Isotopic Niche Overlap Research Articles

Effects of lipid and urea extraction on stable isotope values (δ13C and δ15N) of two batoids: A call for more species‐specific investigations

AbstractThe presence of lipids and urea in elasmobranch tissues can affect carbon (δ13C) and nitrogen (δ15N) stable isotope values, introducing bias in food web interpretations. Information on how lipids and urea affect δ13C/δ15N is only available for < 5% of ~ 1150 described elasmobranch species and results are highly variable among existing studies. Here, we investigate the effects of lipid and urea extraction on δ13C, δ15N, and C:N ratios in muscle and blood plasma of two batoids (Pastinachus ater and Himantura australis) and examine the influence of lipid/urea extraction on isotopic niche size and overlap. Lipid extraction (LE) did not affect δ13C, δ15N, or C:N ratios, suggesting low lipid content. Urea extraction (UE), however, increased δ15N and C:N in muscle. The ~ 1‰ δ15N increase represents a shift of ~ 0.5 trophic levels relative to bulk samples, highlighting the importance of UE to accurately assess trophic positions. Although there was no effect of any treatment on niche size, the probability of P. ater occurring within the niche of H. australis increased following UE. Overall, results suggest that urea should be removed from muscle prior to analysis, but LE is not required. Given the interspecific variability in the effects of lipid/urea on elasmobranch δ13C/δ15N, more studies are needed to assess the effects of lipid and urea on a broader range of species to produce a generalized understanding. Where no species‐specific data are available, we recommend pilot samples are analyzed to determine if LE is needed prior to preparation of the overall sample set.

Effects of Multiple Nonnative Fish on an Imperiled Cyprinid, Hornyhead Chub

AbstractNonnative fish can have substantial negative effects on the abundance and distribution of native fishes through predation and competition. Nonnative predators are of particular interest because they represent novel threats to native prey species that are not adapted to their presence. Prey species with limited distributions or population sizes may be particularly vulnerable to the effects of nonnative predators. In the Laramie River, four nonnative predators—Brown TroutSalmo trutta, Rainbow TroutOncorhynchus mykiss, Brook TroutSalvelinus fontinalis,and Smallmouth BassMicropterus dolomieuare present along with a state‐imperiled population of Hornyhead ChubNocomis biguttatus. The abundance of Hornyhead Chub has declined with increasing abundance of nonnative predators, with the probability of occurrence of Hornyhead Chub dropping drastically when Smallmouth Bass were present. All four nonnative species preyed on native cyprinids, but Smallmouth Bass relied most heavily on fish as a prey item. Isotopic niche overlap occurred between Hornyhead Chub and all of the nonnative predator species. Our results demonstrate that nonnative predators have the potential to negatively affect the abundance and distribution of Hornyhead Chub through the mechanisms of predation and competition, and predator identity is important in determining the extent of effects. Smallmouth Bass are a greater concern than nonnative salmonids because of their more piscivorous behavior, and their recent upstream expansion may be limiting the downstream distribution of Hornyhead Chub in the Laramie River.

Inter-regional variation in feeding patterns of skipjack tuna (Katsuwonus pelamis) inferred from stomach content, stable isotope and fatty acid analyses

