Isotopic Niche Research Articles

Longitudinal changes on ecological diversity of Neotropical fish along a 1700 km river gradient show declines induced by dams

In naturally flowing waters, communities are expected to increase their taxonomic and functional diversity as well as increase the complexity of food web architecture along the longitudinal gradient from headwaters to mouth. However, these theories do not necessarily apply to dammed rivers. We analysed fish communities at 12 locations along the 1700 km stretch of the transnational Uruguay River. We determined species richness, beta-diversity and its components (turnover and nestedness), trophic diversity, and isotopic niches at community level, with particular focus on areas upstream and downstream of the four existing dams. We estimated trophic diversity metrics based on diet analysis, and isotopic niche breadth based on Bayesian isotopic niche models. We detected consistent longitudinal changes, supporting predictions from the River Continuum Concept. However, taxonomic richness, trophic diversity, and isotopic niche decreased abruptly immediately downstream of dams, and the component contributing the most to beta diversity was turnover, with the highest values occurring mainly after the cascade of dams in the upper Uruguay River, fitting expectations from the Serial Discontinuity Concept. The dam-free stretches of the Uruguay River allowed the recovery of ecological diversity as fish richness and trophic and isotopic niche increased. The negative impacts of dams should urgently be considered in the Neotropics, where many large dams are projected.

Variations in inter-specific and sex-related niche partitioning in pelagic boobies during their annual cycle.

Animals that co-occur in a region (sympatry) may share the same environment (syntopy), and niche differentiation is expected among closely related species competing for resources. The masked booby (Sula dactylatra) and smaller congeneric red-footed booby (Sula sula) share breeding grounds. In addition to the inter-specific size difference, females of both species are also larger than the respective males (reversed sexual size dimorphism). Although both boobies consume similar prey, sometimes in mixed-species flocks, each species and sex may specialize in terms of their diet or foraging habitats. We examined inter- and intra-specific differences in isotopic values (δ13C and δ15N) in these pelagically feeding booby species during the incubation period at Clarion Island, Mexico, to quantify the degrees of inter- and intra-specific niche partitioning throughout the annual cycle. During incubation, both species preyed mainly on flyingfish and squid, but masked boobies had heavier food loads than red-footed boobies. There was no overlap in isotopic niches between masked and red-footed boobies during breeding (determined from whole blood), but there was slight overlap during the non-breeding period (determined from body feathers). Female masked boobies had a higher trophic position than conspecific males during breeding; however, no such pattern was detected in red-footed boobies. These results provide evidence of inter- and intra-specific niche partitioning in these tropical seabird species, particularly during the breeding period and in the more-dimorphic species. Our results suggest that these closely related species use different strategies to cope with the same tropical marine environment.

Resource partitioning in hammerhead shark species out-migrating from coastal ecosystems in the Gulf of California

Juveniles of large hammerhead shark species occupy coastal nurseries before migrating offshore to reproduce. In the central Gulf of California, artisanal elasmobranch fisheries have reported catches of juvenile scalloped Sphyrna lewini and smooth S. zygaena hammerhead sharks, but their local foraging habits are yet to be fully understood. In this study, the trophic niches of both hammerhead species as well as of sympatric Pacific sharpnose sharks Rhizoprionodon longurio were investigated using stable isotope values (δ13C, δ34S and δ15N) and fatty acid compositions in whole blood and muscle tissues. Despite interspecific similarities among trophic niches, smooth hammerheads were characterized by lower δ13C, higher δ34S and greater proportion of docosahexaenoic acid (DHA) in both tissues, suggesting they were already partly relying on offshore pelagic resources. For scalloped hammerheads, muscle reflected coastal dietary resources, while offshore trophic markers were detected in blood integrating prey signal over shorter time periods, indicating their more recent initiation of ontogenetic migration. Multidimensional niche calculation revealed low overlap between hammerhead shark trophic niches, implying that potential fine-scale differences in habitat use could reduce competition between these morphologically and ecologically similar species. In the meantime, the isotopic niches of juvenile scalloped and smooth hammerheads were smaller than that of Pacific sharpnose sharks, suggesting they could be more specialized consumers. Overall, the identification of foraging grounds for juvenile hammerhead sharks calls for a future characterization of their residency time in coastal ecosystems to further understand their interactions with fishing pressure in the Gulf of California.

