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Niche overlap among the three shark species in the present study based on the specific convex hull estimates generated via Bayesian modeling in the SIAR package in R
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Predation is one of the most fundamental and unifying concepts in ecology, and we are beginning to obtain a more complete understanding of how predators drive community structure and ecosystem function through their impacts on prey. We know considerably less about how predators affect each other through intraguild interactions, which is surprising...
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... Additionally, seagrass was found to make up a large component of bonnethead shark Sphyrna tiburo diet with mass gut content consisting of 62.1% seagrass (Leigh et al., 2018). Stable isotopes also revealed the community-wide importance of seagrass and mangrove resources for sharks across a variety of locales, including coastal Florida, USA (Gallagher et al., 2017b;Shipley et al., 2019) and Shark Bay, Western Australia (Vaudo and Heithaus, 2011). At the individual level, the presence of kelp BCEs elevated the trophic position of male redspotted catsharks Schroederichthys chilensis (Vaśquez-Castillo et al., 2021). ...
Climate change is threatening marine ecosystems and the distribution of species which rely on them. Due to their capacity to sequester vast amounts of carbon, blue carbon ecosystems (BCEs; seagrass, mangroves, salt marshes, kelp forests) are becoming increasingly recognized as key nature-based solutions to climate change. Sharks are mobile species that can exert strong control of food webs and are also key to conservation efforts. BCEs are known to support shark life histories, but the drivers of these relationships remain poorly understood. Here, we highlight two key behavioural pathways directly linking BCEs and sharks (foraging and reproductive activities) and explore the influence of climate on them. Our evaluation of the literature suggested that the physical attributes of the blue carbon plant tissue itself serves as the key link between sharks and BCEs, facilitating high rates of prey biodiversity and a platform for reproductive behaviours. We revealed that shark body size likely has an influence on the nature of these relationships, and that climate may be a modulator of key interactions. We also use basic network theory to explore how ecological information flows throughout BCEs, with sharks as a prominent actor. We identify ways in which future studies can fill knowledge gaps; namely, a focus on smaller endemic species, and empirical assessments between sharks and organic carbon stocks. Maintaining the integrity of these relationships should preserve blue natural capital: BCEs’ capacity to sequester carbon, support local biodiversity, and the role of sharks in preserving resilience.
... Other studies have shown high niche overlap and relaxed competition states between top consumers when food items are abundant, leading to stable resource use (e.g. Gallagher et al. 2017). Although pre-colonial environments in southern Brazil were likely to be resource-abundant, more factors would be at play in shaping the patterns of niche overlap observed. ...
... Furthermore, high niche overlap among sharks may not be an uncommon occurrence (e.g. Vaudo and Heithaus 2011;Gallagher et al. 2017;Shiffman et al. 2019), although other ecological dynamics may be involved in these examples, this corroborates the plausibility of our findings. The calculation of a trophic level using a single discrimination factor obscures much of the real complexity behind trophic interactions (Zeichner et al. 2017), whereas scaled discrimination factors may be more accurate (Hussey et al. 2014). ...
Sharks are essential components of marine communities, and their removal might simplify ecosystems and lead to unpredictable and detrimental effects on the food web. Comparing the isotopic niche of species between archaeological and modern communities can provide information to assess temporal changes in the ecological dynamics of communities. Here, stable isotope analysis was used to compare food web topology metrics between two shark guilds or communities, a late Holocene archaeological community (AC) dating from 700 to 500 years ago and a modern community (MC) trophic web, both from South Brazil. In the same line of comparison, we assess the trophic position of a top predator, Carcharias taurus. Results showed topological temporal differences such as higher trophic redundancy and higher patterns of niche overlap in the AC. Higher trophic redundancy could be expected in late Holocene food webs, as similar studies also observed these findings. In contrast, the MC showed less dense packing of species within the isotopic niche space, thus higher niche partitioning and higher trophic diversity, suggesting changing ecological interaction dynamics. We found that Carcharias taurus has increased its trophic level, possibly due to a release from the intraspecific competition and a dietary shift towards larger prey items. We suggest that anthropic impacts, such as overfishing, may cause these differences in the trophic position of this species. We also advocate that this study method might help future trophic reconstructions using shark teeth, as information about past marine environments is scarce, and could serve as a baseline for future studies.