Foraging habits of skipjack tuna, SKJ (Katsuwonus pelamis), were investigated in three Spanish marine regions (Balearic Sea, Alboran Sea and Gulf of Cadiz) using stomach content (SCA), stable isotope (SIA), and fatty acid (FA) analyses. The three methodological approaches yielded significant differences among locations. All the studied areas appear to serve as important foraging grounds for SKJ, but the diet composition significantly varied among them. The predominant prey species in the respective locations were the North Atlantic krill, Meganyctiphanes norvegica, in the Balearic Sea, the flyingfish, Exocœtus volitans, in the Alboran Sea, and the anchovy, Engraulis encrasicolus, in the Gulf of Cadiz. Regional differences were also found in δ13C and δ15N values; furthermore, the analysis of standard ellipse areas (SEAc) based on isotopic data showed that the broadest niche corresponded to SKJ from the Gulf of Cadiz, with no significant isotopic niche overlap between areas. Consistent with the SCA and stable isotope results, the FA composition varied among areas. Docosahexaenoic acid (DHA, 22:6n-3) was the most abundant FA in the SKJ muscle, showing its highest values in samples from the Gulf of Cadiz; this FA was the most effective in segregating SKJ geographical groups. In conclusion, this study provides evidence that SKJ in eastern Atlantic and western Mediterranean waters can easily adapt their diet to local prey availability. Furthermore, stable isotope signatures and FA profiles of muscle tissue prove to be reliable trophic markers that allow separation of populations occurring in different marine areas.

Niche dynamics of sympatric non‐breeding shearwaters under varying prey availability

Variation in prey availability can cause changes in species interactions among marine predators. Foraging theory predicts that niche breadth will expand when resources become limited, possibly leading to higher niche overlap among sympatric species; however, a species’ niche can become constrained by interactions with other similar species, resulting in an inability to shift niche breadth or position. In coastal Newfoundland, Capelin Mallotus villosus is the main forage fish species and its availability (i.e. biomass) during the summer has varied considerably following a population collapse in the 1990s. During the summer, non‐breeding Great and Sooty Shearwaters Ardenna gravis and A. grisea migrate and aggregate at annually persistent Capelin spawning sites. We used stable isotope ratios (δ13C, δ15N) of blood components (plasma, red blood cells) to investigate variation in isotopic niche breadth (95% prediction ellipse areas) and overlap of the two shearwater species during 2014, 2015 and 2016. Capelin availability varied among years, illustrated by lower peak biomass in 2015 (0.126 g/m²) and 2016 (0.027 g/m²) relative to 2014 (0.254 g/m²). The isotopic niche breadth (plasma) of both shearwater species expanded similarly from 2014 (0.65–0.66‰²) to 2015 (2.22–2.57‰²) and 2016 (1.15–1.42‰²), suggesting the incorporation of alternative prey types into their diet during years of lower prey availability. Isotopic niche overlap between Great and Sooty Shearwaters remained high across years (44–63%), however, providing little evidence for dietary niche partitioning during years of lower prey availability. Findings suggest that both shearwater species are flexible foragers and can modify their diet during the non‐breeding season to accommodate fluctuations in prey availability.

Trophic interactions between the Kelp Gull (Larus dominicanus) and Royal and Cayenne terns (Thalasseus maximus maximusandThalasseus sandvicensis eurygnathus, respectively) in a human-modified environment

Many closely related seabirds nest in mixed colonies, and this association may result in interspecific interactions such as competition for common resources and kleptoparasitism. Trophic interactions were evaluated between the Kelp Gull (Larus dominicanus Lichtenstein, 1823) and Royal and Cayenne terns (Thalasseus maximus maximus (Boddaert, 1783) and Thalasseus sandvicensis eurygnathus (Saunders, 1876), respectively) nesting at a mixed-species colony in an area with high availability of recreational fishery waste for the opportunistic Kelp Gull. Diet analyses were based on gull chick stomach content samples and direct observations of food delivered to tern chicks in 2013 and 2014, complemented in 2014 with carbon and nitrogen stable isotope analysis of chick whole-blood samples. Main prey species of Kelp Gull chicks were Cynoscion guatucupa (Cuvier, 1830), a demersal species obtained from recreational fishery waste, Argentine anchovies (Engraulis anchoita Hubbs and Marini, 1935), and insects. Engraulis anchoita and Odontesthes spp. were the main prey of both tern species. Trophic niche and isotopic niche overlap between the Kelp Gull and Royal and Cayenne terns was low. Kelp Gull kleptoparasitism on Royal and Cayenne terns was ≤2.5% and <0.6%, respectively. The use of anthropogenic food subsidies by Kelp Gulls may be mediating the trophic relationships among species, favouring their use of predictable and abundant fishery waste over a more unpredictable pelagic schooling fish such as E. anchoita.