Individuals matter more than replicates: distribution of sampling effort in isotopic niche estimation

Individuals matter more than replicates: distribution of sampling effort in isotopic niche estimation

Trophic niche overlap of deep-sea fish species revealed by the combined approach of stomach contents and stable isotopes analysis in the Central Tyrrhenian Sea

Understanding the trophic ecology of marine ecosystems is a key challenge given that they are subject to anthropogenic pressures that can alter the integrity of the food web. The bathyal zone of the Central Tyrrhenian Sea is characterized by an high level of community biodiversity, heavy anthropogenic pressure and thus represents a fundamental environment for the study of trophic relationships between demersal fish species which live in sympatry, but this basin still remains data deficient. To fill this information gap, we investigated the trophic niche overlap of four deep-sea fish species, Galeus melastomus, Helicolenus dactylopterus, Lepidorhombus boscii, and Trisopterus capelanus, in the Central Tyrrhenian Sea using an integrated approach of stomach content and stable isotopes analysis (δ13C and δ15N). Our analysis revealed that the blackmouth catshark G. melastomus displays a wide trophic niche with considerable overlap with other fish species, as it consumed diverse prey including fish, crustaceans, and cephalopods, thus proving a generalist and opportunistic feeding behavior. The blackbelly rosefish H. dactylopterus exhibits a narrower isotopic trophic niche comprising mainly benthic crustaceans and so suggesting a marked specialism in feeding strategy. The diet of the four-spotted megrim L. boscii showed a marked overlap with that H. dactylopterus, as these species shared a similar benthic habit and relied upon a similar pool of resource. The poor cod T. capelanus showed a diverse diet comprising both prey captured close to the bottom, especially decapod and mysid crustaceans, and bathypelagic prey, mainly Osteichthyes, Myctophidae, and amphipod Hyperiidae, with an intermediate partially segregated isotopic niche width. The combination of stomach content analysis and stable isotopes analysis provided the first characterization of the trophic relationships, shedding light on the trophic niche overlap of these four ecologically important deep sea fish species. The diets of the four investigated species showed similarities in the composition of prey, but also differences which allows them to partially reduce competition. This information may be of crucial relevance for the development of effective management and conservation strategies for the bathyal Mediterranean environment.

A longitudinal study of endocrinology and foraging ecology of subadult gray whales prior to death based on baleen analysis

Individual-level assessments of wild animal health, vital rates, and foraging ecology are critical for understanding population-wide impacts of exposure to stressors. Large whales face multiple stressors, including, but not limited to, ocean noise, pollution, and ship strikes. Because baleen is a continuously growing keratinized structure, serial extraction, and quantification of hormones and stable isotopes along the length of baleen provide a historical record of whale physiology and foraging ecology. Furthermore, baleen analysis enables the investigation of dead specimens, even decades later, allowing comparisons between historic and modern populations. Here, we examined baleen of five sub-adult gray whales and observed distinct patterns of oscillations in δ15N values along the length of their baleen plates which enabled estimation of baleen growth rates and differentiation of isotopic niche widths of the whales during wintering and summer foraging. In contrast, no regular patterns were apparent in δ13C values. Prolonged elevation of cortisol in four individuals before death indicates that chronic stress may have impacted their health and survival. Triiodothyronine (T3) increased over months in the whales with unknown causes of death, simultaneous with elevations in cortisol, but both hormones remained stable in the one case of acute death attributed to killer whale predation. This parallel elevation of cortisol and T3 challenges the classic understanding of their interaction and might relate to increased energetic demands during exposure to stressors. Reproductive hormone profiles in subadults did not show cyclical trends, suggesting they had not yet reached sexual maturity. This study highlights the potential of baleen analysis to retrospectively assess gray whales' physiological status, exposure to stressors, reproductive status, and foraging ecology in the months or years leading up to their death, which can be a useful tool for conservation diagnostics to mitigate unusual mortality events.

