 ‰2), and WV-August (3.62 ‰2; p<0.05), respectively. Based on the protein expression patterns, the vent crabs exhibited three groups, i.e., WV-July and YV-July, WV-August, and YV-August, respectively. Our results indicated that the dwelling crabs were associated with their living vent, and within-vent variability was more noticeable in YV compared to WV. We suggested that vent crabs inhabit their resident vent. Even at a scale of meters, trans-vent movement is probably rare as an adaptation to minimize predation risk.

Shark teeth zinc isotope values document intrapopulation foraging differences related to ontogeny and sex

Trophic ecology and resource use are challenging to discern in migratory marine species, including sharks. However, effective management and conservation strategies depend on understanding these life history details. Here we investigate whether dental enameloid zinc isotope (δ66Znen) values can be used to infer intrapopulation differences in foraging ecology by comparing δ66Znen with same-tooth collagen carbon and nitrogen (δ13Ccoll, δ15Ncoll) values from critically endangered sand tiger sharks (Carcharias taurus) from Delaware Bay (USA). We document ontogeny and sex-related isotopic differences indicating distinct diet and habitat use at the time of tooth formation. Adult females have the most distinct isotopic niche, likely feeding on higher trophic level prey in a distinct habitat. This multi-proxy approach characterises an animal’s isotopic niche in greater detail than traditional isotope analysis alone and shows that δ66Znen analysis can highlight intrapopulation dietary variability thereby informing conservation management and, due to good δ66Znen fossil tooth preservation, palaeoecological reconstructions.

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.

Tidewater glaciers as “climate refugia” for zooplankton-dependent food web in Kongsfjorden, Svalbard

With climate warming, many tidewater glaciers are retreating. Fresh, sediment-rich sub-glacial meltwater is discharged at the glacier grounding line, where it mixes with deep marine water resulting in an upwelling of a plume visible in front of the glacial wall. Zooplankton may suffer increased mortality within the plume due to osmotic shock when brought in contact with the rising meltwater. The constant replenishment of zooplankton and juvenile fish to the surface areas attracts surface-foraging seabirds. Because access to other feeding areas, such as the marginal ice zone, has become energetically costly due to reduced sea-ice extent, glacial plumes may become increasingly important as “climate refugia” providing enhanced prey availability. Here, we investigated zooplankton concentrations within the plume and adjacent waters of four tidewater glaciers in Kongsfjorden, Svalbard, in early August 2016 and late July 2017. Our aim was to compare the zooplankton composition, abundance, and isotopic signatures within the plumes to those in adjacent fjord and shelf waters. Our hypothesis was that the plumes resulted in increased zooplankton mortality through osmotic shock and increased prey availability to predators. The mortality due to osmotic shock in the glacial plume was low (<5% dead organisms in samples), although slightly higher than in surrounding waters. This indicates that plumes are inefficient “death traps” for zooplankton. However, the high abundance and biomass of zooplankton within plume areas suggest that the “elevator effect” of rising glacial water supplies zooplankton to the sea surface, thereby enhancing prey availability for surface-feeding seabirds. Thus, our study provides evidence that glacial plumes are important as “climate refugia” for foraging seabirds. Stable isotope signatures showed that the glacial bay zooplankton and fish community represent a distinct isotopic niche. Additionally, zooplankton mortality associated with the plume estimated over 100-days of melt season supports a flux of 12.8 tonnes of organic carbon to benthic communities in the glacial bays. Benthic scavengers, such as Onisimus caricus and Anonyx nugax, were abundant in the glacial bay, where they feed on sinking organic matter.

Opportunistic consumption of marine pelagic, terrestrial, and chemosynthetic organic matter by macrofauna on the Arctic shelf: a stable isotope approach.