Comparison of isotopic niches of four sea cucumbers species (Holothuroidea: Echinodermata) inhabiting two seagrass meadows in the southwestern Mediterranean Sea (Mostaganem, Algeria)

Among the fauna inhabiting the Posidonia oceanica seagrass meadow, holothurians are particularly abundant and provide essential ecological roles, including organic matter recycling within seagrass sediments. This study aimed to investigate the trophic niche of four holothurians of the order Holothuriida [Holothuria poli (Delle Chiaje, 1824), Holothuria tubulosa (Gmelin, 1791), Holothuria sanctori (Delle Chiaje, 1823) and Holothuria forskali (Delle Chiaje, 1823)] inhabiting P. oceanica meadows, through the measurement of nitrogen and carbon stable isotope ratios. Two shallow and contrasting sites of the littoral region of Mostaganem (North West Algeria) were chosen. The first site, located in Stidia, is weakly impacted by human activities. The second site, located in Salamandre, is highly impacted by human activities (industries, harbor facilities). High values of δ15N in holothurians and their food sources were observed at both sites. The δ13C values showed a lower contribution from detritic Posidonia than in other areas. This could be a consequence of P. oceanica bed degradation in the studied area. The stable isotope approach did not reveal dietary differences between species, and the four holothurians species exhibited significant isotopic niche overlap. However, niche sizes differed between species showing more variable individual trophic diversity in some species (H. tubulosa and H. sanctori in Salamandre; H. forskali in Stidia). If niche segregation does occur, it is not in terms of general resource use. More likely, it would be the abundance of food sources, the different life habits and their micro-habitats that may explain their co-existence in the P. oceanica seagrass meadow.

Trophic resource partitioning of two snook fish species (Centropomidae) in tropical estuaries in Brazil as evidenced by stable isotope analysis

We investigated the trophic niches and the resource partitioning of two snook species, the common (Centropomus undecimalis) and the fat snook (C. parallelus), in four tropical estuaries of the northeastern Brazil, using stable isotope analyses of carbon (δ13C) and nitrogen (δ15N). Fish specimens, mainly juveniles, basal resources and a primary consumer were collected in 2015 during dry and rainy seasons in all estuaries, which exhibited differences in system size, geomorphologic shapes, levels of sea access and anthropic pressures. Potential effect of factors like fish body length, estuary and seasonality on isotope ratios were investigated. Positive relationships between the size of fish and δ15N values were found, regardless the species. Our results indicated that snooks can be characterized as secondary consumers and have close trophic niches in most estuaries. Trophic overlaps were more pronounced within the largest estuaries (Catuama and Santa Cruz), whereas smaller systems that have restricted connections to the sea presented low isotopic niche overlap between both species (Suape and Sirinhaém). Moreover, a higher variability of δ13C and δ15N values in snooks was found in larger estuaries undergoing stronger influences from coastal adjacent waters. Although we were not able to detect clear seasonal effects, greater isotopic overlaps were found during the rainy season. Despite the lack of intrinsic differences in life history traits, both species, dominated by juveniles, shared similar trophic niches in these environments.

Diet and isotopic niche overlap elucidate competition potential between seasonally sympatric phocids in the Canadian Arctic

