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Patterns of resource use and isotopic niche overlap among three species of sharks occurring within a protected subtropical estuary

DOI:10.1007/s10452-017-9627-2
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Austin J Gallagher at Beneath the Waves
Austin J Gallagher
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David S Shiffman
David S Shiffman
David S Shiffman
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Evan Byrnes at Simon Fraser University
Evan Byrnes
Caroline Hammerschlag-Peyer at Florida International University
Caroline Hammerschlag-Peyer
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Abstract and Figures

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 considering predators often occur simultaneously and may compete for resources while avoiding being killed themselves. In the present study, we examined aspects of inter- and intra-specific resource use among three species of large-bodied predatory sharks (blacktip, bull, lemon) co-occurring within a subtropical, protected bay in the southeastern USA. Specifically, we inferred relative trophic position, isotopic niche overlap, and patterns of resource use of sharks using stable isotope analysis of carbon-13 and nitrogen-15 from blood and fin cartilage samples. We also combined these approaches with estimates of abundance and occurrence from empirical shark surveys to consider whether these species may exhibit resource partitioning in space and time. We found that all three species overlapped in space, and there was some isotopic niche overlap between the species. We also found evidence of temporal isotopic niche stability, suggesting that co-occurring shark species may compete for available prey resources, but individuals of those species may have similar patterns of resource use over time. We discuss our findings as they relate to the ecologies of the species in question and how sound conservation and management of ecosystems can allow for predator diversity, sympatry, and stable use of resources at the top of the food chain.
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Patterns of resource use and isotopic niche overlap
among three species of sharks occurring within a protected
subtropical estuary
Austin J. Gallagher .David S. Shiffman .
Evan E. Byrnes .C. M. Hammerschlag-Peyer .
N. Hammerschlag
Received: 28 November 2016 / Accepted: 10 May 2017 / Published online: 19 May 2017
ÓSpringer Science+Business Media Dordrecht 2017
Abstract Predation is one of the most fundamental
and unifying concepts in ecology, and we are begin-
ning 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 surpris-
ing considering predators often occur simultaneously
and may compete for resources while avoiding being
killed themselves. In the present study, we examined
aspects of inter- and intra-specific resource use among
three species of large-bodied predatory sharks (black-
tip, bull, lemon) co-occurring within a subtropical,
protected bay in the southeastern USA. Specifically,
we inferred relative trophic position, isotopic niche
overlap, and patterns of resource use of sharks using
stable isotope analysis of carbon-13 and nitrogen-15
from blood and fin cartilage samples. We also com-
bined these approaches with estimates of abundance
and occurrence from empirical shark surveys to
consider whether these species may exhibit resource
partitioning in space and time. We found that all three
species overlapped in space, and there was some
isotopic niche overlap between the species. We also
found evidence of temporal isotopic niche stability,
suggesting that co-occurring shark species may com-
pete for available prey resources, but individuals of
those species may have similar patterns of resource
use over time. We discuss our findings as they relate to
the ecologies of the species in question and how sound
conservation and management of ecosystems can
allow for predator diversity, sympatry, and stable use
of resources at the top of the food chain.
Keywords Feeding Intraguild Partitioning
Predation risk Predator Shark
Introduction
Foraging activities of predators can initiate trophic
cascades via both predation and risk effects on their
prey (Werner and Peacor 2003; Creel and Christianson
Handling Editor: Piet Spaak.
A. J. Gallagher (&)N. Hammerschlag
Rosenstiel School of Marine and Atmospheric Science,
University of Miami, Miami, FL 33149, USA
e-mail: agallagher@rsmas.miami.edu
A. J. Gallagher
Beneath the Waves, Inc., Miami, FL 3313, USA
A. J. Gallagher D. S. Shiffman N. Hammerschlag
Leonard and Jayne Abess Center for Ecosystem Science
and Policy, University of Miami, Coral Gables, FL 33146,
USA
E. E. Byrnes
Department of Biological Sciences, Macquarie
University, North Ryde, NSW 2109, Australia
C. M. Hammerschlag-Peyer
Science Department, Palmer Trinity School, Miami,
FL 33157, USA
123
Aquat Ecol (2017) 51:435–448
DOI 10.1007/s10452-017-9627-2
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... 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). ...