... Understanding the trophic ecology of pelagic predators is essential as they can profoundly regulate the structure of marine communities (Baum et al., 2003). Studies using stable isotopes (SIA) to access foraging behaviors and migration patterns of pelagic predators are increasing (Li et al., 2016;Gallagher et al., 2017;Bird et al., 2018;Wyatt et al., 2019;Prieto-Amador et al., 2022). These applications are based on the premise that the stable carbon and nitrogen isotope ratios (d 13 C and d 15 N, respectively), fractionate systematically throughout the food web as predators consume preys (Peterson and Fry, 1987;Boecklen et al., 2011). ...
Stable isotope analysis (SIA) has proven to be a powerful tool in reconstructing diets and characterizing trophic relationships for pelagic predators. Ethanol has been a common preservative solution for biopsy samples from remote areas and archived collections. It is still under debate whether the effects of ethanol (ET) would bias the trophic interpretation of the stable isotope values. Further, lipid extraction (LE) is becoming more popular as a general treatment for standardization prior to SIA, particularly for investigating intra and interspecific variation of sympatric species, because lipids have lower δ13C values. In this study, the long-term (up to 448 days) effects of treatment ET and combined treatments ET and LE (ET+LE) on stable carbon and nitrogen isotope values (δ13C and δ15N, respectively) of twelve pelagic predators from the open ocean were evaluated. Results showed that compared with control values, δ15N values displayed a positive change (δ15Nmean offset was 0.71 ± 0.56‰) but δ13C values had variable results (δ13Cmean offset was 0.42 ± 0.64‰) among all species following treatment with ET during the first 28 days and then remained stable throughout the experiment. Compared with treatment LE results, no difference was observed in δ13C, δ15N values, and C/N ratios through time following treatment ET+LE. These results indicated that treatment ET may have species-specific effects on stable isotope values, and the shifts from treatment LE could counter the changes caused by treatment ET. In addition, after 28 days of preservation, the values following treatment ET were similar to those following treatment LE in low C/N species (C/N<3.5), which suggested ethanol may also affect some of lipid contents from muscle tissues. Nevertheless, further research is needed to focus on the mechanisms that control changes in stable isotope composition in tissues stored in ethanol. Given the effects on pelagic predators, muscle tissue samples stored in ethanol from the open ocean or a museum after LE treatment could be used to develop SIA.
... Under competitive situations, larger shark species exhibited a reduction in activity space and habitat use, whereby smaller individuals increased their activity space and were pushed out to avoid predation. Among larger predatory fishes, it has been assumed that subtle changes in individual distribution, as well as dietary specificity, drive resource partitioning that may explain competitive coexistence(Gallagher et al., 2017b;Papastamatiou et al., 2018a). Pulses in ephemeral or opportunistic resources offer large predatory fishes valuable energetic incentives to scavenging, but at the cost of acutely intense competition. ...
Movement of fishes in the aquatic realm is fundamental to their ecology and survival. Movement can be driven by a variety of biological, physiological, and environmental factors occurring across all spatial and temporal scales. The intrinsic capacity of movement to impact fish individually (e.g., foraging) with potential knock‐on effects throughout the ecosystem (e.g., food web dynamics) has garnered considerable interest in the field of movement ecology. The advancement of technology in recent decades, in combination with ever‐growing threats to freshwater and marine systems, have further spurred empirical research and theoretical considerations. Given the rapid expansion within the field of movement ecology and its significant role in informing management and conservation efforts, a contemporary and multidisciplinary review about the various components influencing movement is outstanding. Using an established conceptual framework for movement ecology as a guide (i.e., Nathan et al., 2008 PNAS. 105:19052), we synthesize the environmental and individual factors that affect the movement of fishes. Specifically, internal (e.g., energy acquisition, endocrinology, and homeostasis) and external (biotic and abiotic) environmental elements are discussed, as well as the different processes that influence individual‐level (or population) decisions, such as navigation cues, motion capacity, propagation characteristics, and group behaviours. In addition to environmental drivers and individual movement factors, we also explore how associated strategies help survival by optimizing physiological and other biological states. Next, we identify how movement ecology is increasingly being incorporated into management and conservation by highlighting the inherent benefits that spatio‐temporal fish behaviour imbues into policy, regulatory, and remediation planning. Finally, we consider the future of movement ecology by evaluating ongoing technological innovations and both the challenges and opportunities that these advancements create for scientists and managers. As aquatic ecosystems continue to face alarming climate (and other human‐driven) issues that impact animal movements, the comprehensive and multidisciplinary assessment of movement ecology will be instrumental in developing plans to guide research and promote sustainability measures for aquatic resources. This article is protected by copyright. All rights reserved.
