Sharks are considered the apex predator of coral reefs, but the consequences of 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 populations of sharks is important and can yet deliver ecological surprise.

The Ecological Role of Sharks on Coral Reefs

Arguments for the need to conserve aquatic predator (AP) populations often focus on the ecological and socioeconomic roles they play. Here, we summarize the diverse ecosystem functions and services connected to APs, including regulating food webs, cycling nutrients, engineering habitats, transmitting diseases/parasites, mediating ecological invasions, affecting climate, supporting fisheries, generating tourism, and providing bioinspiration. In some cases, human-driven declines and increases in AP populations have altered these ecosystem functions and services. We present a social ecological framework for supporting adaptive management decisions involving APs in response to social and environmental change. We also identify outstanding questions to guide future research on the ecological functions and ecosystem services of APs in a changing world.

Ecosystem Function and Services of Aquatic Predators in the Anthropocene.

Extreme Inverted Trophic Pyramid of Reef Sharks Supported by Spawning Groupers

Trophic cascades occur in many ecosystems, but the factors regulating them are still elusive. We suggest that an overlooked factor is that trophic interactions (TIs) are often scale-dependent and p ...

https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2017.0045

A cross-scale trophic cascade from large predatory fish to algae in coastal ecosystems.

Sharks are a diverse group of mobile predators that forage across varied spatial scales and have the potential to influence food web dynamics. The ecological consequences of recent declines in shark biomass may extend across broader geographic ranges if shark taxa display common behavioural traits. By tracking the original site of photosynthetic fixation of carbon atoms that were ultimately assimilated into muscle tissues of 5,394 sharks from 114 species, we identify globally consistent biogeographic traits in trophic interactions between sharks found in different habitats. We show that populations of shelf-dwelling sharks derive a substantial proportion of their carbon from regional pelagic sources, but contain individuals that forage within additional isotopically diverse local food webs, such as those supported by terrestrial plant sources, benthic production and macrophytes. In contrast, oceanic sharks seem to use carbon derived from between 30° and 50° of latitude. Global-scale compilations of stable isotope data combined with biogeochemical modelling generate hypotheses regarding animal behaviours that can be tested with other methodological approaches.

/pdf/a-global-perspective-on-the-trophic-geography-of-sharks-6dgnbq1k5o.pdf

A global perspective on the trophic geography of sharks

In our recent review of the ecological roles of sharks on coral reefs [1], we concluded that the evidence to support hypothesised shark-driven trophic cascades on coral reefs was weak and equivocal. In their response to our review, Ruppert et al. [2] assert that a major issue with our approach was that we primarily reviewed evidence from correlative observational studies to reach this conclusion. This is incorrect, because our conclusion is based upon multiple lines of evidence, including observational studies, food-web models, dietary and stable isotope analysis, trophic position, habitat use, and behavioural evidence [1].

Eleanor Aurellado

Papers

The Ecological Role of Sharks on Coral Reefs

Reassessing Shark-Driven Trophic Cascades on Coral Reefs: A Reply to Ruppert et al.