Trophic interactions between bullet tuna (Auxis rochei) and frigate tuna (A. thazard) in the Gulf of Cadiz

Bullet tuna (BLT), Auxis rochei, and frigate tuna (FRI), Auxis thazard, are globally exploited wide-ranging species. These anatomically similar tunas may share pelagic habitats in the Atlantic Ocean and Mediterranean Sea. Feeding patterns were investigated in BLT and FRI individuals caught in the Gulf of Cadiz (East Atlantic) in 2019–2021 with the aim to decipher trophic interactions. Stomach content analyses showed a predominantly piscivorous diet, dominated by anchovy (Engraulis encrasicolus). Further cluster analysis coupled to a similarity profile permutation test revealed interspecific dietary differences in 2019 and 2021, as well as interannual differences in BLT. The trophic niche estimated from muscle and liver isotopic data was broader in BLT, and no considerable overlap of isotopic niches was found between the two species. Trophic positions (TP) were estimated in 2020 and 2021 using isotopic data of mesozooplankton as the food web baseline of the area. Muscle data showed that BLT occupied a higher TP than FRI in both 2020 and 2021, whereas liver data estimated a higher TP for FRI in 2020. Differences in isotopic compositions of BLT from the Gulf of Cadiz compared to individuals from the western Mediterranean suggest low mixing rates between populations.

Vertebral microchemistry as an indicator of habitat use of the oceanic whitetip shark Carcharhinus longimanus in the central and eastern Pacific Ocean.

The oceanic whitetip shark, Carcharhinus longimanus, is a highly migratory, epipelagic top predator that is classified as critically endangered. Although this species is widely distributed throughout the world's tropical oceans, its assumed mobility and pelagic behavior limit studies to derive required lifetime data for management. To address this data deficiency, we assessed variation in the habitat use of C. longimanus by oceanic region and over ontogeny through time series trace element and stable isotope values conserved along the vertebral centra (within translucent annulus bands) of 13 individuals sampled from the central and eastern Pacific Ocean. Elemental ratios of Mg:Ca, Mn:Ca, Fe:Ca, Zn:Ca, and Ba:Ca varied significantly among individuals from both sampling regions while principal component analysis of combined standardized elements revealed minimal overlap between the two areas. The limited overlap was also in agreement withstable isotope niches. These findings indicate that C. longimanus exhibit a degree of fidelity to sampling regions but also connectivity in a proportion of the population. The relatively stable Sr:Ca ratio supports its occurrence in oceanic environments. The decreasing trends in Ba:Ca, Mn:Ca, and Zn:Ca ratios, as well as in carbon and nitrogen isotope values along vertebral transects, indicate that C. longimanus undergo a directional habitat shift with age. Combined elemental and stable isotope values in vertebral centra provide a promising tool for elucidating lifetime data for complex pelagic species. For C. longimanus, management will need to consider subpopulation movement behavior in the Pacific to minimize the potential for localized depletions. Further work is now required to sample individuals across the entire Pacific and to link these findings with genetic and movement data to define population structure.

Stable isotopes delineate regional pelagic food web structure in British Columbia’s coastal ocean

The pelagic food webs of British Columbia’s (BC) coastal oceans have never had a comprehensive review of their trophic structure. In this study, we analyzed carbon (δ13C) and nitrogen (δ15N) stable isotope ratios of pelagic food web components collected from four regions in southern BC: Juan de Fuca Strait, the Strait of Georgia, Queen Charlotte Strait, and Queen Charlotte Sound during an August 2019 survey. In addition, conductivity, temperature, and depth (CTD), chlorophyll, and nutrient data were collected to assess the oceanographic basis for regionalization. Between regions, we observed differences in the isotopic baseline driven by regional oceanography. Likewise, we also observed oceanography-driven differences in regional food chain length, carbon range, and isotopic overlap. Species-specific trophic level and isotopic niche were determined. For common pelagic species, we described how trophic level varied regionally, was not always in line with previously published dietary data, and instead largely tracked regional changes in food chain length. We conclude that variable food web properties and trophic ecology can manifest across the small spatial scales of the BC coast’s discrete regions.

