Reassessing the trophic role of reef sharks as apex predators on coral reefs
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.
KeywordsElasmobranch Food web Stable isotope analysis Top-down control Trophic ecology
We thank R. Baker, B. Bergseth, J. Frisch, S. Frisch and B. Bauerle for technical assistance and N. Hussey for reviewing an earlier draft of the manuscript. Funding was provided by a Lizard Island Research Station John and Laurine Proud Fellowship (AJF) and a grant from the Save Our Seas Foundation (JRR). This research was undertaken with permission from the GBR Marine Park Authority (permit number G12/34941.1), Fisheries Queensland (permit number 152940) and the James Cook University Animal Experimentation Ethics Committee (approval number A1742).
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