Interspecific interactions may be altered as a result of poleward species range shifts caused by climate change. In recent decades, Cumberland Sound, Nunavut, in the Canadian Arctic, has undergone concurrent increases in the availability of the forage fish capelin (Mallotus villosus) and the number of migratory harp seals (Pagophilus groenlandicus) during the open-water period; however, the impacts of these changes on endemic Arctic species, such as ringed seals (Pusa hispida), have received little attention. We coupled stomach contents with stable isotope analysis (δ13C and δ15N) of muscle and liver to determine the extent of potential competition between ringed seals (n = 91) and harp seals (n = 39) in Cumberland Sound. Isotopic niche breadth was greater for ringed seals (95% ellipse: 10.08‰2) than for harp seals (95% ellipse: 8.00‰2), and harp seal isotopic niche overlapped more with ringed seal isotopic niche than vice versa, suggesting asymmetrical competition potential. Although there was high overlap in isotopic niche breadth (range 50.3–91.0%) and prey species consumed (Schoener’s Index 0.60), stomach content analysis revealed differences in prey species proportions and size composition, thereby reducing the degree of realized niche overlap. Harp seals consumed a higher biomass of fish (66.7%) than did ringed seals (31.9%), and harp seals also consumed larger capelin (64–200 mm), polar cod (Boreogadus saida; 28–194 mm), and Liparidae (55–115 mm) than ringed seals (63–154 mm, 20–189 mm, and 16–128 mm, respectively). With climate change and range shifts predicted to continue into the future, our results provide an important baseline for future studies examining interspecific interactions.

Show your beaks and we tell you what you eat: Different ecology in sympatric Antarctic benthic octopods under a climate change context

Sympatry can lead to higher competition under climate change and other environmental pressures, including in South Georgia, Antarctica, where the two most common octopod species, Adelieledone polymorpha and Pareledone turqueti, occur side by side. Since cephalopods are typically elusive animals, the ecology of both species is poorly known. As beaks of cephalopods are recurrently found in top predator's stomachs, we studied the feeding ecology of both octopods through the evaluation of niche overlapping and specific beak adaptations that both species present. A multidisciplinary approach combining carbon (δ13C) and nitrogen (δ15N) stable isotope signatures, mercury (Hg) analysis and biomaterials' engineering techniques was applied to investigate the beaks. An isotopic niche overlap of 95.6% was recorded for the juvenile stages of both octopod species, dropping to 19.2% for the adult stages. Both A. polymorpha and P. turqueti inhabit benthic ecosystems around South Georgia throughout their lifecycles (δ13C: −19.21 ± 1.87‰, mean ± SD for both species) but explore trophic niches partially different during adult life stages (δ15N: 7.01 ± 0.40‰, in A. polymorpha, and 7.84 ± 0.65‰, in P. turqueti). The beaks of A. polymorpha are less dense and significantly less stiff than in P. turqueti. Beaks showed lower mercury concentration relative to muscle (A. polymorpha - beaks: 0.052 ± 0.009 μg g−1, muscle: 0.322 ± 0.088 μg g−1; P. turqueti - beaks: 0.038 ± 0.009 μg g−1; muscle: 0.434 ± 0.128 μg g−1). Overall, both octopods exhibit similar habitats but different trophic niches, related to morphology/function of beaks. The high Hg concentrations in both octopods can have negative consequences on their top predators and may increase under the present climate change context.

The influence of body size and season on the feeding ecology of three freshwater fishes with different diets in Lake Erie

Isotopic niche and length-isotope relationships were quantified across the body size of three similarly-sized fish species with different feeding strategies as adults (benthivore: freshwater drum, Aplodinotus grunniens; piscivore: walleye, Sander vitreus; and omnivore: white perch, Morone americana) in Lake Erie's western basin using δ13C, δ15N and δ34S. Stable isotopes demonstrated that resource utilization changed with body size for all three species and length-isotope relationships varied with season. Isotopic niche overlap was lower when modelled with three isotopes (δ13C, δ15N, and δ34S) than with two (δ13C and δ15N), providing greater resolution of feeding ecology among the species. Based on isotopic niches, there was significant overlap in resource use among species and size classes in spring, but overlap decreased in the fall for both. In this study, freshwater fish species with different adult feeding strategies partition resources through different mechanisms that vary through body size and season. Isotopes supported the generally identified feeding ecology of each species but demonstrated that each species underwent significant changes in feeding ecology with increasing body size. Changes in isotopes across season, and body size for each species demonstrate the need for a more thorough understanding of how resource use changes with body size and season in freshwater fish.