... Stable Isotopes Analysis (SIA) is widely used across broad areas of ecology for the study of isotopic niche (Zambrano et al., 2010;Layman and Allgeier, 2012;Newsome et al., 2012), fish trophic ecology (Gu et al., 1996;Abrantes et al., 2014;Córdova-Tapia et al., 2015;Costantini et al., 2018;Cicala et al., 2020), to quantify food web structure and functioning (Layman et al., 2007a;Perkins et al., 2014), and to evaluate the impacts of biological invasions on the trophic structures of the recipient communities (Vander Zanden et al., 1999;Sagouis et al., 2015). The isotopic composition of carbon (δ 13 C) and nitrogen (δ 15 N) give information about the potential resources exploited by a consumer and its trophic position in the food web, respectively (Post, 2002;Inger and Bearhop, 2008;Gallagher et al., 2017). Moreover, δ 13 C and δ 15 N ratios of consumers within a given environment can be plotted on an isotopic biplot, allowing the quantification of isotopic niche metrics (Layman et al., 2007a;Jackson et al., 2011), which can provide useful insight on the trophic structure of a population and on potential interspecific interactions (Jackson et al., 2012). ...
The introduction and establishment of non-native fish species is a widespread phenomenon in freshwater ecosystems, including small and shallow lakes. However, these ecosystems are often not considered in conservation and ecological studies and a few information is available on their fish communities and the impacts of biological invasions. Here, standardized fish surveys (gillnetting and electrofishing) and stable isotopes analysis (SIA) of carbon and nitrogen (δ¹³C and δ¹⁵N) were combined to assess fish community composition, trophic structure and trophic diversity, and to characterize the interspecific trophic interactions (as isotopic niches and their relative asymmetric overlap) of the native and non-native species (NS and NNS, respectively) found in two protected small and shallow lakes, San Michele and Campagna (northern Italy). In San Michele, 92 % of the fish caught were NNS and both the individual and biomass per unit effort were dominated by NNS, particularly by the invasive Lepomis gibbosus L. and Ictalurus punctatus (Rafinesque, 1818). In Campagna, 5.6 % of the fish in the total catches belonged to NNS and the numeric and biomass abundances of the assemblage were dominated by the NS Alburnus alborella (Bonaparte, 1841). SIA revealed that NNS had greater trophic structure and were exploiting a wider range of resources (as per Layman metrics) and had a higher asymmetric overlap than NS assemblages in both lakes. This was also evident when species-specific isotopic ecology was considered, underlining that, in both lakes, NNS (particularly the NNS Ameiurus melas (Rafinesque, 1820)) may have a competitive advantage over NS in case of limiting resources. The results thus pointed to a worrying conservation status of these lakes that may mirror the condition of other Italian shallow and small lakes highlighting the need to implement management actions to preserve these valuable ecosystems.
... Pethybridge et al., 2018;Meyer et al., 2019). For instance, stable isotopes of carbon (δ 13 C) have been used to inform individual foraging habitat (e.g., inland mangroves vs. coastal neritic, Shipley et al., 2019), while nitrogen isotopes (δ 15 N) have been used to infer the trophic position, as it is gradually enriched though trophic transfer up the food web (Gallagher et al., 2017;Shiffman et al., 2019). Fatty acids can also provide insights in to trophic ecology, since they remain relatively unchanged from prey to predator, and consequently reflect trophic interactions and basal food chain dependencies (dinoflagellates, bacteria, diatoms; Budge et al., 2006;Gomes et al., 2021). ...