Dietary partitioning among three cryptobenthic reef fish mesopredators revealed by visual analysis, metabarcoding of gut content, and stable isotope analysis

AbstractUnderstanding how mesopredators partition their diet and the identity of consumed prey can assist in understanding the ecological role predators and prey play in ecosystem trophodynamics. Here, we assessed the diet of three common coral reef mesopredators; Pseudochromis flavivertex, Pseudochromis fridmani, and Pseudochromis olivaceus from the family Pseudochromidae, commonly known as dottybacks, using a combination of (i) visual stomach content analysis, (ii) stomach content DNA metabarcoding (18S, COI), and (iii) stable isotope analysis (δ15N, δ13C). In addition, P. flavivertex is found in two distinct color morphs in the Red Sea, providing an opportunity to analyze intra‐morph differences. These techniques revealed partitioning in the dietary composition and resource use among species. Arthropods comprised the main dietary component of P. flavivertex (18S > 60%; COI > 10%) and P. olivaceus (18S = 57.2%), while P. fridmani ingested predominantly mollusks (18S = 51.3%, COI = 24.6%). Despite being small predators, microplastics were found in the gut content of some of these fishes. Stable isotope analysis showed differences in species' isotopic niche breadth and trophic position. Pseudochromis olivaceus presented the largest isotopic niche (SEAC = 1.61‰2), while P. fridmani showed the smallest isotopic niche (SEAC = 0.45‰2) among species. Although the two techniques used for stomach content analysis did not show differences in the diet within color morphs of P. flavivertex, they differed in the isotopic niche and resource use. Despite our limited sampling, our findings provide evidence of species‐specific differences in the trophic ecology of dottybacks and demonstrate their important role as predators of cryptic invertebrates and small fishes. This study highlights the importance of combining several approaches (short‐term: visual analysis and DNA metabarcoding; and long‐term: isotope analysis) when assessing the feeding habits of coral reef fish, as they provide complementary information necessary to delimit their niches and understand the role that small mesopredators play in coral reef ecosystems.

Trophic niche partitioning of golden cuttlefish (Sepia esculenta) during ontogeny revealed by stable isotope analysis and DNA metabarcoding

Cephalopods are becoming more important in global fishery. Understanding the trophic ecology partitioning during their ontogeny is critical to the stock assessment and fishery management. The golden cuttlefish (Sepia esculenta), one of commercially important cephalopod species has been found at different life-history stages that co-inhibit in the coastal waters of Qingdao, China. In the present study, the trophic ecology of S. esculenta was assessed by using stable isotope analysis (SIA: based δ13C and δ15N from muscle and liver tissue) and DNA metabarcoding of the stomach contents through high-throughput sequencing (HTS). SIA uncovered the variation in isotope niche and trophic position of S. esculenta during ontogeny. DNA metabarcoding showed a higher resolution of prey identification than before. The combined analysis of SIA and DNA metabarcoding indicated that diet shifts occurred in the ontogeny of S. esculenta. Paralaval and juvenile prefer zooplankton, while sub-adult and adult prefer demersal fishes. The standard ellipse area (SEAc) indicated that low isotope niche overlaps among paralaval, juvenile and sub-adult of S. esculenta, which reflect the squid foraging strategy during ontogeny. The study confirmed that S. esculenta resources can be partitioned into different life stages, suggesting a survival strategy to maintain population dynamics.

Living with voracious roommates: Factors that explain isotopic niche variation in a mixed colony of insectivorous bats.