Intraspecific differences in relative isotopic niche area and overlap of co-occurring sharks

Few studies have assessed whether patterns of resource partitioning among sympatric marine predators vary between different habitats. This type of data is important for understanding food web functioning as well as for supporting an ecosystem-based fisheries management plan. In this study, we used δ13C and δ15N stable isotope analysis to assess the relative isotopic niche area (i.e., which species has the largest and smallest isotopic niche area within a study site) and core isotopic niche overlap between 299 sharks of 11 co-occurring shark species that vary in size, diet and behavior in three coastal study areas in South Florida. Overall, results show that the relative isotopic niche area and patterns of core isotopic niche overlap between some sympatric shark species varied between sites, suggesting plasticity in resource use and competitive interactions between shark species (e.g., 85% of blacktip shark Carcharhinus limbatus standard ellipse area overlapped with the blacknose shark C. acronotus ellipse in Biscayne Bay, but there was no overlap between these species’ ellipses in nearby Florida Bay). Therefore, patterns of resource use and trophic interactions documented among species from one site may not be applicable to a neighboring location. These findings may have implications for food web models that incorporate trophic data from different areas for a species when local data are unavailable.

Stomach content and stable isotope analyses reveal resource partitioning between juvenile bluefin tuna and Atlantic bonito in Alboran (SW Mediterranean)

The Alboran Sea holds one of the highest biological productivity rates in the Mediterranean, and thus represents an important nursing area for pelagic fishes including tunas. Trophic interactions between sympatric populations of age-0 bluefin tuna (ABFT) and Atlantic bonito (BON) were investigated in Alboran by integrating stomach content analysis (SCA) and stable isotope analysis of bulk muscle (BSIA) and individual amino acids (AA-CSIA). BON and ABFT individuals were collected in the Bay of Malaga; additionally, ABFT were caught off Carboneras. SCA showed that BON fed only on teleost fishes, whereas ABFT diet also comprised invertebrates. We found significant dietary differences between groups using SCA and BSIA data. Low values of niche width estimated from SCA and BSIA indicate poor dietary diversity in the two sampling sites. There was no isotopic niche overlap between groups, which may indicate resource partitioning. Trophic positions (TP) estimated from SCA, BSIA and AA-CSIA place BON at higher TP than ABFT.

Behavioral Interactions and Trophic Overlap between Invasive Signal Crayfish Pacifastacus leniusculus (Decapoda, Astacidae) and Native Fishes in Iberian Rivers

The signal crayfish Pacifastacus leniusculus Dana, 1852 is a successful invasive species in the Iberian Peninsula. This is of particular conservation concern, as fish fauna is highly endemic and also threatened within this ecoregion. The aim of this study was to analyze behavioral interactions and trophic overlap between signal crayfish and native fishes in Iberian rivers (northern Spain). Video cameras were used to record fish “dominance/evasion” after spatial encounters with crayfish. Diet composition and isotopic signatures (δ13C and δ15N) were compared to evaluate the niche overlap. Fish were dominant in 25% of the encounters with juvenile crayfish, whereas this percentage was only 4% with adult crayfish. Observations showed a high fish “evasion” response for Pyrenean stone loach Barbatula quignardi (Bǎcescu-Meşter, 1967) (>30%). Dietary results showed a high trophic overlap between signal crayfish with the pelagic Pyrenean minnow Phoxinus bigerri Kottelat, 2007 and the benthic loach. However, the isotopic niche overlap was low, with brown trout Salmo trutta L., 1758 showing the highest area (only 0.1 ‰2). Overall, our findings suggest that interferences may occur with native species for food (i.e., benthic invertebrates). Consequently, measures should be applied to control invasive crayfish in Iberian rivers.

