Report

Coral Reefs

, Volume 35, Issue 2, pp 459-472

First online:

Reassessing the trophic role of reef sharks as apex predators on coral reefs

  • Ashley J. FrischAffiliated withAustralian Research Council Centre of Excellence for Coral Reef Studies, James Cook University Email author 
  • , Matthew IrelandAffiliated withCollege of Marine and Environmental Sciences, James Cook University
  • , Justin R. RizzariAffiliated withAustralian Research Council Centre of Excellence for Coral Reef Studies, James Cook UniversityCollege of Marine and Environmental Sciences, James Cook University
  • , Oona M. LönnstedtAffiliated withCollege of Marine and Environmental Sciences, James Cook UniversityDepartment of Ecology and Genetics, Limnology, Uppsala University
  • , Katalin A. MagnenatAffiliated withCollege of Marine and Environmental Sciences, James Cook University
  • , Christopher E. MirbachAffiliated withCollege of Marine and Environmental Sciences, James Cook University
  • , Jean-Paul A. HobbsAffiliated withDepartment of Environment and Agriculture, Curtin University

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Abstract

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.

Keywords

Elasmobranch Food web Stable isotope analysis Top-down control Trophic ecology