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Full-text available
  • Feb 2016
Apex predators often have strong top-down effects on ecosystem components and are therefore a priority for conservation and management. Due to their large size and conspicuous predatory behaviour, reef sharks are typically assumed to be apex predators, but their functional role is yet to be confirmed. In this study, we used stomach contents and stable isotopes to estimate diet, trophic position and carbon sources for three common species of reef shark (Triaenodon obesus, Carcharhinus melanopterus and C. amblyrhynchos) from the Great Barrier Reef (Australia) and evaluated their assumed functional role as apex predators by qualitative and quantitative comparisons with other sharks and large predatory fishes. We found that reef sharks do not occupy the apex of coral reef food chains, but instead have functional roles similar to those of large predatory fishes such as snappers, emperors and groupers, which are typically regarded as high-level mesopredators. We hypothesise that a degree of functional redundancy exists within this guild of predators, potentially explaining why shark-induced trophic cascades are rare or subtle in coral reef ecosystems. We also found that reef sharks participate in multiple food webs (pelagic and benthic) and are sustained by multiple sources of primary production. We conclude that large conspicuous predators, be they elasmobranchs or any other taxon, should not axiomatically be regarded as apex predators without thorough analysis of their diet. In the case of reef sharks, our dietary analyses suggest they should be reassigned to an alternative trophic group such as high-level mesopredators. This change will facilitate improved understanding of how reef communities function and how removal of predators (e.g., via fishing) might affect ecosystem properties.
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Use of marine protected areas and exclusive economic zones in the subtropical western North Atlantic Ocean by large highly mobile sharks
Article
Full-text available
  • Feb 2016
Study aim and location: Many populations of highly mobile marine fishes, including large sharks, are experiencing declines. The benefits of spatial management zones, such as marine protected areas (MPAs), for such animals are unclear. To help fill this knowledge gap, we examined core habitat use areas (CHUAs) for bull (Carcharhinus leucas), great hammerhead (Sphyrna mokarran) and tiger sharks (Galeocerdo cuvier) in relation to specific MPAs and exclusive economic zones (EEZs) in the western North Atlantic Ocean. Methods: Bull, great hammerhead and tiger sharks (N = 86 total) were satellite tagged and tracked in southern Florida and the northern Bahamas between 2010 and 2013. Filtered and regularized positions from Argos locations of tag transmissions were used to generate CHUAs for these sharks. Overlaps of CHUAs with regional protected areas and exclusive economic management zones were quantified to determine the proportion of each tracked shark’s CHUA under spatial protection from exploitation. Results: A total of 0%, 17.9% and 34.7% of the regional CHUAs for tracked bull, great hammerhead and tiger sharks, respectively, were fully protected from exploitation in the study area. Main conclusions: Expansion of protected areas to include U.S. territorial waters would effectively protect 100% of the CHUAs for all tracked sharks in the study area. This finding is particularly significant for great hammerhead sharks, which are currently overfished, vulnerable to bycatch mortality and are the focus of strident regional conservation efforts. These findings also provide a means to inform decision makers and marine conservation planning efforts as to the types of management actions available and potential efficacy of spatial protections for these marine predators.
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The Ecological Role of Sharks on Coral Reefs
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Sharks are considered the apex predator of coral reefs, but the consequencesof their global depletion are uncertain. Here we explore the ecological roles of sharks on coral reefs and, conversely, the importance of reefs for sharks. We find that most reef-associated shark species do not act as apex predators but instead function as mesopredators along with a diverse group of reef fish. While sharks perform important direct and indirect ecological roles, the evidence to support hypothesised shark-driven trophic cascades that benefit corals is weak and equivocal. Coral reefs provide some functional benefits to sharks, but sharks do not appear to favour healthier reef environments.Restoring popula- tions of sharks is important and can yet deliver ecological surprise.
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