Individual foraging specialization and sexual niche segregation in South American fur seals

Individual variation in habitat and resource use has been reported for many top predators. This variation becomes important when comparing individuals taking into account sex, size, or age classes, since it can influence population dynamics and stability. We evaluated the individual variation and sexual/geographical isotopic niche overlap of the South American fur seal (SAFS) from the western South Atlantic. Whiskers of adult individuals from Brazil (n = 19), Uruguay (n = 29), and Argentina (n = 5) collected between 2005 and 2016 were serially sampled, resulting in 1001 samples, and their carbon and nitrogen isotopic ratios were analyzed longitudinally. According to its length, time integrated by whiskers ranged between 1.4 and 5.6 years. Males had δ13C (− 14.5 ± 0.6‰) and δ15N (18.9 ± 1.2‰) values significantly higher than females (δ13C = − 15.2 ± 0.5‰, δ15N = 17.8 ± 1.2‰). Females from Uruguay and Brazil were isotopically similar, displaying a large isotopic niche overlap (65.2–84%). Contrary, moderate isotopic niche overlaps were observed between males from Uruguay and Brazil (40.1–48.4%), and Uruguay and Patagonia (22.3–27.8%), indicating the use of different prey and/or feeding grounds. The WIC/TNW index of individual specialization pointed a significant specialization in males (0.38 for δ15N and 0.39 for δ13C). Females, on the other hand, are more generalists compared to males (0.53 and 0.71, for δ15N and δ13C, respectively). Differences in the ecological opportunity between sexes can account for these variations. Our study points out that trophic generalist populations of SAFS are composed of specialist and generalist individuals.

Consistently High Trophic Overlap between Invasive White Perch and Native Black Crappies in Southeastern Reservoirs

AbstractThe detrimental impacts of invasive species often occur through deleterious trophic interactions (e.g., competition) with native species. However, the potential for competitive interactions between species may vary among systems, among seasons, or throughout the species’ ontogeny, requiring a thorough examination of trophic niches to determine whether overlap exists and to elucidate the likelihood of competition. The Black Crappie Pomoxis nigromaculatus is a popular game fish species across the United States, but recent invasions of White Perch Morone americana, which exploit a similar trophic niche, into southern reservoirs represent a major concern for the continued sustainability of Black Crappie fisheries. We examined the trophic niches of these species throughout their ontogeny in two North Carolina reservoirs during 2009 and 2010 using a combination of diet content and stable isotope analyses. Extensive and significant similarities in diet composition and isotopic niche overlap existed between Black Crappies and White Perch throughout their ontogeny and across seasons in both systems. Overlap was particularly high between smaller (<200‐mm), invertivorous Black Crappies and all sizes of White Perch. This overlap suggests that if invertebrate resources become limited, competition during the juvenile stage could reduce growth rates and survival of young Black Crappies. By evaluating variation in the trophic niches of each species among seasons, body sizes, and reservoirs, we conclude that the potential for competitive interactions between White Perch and Black Crappie is high in southern reservoirs. Management options to either reduce White Perch abundance or prevent their spread to novel habitats may be necessary to sustain native and supplemented Black Crappie populations that are potentially vulnerable to White Perch invasions.

