REPORT Coral–macroalgal phase shifts or reef resilience: links with diversity and functional roles of herbivorous fishes on the Great Barrier Reef A. J. Cheal • M. Aaron MacNeil • E. Cripps • M. J. Emslie • M. Jonker • B. Schaffelke • H. Sweatman Received: 23 February 2010 / Accepted: 4 July 2010 / Published online: 12 August 2010 Ó Springer-Verlag 2010 Abstract Changes from coral to macroalgal dominance following disturbances to corals symbolize the global degradation of coral reefs. The development of effective conservation measures depends on understanding the cau- ses of such phase shifts. The prevailing view that coral– macroalgal phase shifts commonly occur due to insufficient grazing by fishes is based on correlation with overfishing and inferences from models and small-scale experiments rather than on long-term quantitative field studies of fish communities at affected and resilient sites. Consequently, the specific characteristics of herbivorous fish communities that most promote reef resilience under natural conditions are not known, though this information is critical for identifying vulnerable ecosystems. In this study, 11 years of field surveys recorded the development of the most persistent coral–macroalgal phase shift ( [ 7 years) yet observed on Australia’s Great Barrier Reef (GBR). This shift followed extensive coral mortality caused by thermal stress (coral bleaching) and damaging storms. Comparisons with two similar reefs that suffered similar disturbances but recovered relatively rapidly demonstrated that the phase shift occurred despite high abundances of one herbivore functional group (scraping/excavating parrotfishes: Labri- dae). However, the shift was strongly associated with low fish herbivore diversity and low abundances of algal browsers (predominantly Siganidae) and grazers/detriti- vores (Acanthuridae), suggesting that one or more of these factors underpin reef resilience and so deserve particular protection. Herbivorous fishes are not harvested on the GBR, and the phase shift was not enhanced by unusually high nutrient levels. This shows that unexploited popula- tions of herbivorous fishes cannot ensure reef resilience even under benign conditions and suggests that reefs could lose resilience under relatively low fishing pressure. Pre- dictions of more severe and widespread coral mortality due to global climate change emphasize the need for more effective identification and protection of ecosystem com- ponents that are critical for the prevention of coral reef phase shifts. Keywords Coral reef Á Herbivorous fishes Á Macroalgae Á Phase shift Á Diversity Á Resilience Introduction Phase shifts are dramatic changes in ecological systems to an alternative state (Scheffer et al. 2001; Folke et al. 2004; de Young et al. 2008; Mumby 2009). The new state, characterized by altered community composition and structure (Nystro ¨m et al. 2008), often provides fewer eco- system services and hence fewer benefits for many asso- ciated organisms, including humans (Brock and Carpenter 2006). Phase shifts occur when the magnitude of an external force exceeds the internal resilience of a system, resilience that is likely to be eroded by the removal of Communicated by Ecology Editor Prof. Mark Hay Electronic supplementary material The online version of this article (doi:10.1007/s00338-010-0661-y) contains supplementary material, which is available to authorized users. A. J. Cheal (&) Á M. A. MacNeil Á E. Cripps Á M. J. Emslie Á M. Jonker Á B. Schaffelke Á H. Sweatman Australian Institute of Marine Science, Townsville, QLD 4810, Australia e-mail: a.cheal@aims.gov.au E. Cripps School of Mathematics and Statistics, University of Western Australia, Crawley, WA 6009, Australia 123 Coral Reefs (2010) 29:1005–1015 DOI 10.1007/s00338-010-0661-y