Macrofauna can contribute substantially to the organic matter cycling on the seafloor, yet the role of terrestrial and chemosynthetic organic matter in the diets of microphagous (deposit and suspension) feeders is poorly understood. In the present study, we used stable isotopes of carbon and nitrogen to test the hypothesis that the terrestrial organic matter supplied with river runoff and local chemosynthetic production at methane seeps might be important organic matter sources for macrofaunal consumers on the Laptev Sea shelf. We sampled locations from three habitats with the presumed differences in organic matter supply: "Delta" with terrestrial inputs from the Lena River, "Background" on the northern part of the shelf with pelagic production as the main organic matter source, and "Seep" in the areas with detected methane seepage, where chemosynthetic production might be available. Macrobenthic communities inhabiting each of the habitats were characterized by a distinct isotopic niche, mostly in terms of δ13C values, directly reflecting differences in the origin of organic matter supply, while δ15N values mostly reflected the feeding group (surface deposit/suspension feeders, subsurface deposit feeders, and carnivores). We conclude that both terrestrial and chemosynthetic organic matter sources might be substitutes for pelagic primary production in the benthic food webs on the largely oligotrophic Laptev Sea shelf. Furthermore, species-specific differences in the isotopic niches of species belonging to the same feeding group are discussed, as well as the isotopic niches of the symbiotrophic tubeworm Oligobrachia sp. and the rissoid gastropod Frigidoalvania sp., which are exclusively associated with methane seeps.

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

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

Ecological traits of the franciscana dolphin (Pontoporia blainvillei) from the Late Pleistocene to the present days based on stable isotope analysis

AbstractLate Pleistocene fossils from southern Brazil provide a unique opportunity to study the diet and habitat use of the extant franciscana dolphin (Pontoporia blainvillei) prior to human influence. Here, we subject 19 fossil and 21 contemporary specimens to stable carbon and oxygen isotope analysis (after first ruling out diagenesis via Fourier transform infrared spectroscopy). The two groups differ in their isotopic composition (PERMANOVA, p < .01), with fossils yielding more disparate δ13C values and recent samples more disparate δ18O values. Their isotopic niches show an overlap of ~20%, with that of the fossils being slightly wider (SEAc = 2.25‰2 versus 1.84‰2). We attribute these differences to impoverished modern fish communities and temporal changes in freshwater influx.

Specialist shorebird respond to prey and habitat availability through trophic plasticity

Spatiotemporal variations in food availability represent a challenge to the persistence of specialist species. The American oystercatcher (Haematopus palliatus) is a shorebird regarded as a bivalve specialist, although foraging habitats and prey species may vary along its distribution. Here, we studied American oystercatcher breeding in sites with variable landscapes to test the effect of temporal and spatial variations in food availability and dietary aspects. Between 2017 and 2021, we sampled oystercatchers (n = 100) and macroinvertebrates at the mesolitoral zone in five foraging areas in southern Brazil, three composed by sand and rock substrates (mixed), and two by sandy beach only. We obtained biological samples from oystercatchers and macroinvertebrates for carbon (δ13C) and nitrogen (δ15N) stable isotope analysis. In addition, we carried out systematic sampling of macroinvertebrates in sandy beaches at foraging areas to assess prey availability. Main findings showed the oystercatcher diet to be influenced both by the heterogeneity of foraging habitats and temporal fluctuations in availability of food resources. Bivalves contributed ∼60% to the diet of oystercatchers, but differences in the preferred bivalve species were detected among areas. In sites with mixed substrates, oystercatchers had a wider isotopic niche, suggesting habitat heterogeneity induced a more varied diet. Finally, we also observed interannual variation in the diet that may be associated with variation in macroinvertebrate availability on sandy beaches, especially for non-bivalve prey. Therefore, both temporal variations in food availability and foraging habitat heterogeneity seem to shape the foraging ecology of oystercatchers in the coastal zone, evidencing trophic plasticity in this specialist shorebird.