Diet composition of expanding breeding populations of the Magellanic Penguin

ABSTRACTThe Magellanic Penguin (Spheniscus magellanicus) is the most abundant and widely distributed seabird breeding on the Patagonian Coast of Argentina. We combined conventional stomach content and stable isotope analysis to assess Magellanic Penguin diet during the chick rearing stage at the two northernmost colonies in an area subject to fisheries. In 2011 and 2013, Thornfish (Bovichtus argentinus) was the main prey by mass at Complejo Islote Lobos (63.0% and 32.3%, respectively) and Argentine Anchovy (Engraulis anchoita) at Estancia San Lorenzo (85.2 and 63.3%, respectively). Magellanic Penguins from both colonies showed low isotopic niche overlap in 2011 (36%) and no overlap in 2013, suggesting a different use of prey species and/or foraging areas. Stable isotope mixing models showed that Argentine Anchovy (52.8%) and Thornfish (65.9%) were the main prey at Complejo Islote Lobos in 2011 and 2013, respectively, while Patagonian Redfish (Sebastes oculatus) (46.4%) and Squat Lobster (Munida gregaria) (50%) were the main prey at Estancia San Lorenzo in 2011 and 2013, respectively. Results show that in addition to Argentine Anchovy, previously recognized as main prey for breeding Magellanic Penguins in northern Patagonia, other juvenile or small sized fish are important diet items. Diet results suggest different scenarios of food conditions for each colony, despite the relative short distance between breeding locations. The low contribution of Argentine Hake (Merluccius hubbsi) and Argentine Shortfin Squid (Illex argentinus) suggests a low trophic overlap with commercial fisheries. The information provided is key to understand changes in the marine ecosystem and potential penguin-fishery interactions.

Show us your beaks and we tell you what you eat: Different ecology in sympatric Antarctic benthic octopods under a climate change context

Show us your beaks and we tell you what you eat: Different ecology in sympatric Antarctic benthic octopods under a climate change context

Teasing apart cryptic species groups: Nutritional ecology and its implications for species‐specific conservation of the Myotis mystacinus group

AbstractThe increasing loss of biodiversity and the need to delay the sixth mass extinction on earth resulted in several international conventions aiming at protecting threatened species. Protection improves with increasing knowledge of the biology and ecology of these species. However, species numbers rapidly increase with the ongoing description of cryptic species, resulting in a lack of ecological knowledge for each of these species. We studied the nutritional ecology of three cryptic bat species of the Myotis mystacinus group using stable isotope analysis and molecular fecal analysis. Inter‐ and intraspecific variations were compared by calculating the isotopic and dietary niches for each species. We hypothesized (a) that all three species are generalists, but (b) that M. mystacinus and Myotis brandtii are more similar to each other than to Myotis alcathoe. Third we assumed that the occupied niches are intraspecifically more similar than interspecifically. We could show that the members of the M. mystacinus group differ in their nutritional ecology. Our data suggest that M. alcathoe is the most specialized species of the group, occupying a more distinct ecological niche compared to the other two species. Interestingly, the dietary and isotopic niche overlap was higher interspecifically than intraspecifically. Furthermore, considering our findings and those from other studies we illustrate implications for the conservation of these cryptic species.

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

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

Comparative study of trophic and elemental characteristics of zooplankton in deep (500–3500 m) and shallow (0–200 m) layers

The deep sea has limited food resources and is considered one of the most rigorous environments on Earth. In contrast, zooplankton and their food supply are abundant in shallow sea layers. In this study, the trophic and elemental characteristics of zooplankton taxa in the deep and shallow layers were explored with respect to trophic position, trophic niche overlap among taxa, and growth performance. Our results showed that higher values of δ15N were detected in most taxa or taxa groups in the deep layer compared to similar taxa in the shallow layer, indicating that depth-dependent δ15N values existed in most zooplankton taxa. A lower isotopic niche overlap was detected among taxa in the shallow layer, since the overlap of SEAc (standard ellipse area corrected for small sample sizes) between pairs of taxa was lower in the shallow layer than in the deep layer. In the deep layer, SEAc values of taxa were higher than those in the shallow layer respectively, showing higher isotopic niche widths. This could be due to the high trophic diversity of each taxa in the deep layer and the area sampled actually including at least two zones (mesopelagic and bathypelagic). From an ecological stoichiometry perspective, most taxonomic groups in the deep layer exhibited higher C:P and N:P ratios than similar taxa in the shallow layer, suggesting lower maximal growth rates in the deep layer. Lower growth rates potentially reduce the consumption of limited resources in the deep layer, and higher growth rates in the shallow layer facilitate the rapid exploitation of abundant resources and allow zooplankton to flourish.