Theory predicts that in resource-limited environments, coexisting species may overlap their niche dimensions but must differ in at least one to avoid competitive exclusion. Specifically, it has been suggested that the coexistence of competing species within a guild, could be sustained with mechanisms of resource partitioning, such as segregation along a trophic dimension. Among the most gregarious mammals are bats, which present diversification in their diet based on habitat choice and body size. Despite differences that could explain specialization in prey selection, there are insufficient studies that explore food overlap in mixed bat colonies and the factors that determine the selection of prey, both at intra- and inter-specific levels. To fill this gap, we analyzed the isotope signal (δ13C and δ15N) in feces collected in a mixed colony of Tadarida brasiliensis and Myotis chiloensis. To understand how several factors could influence these isotopic signals, intrinsic explanatory variables were analyzed, including body mass, body length, age, and sex. Also, extrinsic variables were analyzed, including monthly temporality and moonlight intensity. Our findings support age-dependent specialization in M. chiloensis, with a significant role of moonlight intensity and sex on δ15N. In T. brasiliensis, we identified a significant effect of size, sex, and ear length on δ15N. Our analysis indicates that both species of bats experience diverse degrees of overlap through austral summer months, affected by several factors that explain the variability in their fecal isotopic signals.

Filling the gap: The unique isotopic niche of medium-sized and large sharks in the northwestern Mediterranean Sea

The Mediterranean Sea is considered a biodiversity hotspot and hosts a broad diversity of shark species. Unfortunately, many shark populations have suffered a long story of overexploitation and their abundance has strongly declined over the last decades. A few studies have addressed the trophic role of sharks in the northwestern Mediterranean Sea, yet little is known about their isotopic niches. In this study, the isotopic niches of six shark species (Alopias vulpinus, Galeorhinus galeus, Hexanchus griseus, Isurus oxyrinchus, Prionace glauca and Squalus acanthias) from the northwestern Mediterranean Sea were determined and compared with those of sympatric predatory teleosts and marine mammals. Overall, δ13C and δ15N values were similar amongst all the shark species except for the three individuals of G. galeus, whose high δ15N values indicate recent immigration from a distinct area with a different isotopic baseline and hence were excluded from further analysis. The mixing models identified cephalopods as the dominant prey of the other five shark species and revealed that the medium-sized and large shark species studied here filled unique isotopic niches. Their trophic positions were similar to those of predatory teleosts and small dolphins but lower than those of larger dolphins, pilot whales and beaked whales. The high contribution of cephalopods to the diet of sharks in the northwestern Mediterranean Sea might be indicative of fish scarcity and deserves further research in the context of potential ontogenetic diet changes.

The foraging ecology of green turtles (Chelonia mydas) in New Caledonia

Green sea turtles (Chelonia mydas) face many threats from anthropogenic activity and are listed as Endangered on the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. They spend the majority of their lives in coastal foraging areas, and show strong fidelity to narrow foraging ranges. Understanding the foraging ecology of green turtles is therefore important for their management and conservation. Using stable isotopes analysis (n = 200) and identification of stomach contents (n = 21), this study investigated the foraging ecology of four distinct green turtle foraging aggregations (Goro, Isle of Pines, Ouen Island and Uo/Mato Islands) within the Grand Lagon Sud. This World Heritage Area located in New Caledonia supports regionally important nesting and foraging green turtle populations. The δ13C and δ15N values of skin samples ranged from − 19.3 to − 7.3‰ and 2.8–15.9‰, respectively. Metrics of isotopic niche and regression analyses were used to assess the degree of omnivory, trophic diversity and potential ontogenetic diet shifts in the different aggregations of foraging green turtles in this location. Each of the four sites and four age-classes had distinct isotopic niches, with evidence of omnivory and ontogenetic shifts, particular in new recruits and juveniles, and at Goro and Uo/Mato Islands. This was generally supported by the stomach contents analyses of a small number of green turtles opportunistically sampled from traditional ceremonies. This study provides important information on the foraging ecology of green turtles in New Caledonia, supporting management outcomes in this region. This collaboration with the tribes of the Grand Lagon Sud is also the first step towards the integrated management of the resource.