Paleodiets of hunter‐gatherers from the central Patagonian coast: Reviewing scopes and limitations of stable isotope analyses

AbstractThis work reviews paleodietary reconstructions of hunter‐gatherers from the northeast province of Chubut, Argentina. The isotopic record of human bone can complement information from zooarchaeological and technological studies with its particular resolution on the long‐term diet of individuals. Previous δ13C and δ15N studies found dietary differences between the coastal area and the lower basin of the Chubut River, both in the proportion of marine resources in the diet and the trophic positions of prey. However, it is challenging to test hypotheses concerning human diets in northeastern Patagonia, where C3 and CAM plants grow and terrestrial, riverine, and marine prey are available. Here, we focus on the local isotopic ecology to assess the scopes and limitations of δ13C and δ15N stable isotopes analyses. We provide new data on native faunas spanning different trophic levels of terrestrial, riverine, and marine environments. After establishing the basic patterns in the local isotope record, we reanalyze the available δ13Ccol, δ15N, and δ13Capa measurements from 23 individuals studied 15 years ago. That analysis reveals a relatively continuous distribution of several overlapping resources on the coast of northeastern Patagonia. Bayesian mixing models identified a combination of terrestrial and riverine resources aggregated as the main component of long‐term diets. However, it generated diffuse estimations. The Bayesian ellipses, describing isotopic niches, emphasized more the spatial variations in diet between coastal and valley groups.

Movement and population dispersal of dolphinfish (Coryphaena hippurus) across the Eastern Tropical Pacific inferred from carbon and nitrogen stable isotope analyses.

The dolphinfish (Coryphaena hippurus) is a globally distributed marine predator that supports one of the most important coastal fisheries along the Eastern Tropical Pacific (ETP), but its spatial movements in this area are poorly understood. Stable isotope values (δ13 C and δ15 N) of white muscle from dolphinfish (n = 220) captured at different locations across the ETP (i.e., Mexico, Costa Rica, Ecuador, Peru and oceanic areas) were normalized to copepod baseline stable isotope values to estimate dolphinfish trophic position, movements and population dispersal. Movement or residence patterns were inferred from the difference in δ15 N values (Δ15 Ndolphinfish-copepod ‰) between copepods and dolphinfish muscle. Baseline corrected isotope values (δ13 Cdolphinfish-copepod and δ15 Ndolphinfish-copepod ) of dolphinfish muscle were used to estimate isotopic niche metrics and infer population dispersal across isoscapes. Values of δ13 C and δ15 N differed between juvenile and adult dolphinfish and across the ETP. Trophic position estimates ranged from 3.1 to 6.0 with a mean of 4.6. Adults and juveniles had similar trophic position estimates, whereas isotopic niche areas (SEA ‰2 ) of adults were greater relative to juveniles in every location. Adult dolphinfish showed "moderate movement by some individuals" in all locations based on Δ15 Ndolphinfish-copepod values, except for Costa Rica where adults were classified with "high degree of movement by some individuals" whereas juveniles showed "limited movement" in all areas except Mexico. Population dispersal based on Δ15 Ndolphinfish-copepod values showed "moderate" and "high" dispersal for adults and "no dispersal" for most juveniles, except for Mexico. This study provides insight into potential dolphinfish spatial mobility across an area of interest for multiple nations, which can help to improve stock assessments and management of the species.

The introduced Arapaima gigas in the Bolivian Amazon: Trophic position and isotopic niche overlap with native species

AbstractNon‐native fish species may generate major ecological impacts on native assemblages. This study aims to assess the potential impact of the introduced Arapaima gigas on native fish assemblages in two oxbow lakes of the Bolivian Amazon. Stable isotope data were used to determine trophic position (TP) and isotopic niche overlap, to evaluate potential predation and competition interactions, respectively. Results suggest that A. gigas is more an omnivore than a top predator, as often claimed. Arapaima gigas occupied an intermediate TP between detritivore/herbivore and piscivore fish species and showed broader isotopic niche compared to most native species analysed. The isotopic niche of A. gigas significantly overlapped with most native fish species in one lake (i.e. Lake Mentiroso), while there was low niche overlap in the second (i.e. Lake Miraflores). Given its omnivorous tendencies, the predation impact of A. gigas on other fish species is likely less than currently claimed and likely varies with the food web structure of the ecosystem. More precise data on resource availability and use are necessary to infer whether niche overlap will have negative impacts on native fish species through potential competitive interactions. Increasing our understanding on the processes generating impact of these introductions on resident communities through food web ecology will pave the way for better resource management and conservation efforts.

