Pollution and overfishing have for some time been major threats to the health of coral-reef ecosystems, but additional environmental stress caused by warming of ocean waters has recently become a key factor in coral-reef stability. The importance of this kind of stress is illustrated by the massive coral-bleaching event of 1997--1998, which impacted 16% of the world's reefs and was particularly damaging in regions of the Pacific and Indian Oceans.

In the new work, the Australian researchers took an innovative approach to studying the recovery of coral reefs after the 1997--1998 event. Instead of simply observing and describing effects of the event and subsequent trajectories of reef health, the authors of the study experimentally tested the ability of reefs to recover under two different conditions: the presence of abundant predatory and herbivorous (plant- and algae-eating) fish, and the absence of significant numbers of these fish. The latter condition, which was achieved by placing large cages over coral reef stands to keep large fish away, mimicked the conditions under which coral reefs would recover from a bleaching event in areas also experiencing chronic overfishing.

The experiment assessed coral recovery in a region of the Great Barrier Reef Marine Park that is strictly protected from fishing, such that only coral stands under experimental cages experienced the depletion of large predatory and herbivorous fish. The researchers found that the two groups of coral underwent very different courses of recovery from the bleaching event: Whereas reefs subjected to grazing by large herbivorous fish species exhibited resilience in recovery, recruiting new corals to the reef and keeping algal growth in check, the reef areas from which large fish were excluded showed a distinct erosion in reef quality, with assemblages of algae and plant life overgrowing the reef and preventing the recruitment of new coral.

The study's findings indicate that grazing by large herbivorous fishes plays a key role in the ability of coral reef ecosystems to recover from bleaching events and maintain resilience in the face of thermal stress due to rises in ocean temperatures. On a practical level, the work strongly suggests that local management efforts aimed at preventing chronic overfishing may not only help ensure reef health under stable conditions, but may also significantly boost the resilience of coral reefs in the face of bleaching events and, potentially, other aspects of environmental thermal stress.

The researchers include Terence P. Hughes, Morgan S. Pratchett, Maria J. Rodrigues, David R. Bellwood, Daniela Ceccarelli, and Bette Willis of the Australian Research Council Centre of Excellence for Coral Reef Studies, James Cook University in Townsville, Australia; Ove Hoegh-Guldberg of the Australian Research Council Centre of Excellence for Coral Reef Studies and Centre for Marine Science, University of Queensland in Brisbane, Australia; Laurence McCook of the Australian Research Council Centre of Excellence for Coral Reef Studies, James Cook University and Great Barrier Reef Marine Park Authority in Townsville, Australia; Natalie Moltschaniwskyj of the University of Tasmania in Tasmania, Australia; Robert S. Steneck of the Australian Research Council Centre of Excellence for Coral Reef Studies, James Cook University in Townsville, Australia and University of Maine in Walpole, Maine.

The researchers thank the Australian Research Council for providing financial support, the Great Barrier Reef Marine Park Authority for granting a research permit, and a small army of student volunteers for helping with routine scrubbing of the experimental cages.

Hughes et al.: "Phase Shifts, Herbivory, and the Resilience of Coral Reefs to Climate Change" Publishing in Current Biology 17, February 20, 2007. http://www.current-biology.com

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