Intracolony variability in winter feeding and migration strategies of Atlantic puffins and black-legged kittiwakes

Polar ecosystems are subjected to many stressors, including climate change, that impact their overall functioning. Seabirds are good bioindicators of these systems as they readily respond to changes in environmental conditions. To quantify how environmental changes affect their life history, data on seabird diet, spatial distribution and body condition are needed to reveal the underlying mechanisms. We explored possible drivers of the winter distribution of single-colony populations of Atlantic puffins Fratercula arctica and black-legged kittiwakes Rissa tridactyla, two of the most numerous seabird species in the North Atlantic. Based on carbon and nitrogen stable isotopic data from feathers moulted during winter migration, we identified three groups of Atlantic puffins and two groups of black-legged kittiwakes occupying different isotopic niches. We then used geolocator tracking data for the same birds to determine if these groups reflected parallel differences in the location of moulting grounds rather than differences in their diet. We found that the isotopic niches of the three Atlantic puffin groups likely resulted from their use of different habitats during winter moult. In contrast, the isotopic niches of the two black-legged kittiwake groups were more likely a result of differences in their diet, as both groups were distributed in the same area. These findings suggest that different winter feeding and/or migration strategies may play a role in shaping the body condition of individuals for their following breeding season. We discuss the role of environmental conditions encountered by seabirds during winter migration to further elucidate such intracolony divergence in strategies. As polar ecosystems experience rapid changes in environmental conditions, the approach presented here may provide valuable information for the development of effective conservation measures (taking both intra- and intercolony variability into account), and to better predict future impacts of climate change.

Assessing the diet of modern and archaeological guanacos from the Great Chaco in Córdoba, Argentina, through stable isotope analysis (δ13C, δ15N) of bone and dentin collagen: Implications for paleoenvironmental and zooarchaeological studies

The application of stable isotope analyses allows a diachronic characterisation of species habitat and feeding behaviour, information of utmost importance for zooarchaeological research. In South America, the former distribution of the guanaco ( Lama guanicoe) encompassed a much larger territory than the current one. Within the Argentinean Great Chaco, only a small native population persists in Northwestern Córdoba province, Central Argentina, where it was once widely distributed. In this paper, we present the first set of dentin and collagen δ13C and δ15N values for this relict population ( N = 18) along with archaeological data of guanaco specimens ( N = 19) dated to the Middle and Late Holocene from the nearby Ongamira valley. Neither deciduous and permanent teeth nor males and females show marked differences within the modern samples. Both modern and archaeological guanaco δ13C values indicate a mixed diet of C3 and, to a lesser extent, C4 plants. Conversely, the δ15N values exhibit distinct signals between the Arid and the Mountain Chaco subregions. The diet breadth of the archaeological guanacos suggests the use of transitional or ecotonal environments, also reflected in the size of their isotopic niche as opposed to modern guanacos. Regarding the timing of the species retraction in the region, there is no evidence of a shift in its habitat during the period between ~4700 and 190014C years BP. We suggest their retraction probably occurred later than previously proposed in regional zooarchaeological models.

Dietary change of North Patagonian guanacos: A historical ecology perspective through the study of stable isotopes

This paper presents the results of a study on the isotopic ecology of guanacos in central western Argentina. We examine the historical population ecology of guanacos using stable isotope analysis of bone collagen from pre-Hispanic and modern guanaco populations ( n = 129), considering variability in two ecoregions: the Monte hot desert and the Andean-Patagonian cold desert. Our study addresses the consistency of guanaco diets over time, evaluating palaeoecology to provide information for conservation of this taxon. We found significant differences in isotopic niche size between modern and archaeological guanacos. When analyzed by ecoregion, there were significant differences in niche size through time, indicating that guanacos had distinctive dietary habits and occupied different ecological niches across the ecoregions. Comparing Standard Ellipse Areas (SEA) through time and across space, we observed that the archaeological SEA for guanacos is smaller than its modern counterpart in the Andean-Patagonia ecoregion. Conversely, in Monte, the archaeological SEA is larger than the one established for modern samples. The contrast between pre-Hispanic and modern populations highlight the impact of human activity and conservation efforts on the distribution and ecology of guanacos. These findings have important implications for understanding guanaco ecology with consequences for conservation policies.