The synergistic effects of coastal urbanization have dramatically impacted biological communities. Yet, few studies have investigated how urbanization can influence the diet quality and trophic ecology of coastal sharks. In a preliminary study, we examined for spatial variation in the nutritional ecology of a highly active marine predator, the blacktip sharks (Carcharhinus limbatus) exposed to regional differences in coastal urbanization in southeast Florida. We used medium-term nutritional indicators (i.e., body condition; whole blood stable isotopes [δ¹⁵N and δ¹³C]) and short-term dietary markers (i.e., plasma fatty acid profiles) to test the hypothesis that the nutritional ecology of marine predators would differ in areas exposed to increased urbanization. Our initial results showed that blacktip sharks sampled in high urbanized area (hereafter, ‘urban sharks’) exhibited relatively higher body condition, blood δ¹⁵N levels, and percentages of saturated fatty acids compared to sharks sampled in low urbanized area (hereafter ‘non-urban sharks’). Collectively, these results suggest a possible positive alteration in the amount of food consumed by sharks and/or in the caloric value of their prey. We also found lower percentages of bacterial markers and higher values of dinoflagellate markers in urban sharks. Compared to more resident species evaluated in the region, we did not detect a reduction in diet quality (in terms of essential fatty acids) in this highly active species exposed to urbanization. Therefore, it is possible that the lifestyle and feeding behavior have an influence on the quality of food consumed by urban sharks, and maybe the impacts of urbanization are more pronounced in resident, sedentary and benthic species.
... Isotope signatures in organisms yield information on timeintegrated assimilated food (Fry, 1988;Vander Zanden and Rasmussen, 1999). Notably, carbon and nitrogen stable isotope ratios in the δ-space have been successfully used to understand the trophic dynamics in marine systems, to trace the pathway of organic matter of different origins through aquatic food webs (Fry and Scherr, 1984;Kaehler et al., 2000;Pinnegar and Polunin, 2000;Briand et al. 2016), to measure the impact of invasive species (O'Farrell et al., 2014), the impact of predators on preys (Gallagher et al., 2017), or to support the resource breadth hypothesis (Rader et al., 2017), to cite a few applications. ...
Ecological (isotopic) niche refers to a surface in a two-dimensional space, where the axes correspond to environmental variables that reflect values of stable isotopes incorporated in an animal's tissues. Carbon and nitrogen stable isotope ratios (δ¹³C-δ¹⁵N) notably provide precious information about trophic ecology, resource and habitat use, and population dynamics. Various metrics allow for isotopic niche size and overlap assessment. In this paper, we advocate α-minimum convex polygons (MCP) - that have long been used for home range estimation – as a relevant tool for isotopic niche size, overlap, and characteristics. The method allows for outlier rejection while being suited to data that are not Gaussian in the bivariate isotopic (δ¹³C-δ¹⁵N) space. The proposed indicators are compared to other existing approaches and are shown to be complementary. Notably an indicator of divergence within the niche is introduced, and allows for comparisons at low (n > 6) and different sample sizes. The R code is made publicly available and will enable ecologists to perform isotopic niche comparison, contraction and expansion assessment, and overlap, based on various methods.
... Following Hammerschlag et al. (2012) and Gallagher et al. (2017), shark relative abundance was calculated using two approaches. First, we calculated the frequency of occurrence for each species expressed as the proportion of drumline sets positive for a given species. ...
Given population declines of many shark species, coupled with increasing conservation efforts and recoveries, there is a need for baseline assessments and continued monitoring of shark populations to support management. The waters of South Florida, USA, are used by a diverse array of sharks that occur among a mosaic of habitats and management zones. Here we conducted standardized drumline surveys from Miami through the middle Florida Keys to examine spatial, seasonal and environmental patterns in shark occurrence, catch per unit effort, composition, and demographic structure. Between 2009 and 2021, a total of 21,755 drumlines were deployed, capturing 3398 sharks, comprising fifteen species. Ginglymostoma cirratum, (n = 1335), Carcharhinus limbatus, (n = 650), Negaprion brevirostris, (n = 314), C. leucas, (n = 253), and Sphyrna mokarran, (n = 238) were the most common species encountered. Overall, the largest shark surveyed was a 450 cm S. mokarran and the smallest shark was a 50 cm Rhizoprionodon terraenovae. At the assemblage level, relative abundance among regions and seasons were generally similar; however clear species-specific patterns of abundance, size structure, and sex-composition were detected by season, region, habitat, and management zones. Of the physical conditions evaluated, habitat type and depth emerged as the most influential parameters affecting abundances and sizes of species captured. While few species exhibited significant differences in catches by management zone, areas with the most restrictive fisheries regulations generally supported higher abundances for which differences were detected. These data serve as a baseline for future monitoring of shark populations in South Florida and assessing their response to environmental change.