Stable isotope ecology and interspecific dietary overlap among dolphins in the Northeast Atlantic

Dolphins are mobile apex marine predators. Over the past three decades, warm-water adapted dolphin species (short-beaked common and striped) have expanded their ranges northward and become increasingly abundant in British waters. Meanwhile, cold-water adapted dolphins (white-beaked and Atlantic white-sided) abundance trends are decreasing, with evidence of the distribution of white-beaked dolphins shifting from southern to northern British waters. These trends are particularly evident in Scottish waters and ocean warming may be a contributing factor. This mobility increases the likelihood of interspecific dietary overlap for prey among dolphin species previously separated by latitude and thermal gradients. Foraging success is critical to both individual animal health and overall population resilience. However, the degree of dietary overlap and plasticity among these species in the Northeast Atlantic is unknown. Here, we characterise recent (2015-2021) interspecific isotopic niche and niche overlap among six small and medium-sized delphinid species co-occurring in Scottish waters, using skin stable isotope composition (δ13C and δ15N), combined with stomach content records and prey δ13C and δ15N compiled from the literature. Cold-water adapted white-beaked dolphin have a smaller core isotopic niche and lower dietary plasticity than the generalist short-beaked common dolphin. Striped dolphin isotopic niche displayed no interspecific overlap, however short-beaked common dolphin isotopic niche overlapped with white-beaked dolphin by 30% and Atlantic white-sided dolphin by 7%. Increasing abundance of short-beaked common dolphin in British waters could create competition for cold-water adapted dolphin species as a significant portion of their diets comprise the same size Gadiformes and high energy density pelagic schooling fish. These priority prey species are also a valuable component of the local and global fishing industry. Competition for prey from both ecological and anthropogenic sources should be considered when assessing cumulative stressors acting on cold-water adapted dolphin populations with projected decline in available habitat as ocean temperatures continue to rise.

Dietary Niche of Three Omnivorous Turtle Species in a Northern Florida River: Insights from Stable Isotope Analysis

Macrochelys suwanniensis (Suwannee Alligator Snapping Turtle) coexists with 2 other native large omnivorous turtle species (Chelydra serpentina [Snapping Turtle] and Trachemys scripta scripta [Yellow-bellied Slider]) in a 9-km section of the Santa Fe River in northern Florida. A major shift in dominant submersed aquatic vegetation prompted us to quantify trophic position and niche overlap among these 3 species. Here, we examine carbon and nitrogen isotopic values of these turtles and their potential food resources within the changing riverine system. We provide evidence of low isotopic niche overlap between M. suwanniensis and C. serpentina, whereas T. s. scripta occupied a discrete niche having lower carbon and nitrogen values.

Fine-scale intraspecific niche partitioning in a highly mobile, marine megafauna species: implications for ecology and conservation.

A species may partition its realized ecological niche along bionomic and scenopoetic axes due to intraspecific competition for limited resources. How partitioning manifests depends on resource needs and availability by and for the partitioning groups. Here we demonstrate the utility of analysing short- and long-term stable carbon and nitrogen isotope ratios from imperiled marine megafauna to characterize realized niche partitioning in these species. We captured 113 loggerhead sea turtles (Caretta caretta) at a high-use area in the eastern Big Bend, Florida, between 2016 and 2022, comprising 53 subadults, 10 adult males and 50 adult females. We calculated trophic niche metrics using established and novel methods, and constructed Bayesian ellipses and hulls, to characterize loggerhead isotopic niches. These analyses indicated that loggerheads partition their realized ecological niche by lifestage, potentially along both bionomic (e.g. trophic) and/or scenopoetic (e.g. habitat, latitude or longitude) axes, and display different characteristics of resource use within their niches. Analysis of stable isotopes from tissues with different turnover rates enabled this first characterization of intraspecific niche partitioning between and within neritic lifestages in loggerhead turtles, which has direct implications for ongoing research and conservation efforts for this and other imperiled marine species.