A test of trophic cascade theory: fish and benthic assemblages across a predator density gradient on coral reefs
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Removal of predators is often hypothesized to alter community structure through trophic cascades. However, despite recent advances in our understanding of trophic cascades, evidence is often circumstantial on coral reefs because fishing pressure frequently co-varies with other anthropogenic effects, such as fishing for herbivorous fishes and changes in water quality due to pollution. Australia’s outer Great Barrier Reef (GBR) has experienced fishing-induced declines of apex predators and mesopredators, but pollution and targeting of herbivorous fishes are minimal. Here, we quantify fish and benthic assemblages across a fishing-induced predator density gradient on the outer GBR, including apex predators and mesopredators to herbivores and benthic assemblages, to test for evidence of trophic cascades and alternative hypotheses to trophic cascade theory. Using structural equation models, we found no cascading effects from apex predators to lower trophic levels: a loss of apex predators did not lead to higher levels of mesopredators, and this did not suppress mobile herbivores and drive algal proliferation. Likewise, we found no effects of mesopredators on lower trophic levels: a decline of mesopredators was not associated with higher abundances of algae-farming damselfishes and algae-dominated reefs. These findings indicate that top-down forces on coral reefs are weak, at least on the outer GBR. We conclude that predator-mediated trophic cascades are probably the exception rather than the rule in complex ecosystems such as the outer GBR.
KeywordsCommunity ecology Fishing Herbivory Marine protected areas Top-down effects
We thank O. Lönnstedt, Lizard Island Research Station, and the crew of the Research Vessel James Kirby for field support. We also thank J. Alvarez Romero, D. E. Burkepile, J. H. Choat, J. S. Lefcheck, S. A. Sandin, one anonymous reviewer, and the Ecological Modelling Research Group at James Cook University for helpful comments and discussions that improved the manuscript. Funds were provided by the ARC Centre of Excellence for Coral Reef Studies and James Cook University. All work was carried out under the Great Barrier Reef Marine Parks Authority Permit No. G13/36059.1.
Author contribution statement
JMC, AHB, MOH, JRR, AJF, and SRC designed the study. JMC, AHB, SJB, MOH, JRR, AJF, and CEM collected the data. JMC and SJB performed modelling. JMC wrote the first draft of the manuscript, and all authors contributed substantially to revisions.
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