... The isotopic niche concept proposed by Newsome et al. (2007), a refinement of Hutchinson's idea (Hutchinson, 1957(Hutchinson, , 1978, suggests the use of isotopic tools to assess the ecological characteristics of organisms that ecologists aim to investigate, such as coexistence and resource sharing. Stable isotopic signatures of carbon (C) and nitrogen (N) have been widely used in this sense (Jackson et al., 2011;Yasue et al., 2014;Gallagher et al., 2017;Jensen et al., 2017;Rader et al., 2017), as the values measured in the consumer's tissues are closely related to the values from its diet. Stable nitrogen isotope ratios in consumers are typically enriched in the heavier isotope ( 15 N) by 2-4‰ per trophic level (Minagawa & Wada, 1984;Peterson & Fry, 1987), making δ 15 N values useful for the definition of trophic positions of consumers (Post, 2002). ...
The stable isotope ratios of carbon (δ ¹³ C) and nitrogen (δ ¹⁵ N) and total mercury concentrations (THg) of the three marine catfish species Aspistor luniscutis , Bagre bagre and Genidens genidens were evaluated to understand their trophic relationship in northern Rio de Janeiro state, south-eastern Brazil. The δ ¹³ C was similar among the three marine catfishes, whereas δ ¹⁵ N was similar in A. luniscutis and B. bagre and lower in G. genidens . THg was higher in G. genidens and lower in B. bagre . The greater assimilation of Sciaenidae fishes and squids by A. luniscutis and B. bagre resulted in smaller isotopic niche areas and trophic diversity but higher isotopic niche overlap, trophic redundancy and evenness. For G. genidens , the similar assimilation of all prey items resulted in the broadest isotopic niche among the marine catfishes. The higher mercury content in G. genidens is consistent with an increased important contribution of prey with a higher Hg burden. The bioaccumulation process was indicated by significant correlations of δ ¹⁵ N and THg with total length and total mass. Additionally, a significant correlation between THg and δ ¹⁵ N reflected the biomagnification process through the food web.
... Indeed, most current studies of dietary specialization, niche partitioning, and competition among individuals, populations, and species rely on MCP and SEA techniques to delineate isotopic niche estimates (Botta et al., 2017;Galetti, Rodarte, Neves, Moreira, & Costa-Pereira, 2016;Gallagher, Shiffman, Byrnes, Hammerschlag-Peyer, & Hammerschlag, 2017;Staudinger, McAlarney, McLellan, & Pabst, 2014; but see Sepulveda, Lowe, & Marra, 2012). ...
... Thus, without knowledge on the Kestrel behavior, interpretation of niche overlap would be erroneous. In this instance the overlap in isotopic signatures of Kestrels was derived from overlapping values of prey not as a result of prey selection.In most isotopic studies, the foraging behavior of individuals is unknown and inferred from the isotopic data themselves(Botta et al., 2017;Galetti et al., 2016;Gallagher et al., 2017; ...
The isotopic niche of consumers represents biologically relevant information on resource and habitat use. Several tools have been developed to quantify niche size and overlap. Nonetheless, methods adapted by spatial ecologists to quantify animal home ranges can be modified for use in stable isotope ecology when data are not normally distributed in bivariate space.
We offer a tool that draws on existing spatial metrics, such as minimum convex polygon (MCP) and standard ellipse area (SEA), and add novel metrics using kernel utilization density (KUD) estimators to measure isotopic niche size and overlap. We present examples using empirical and simulated data to demonstrate the performance of the package kernel isotopic niches in r (rKIN) under various scenarios.
Results of niche size from MCP, SEA and KUD were highly correlated but divergent among datasets. Overall, the KUD method produced the largest niche sizes and was more sensitive to the distribution of the isotopic data. Pairwise estimates of overlap were highly variable, likely because MCP and SEA inherently include or exclude unused areas in the resulting niche estimate. Four bandwidth methods (reference, normal scale, plug‐in and biased cross‐validation) produced comparable estimates of niche size and overlap at various sample sizes (10–40). Niche size and overlap were consistent across sample sizes >15.
Use of rKIN will allow isotope ecologists to quantify niche shifts, expansions or contractions, as well as assess the performance of several estimation methods. The package also can be applied to other data types (e.g. principal component analysis, multi‐dimensional scaling) so long as axes and measurement units are identical and can be converted to Cartesian coordinates.
