Reef Shark Declines: Consequences for the Trophic Structure of Communities of Coral Reef Fishes

On coral reefs, sharks are experiencing rapid declines in abundance due to overfishing. Recent studies suggest that a high biomass of sharks promotes a trophic structure within the fish community very different to that on a reef where sharks are in low numbers. Although this implies that reef sharks have an important top-down role in structuring communities, declines in sharks can occur concomitantly with other anthropogenic (habitat loss) and natural processes (hurricanes, coral bleaching, disease, etc). Thus, the effects of shark removal may be confounded with other natural events that restructure fish communities from a bottom-up perspective.  Here, we use the results of a long-term monitoring program to disentangle the relative effects of top-down and bottom-up processes in structuring fish communities on coral reefs.

We compared the trophic structure of reef fish communities at isolated atolls on the north-west coast of Western Australia using a 16-year data set of species abundances and benthic habitat information.  One set of these atoll reefs is subject to a small-scale fishery that targets sharks (Scott Reef) and the other is protected from all fishing (Rowley Shoals). Because the disturbance history (cyclones, bleaching etc) of these reefs are documented, the effect of the removal of top-order predators could be examined while taking into account the effects of bottom-up processes. We used partial Redundancy Analysis (pRDA) to show that fished reefs that hosted very few top-order predators had fish communities that were composed of more carnivorous and herbivorous fishes than on un-fished reefs.  Permuted MANOVAs showed that shark removal was a significant driver of these differences, but that both disturbances and interactions between shark loss and disturbance also played roles in determining community structure. We also investigated the strength and significance of interactions between top-down and bottom-up processes on fish community structure. Working from a conceptual model we employed structural equation modeling to show that shark density and benthic community structure are important drivers of fish community trophic structure.  Our results suggest that sharks have a lasting and pervasive top-down influence on fish communities in coral reef ecosystems that is distinct from the effects of bottom-up processes.