Hydro-morphological features and functional structure of fish assemblages mediate species isotopic niches in estuaries

Despite the popularity of stable isotope analysis (Carbon and Nitrogen), the drivers of species isotopic niches and their consequences on food web functioning remained poorly described, especially in estuaries. Here, we hypothesised that species niche characteristics are influenced by ecosystem hydro-morphological features and the functional structure of fish assemblages. The trophic niches of four fish species (Dicentrarchus labrax, Pomatoschistus minutus, Sprattus and Platichthys flesus) and overlap between them were compared during two consecutive autumns in eight estuarine ecosystems of the northeast Atlantic Ocean. Our findings demonstrated a relative steadiness of trophic positions of fish in estuaries, despite a high level of interspecific variability. The seabass generally occupied the higher trophic position, followed by the sand goby, while the flounder and sprat were at the lowest position in the food web. Species isotopic niches and overlap were mediated by some estuarine features, including estuary size and intertidal extent, but also by biotic interactions at the intra- and inter-specific levels, as reflected by the influence of fish abundance and functional diversity metrics. Our results support the statement that the intertidal area is a pivotal factor in regulating trophic interactions, by promoting niche partitioning and diversification of trophic resources between species. Moreover, niche size and dietary divergence of species decreased with increasing regularity of trophic strategies within assemblages, suggesting that species occupied more restricted trophic niches when they were less subjected to competition for the same food resources. These findings emphasize the importance of local conditions and the functional composition of assemblages in modulating the trophic resources consumed by fish in estuarine environments.

Movement ecology determines isotopic niche width in the undulate skate Raja undulata

Functional connectivity patterns of threatened species provide critical information for effective management and conservation spatial planning. In this study, we used a non-lethal novel combination of acoustic telemetry and stable isotope analysis (SIA) on the Near Threatened elasmobranch Raja undulata to evaluate shifts in resource/space use in response to differences in residency patterns within a marine protected area (MPA) in NW Spain. By using acoustic telemetry, we classified individuals of a local population of R. undulata as high- or low-residence fish based on estimates of residence index (% of days present within the studied MPA). Individuals with a high residence index (≥50%) showed a narrower isotopic niche width than low-residence fish, as they were exposed to a limited variety of habitats and food resources. Conversely, low-residence fish (mainly females) showed wider isotopic niche widths, indicative of greater diversity of resources from different environments. Therefore, we hypothesize that low-residence individuals play an important role connecting different habitats and even populations, particularly through dispersion patterns of females along the coast. This study provides valuable insights into the movement and resource ecology of R. undulata and their role in the functional connectivity of the coastal ecosystem, with direct applicability to inform management of the studied MPA.

Trophic position and niche overlap of an Asian weatherfish (Misgurnus bipartitus), western tubenose goby (Proterorhinus semilunaris) and native benthic fish species

Co-occurring and morphologically similar species have adapted to differential niches to minimize competition. An invasive alien species can occupy an ‘empty niche’ in introduced ranges. Alternatively, the invader may occupy an overlapping niche and compete with native species to a certain degree. In a Western European lowland brook with high nutrient loads, we studied a benthic community of five fish species, including two alien species: an Asian weatherfish (Misgurnus bipartitus) and the western tubenose goby (Proterorhinus semilunaris). The native species concerned stone loach (Barbatula barbatula), spined loach (Cobitis taenia), and gudgeon (Gobio gobio). Because of the unknown effects of the invaders on native benthic fish species, the trophic position, isotopic niche overlap, and potential food competition among these species were identified using nitrogen and carbon stable isotopes. The trophic levels of the five fish species indicated that they are secondary consumers. The body size of native fish species correlated significantly negatively with their δ15N (‰) signature, in contrast with the invaders indicating that the latter are generalists. Significant isotopic niche overlap was observed among all benthic species. The degree of niche overlap of M. bipartitus was the highest with G. gobio (91.8%). Proterorhinus semilunaris showed the highest degree of niche overlap with B. barbatula (91.2%). It was notable that the observed niche overlap between the native B. barbatula and C. taenia was high (99.2%). Overlap between M. bipartitus and P. semilunaris was low (8.9%), indicating little resource competition between these alien species. Native species showed wider isotopic niches than the invaders. Bayesian mixing models revealed that native and alien species slightly differ in their main diet. The results suggest that the invaders are plastic in their resource use, leading to niche differentiation and promoting the co-existence of benthic fish species.