Ecology, 92(10), 2011, pp. 1959–1971 Ó 2011 by the Ecological Society of America Influence of corallivory, competition, and habitat structure on coral community shifts HUNTER S. LENIHAN, 1,2,4 SALLY J. HOLBROOK, 2,3 RUSSELL J. SCHMITT, 2,3 AND ANDREW J. BROOKS 2 1 Bren School of Environmental Science and Management, University of California, Santa Barbara, California 93106-5131 USA 2 Coastal Research Center, Marine Science Institute, University of California, Santa Barbara, California 93106-6150 USA 3 Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California 93106-9610 USA Abstract. The species composition of coral communities has shifted in many areas worldwide through the relative loss of important ecosystem engineers such as highly branched corals, which are integral in maintaining reef biodiversity. We assessed the degree to which the performance of recently recruited branching corals was influenced by corallivory, competition, sedimentation, and the interactions between these factors. We also explored whether the species-specific influence of these biotic and abiotic constraints helps to explain recent shifts in the coral community in lagoons of Moorea, French Polynesia. Population surveys revealed evidence of a community shift away from a historically acroporid-dominated community to a pocilloporid- and poritid-dominated community, but also showed that the distribution and abundance of coral taxa varied predictably with location in the lagoon. At the microhabitat scale, branching corals grew mainly on dead or partially dead massive Porites (‘‘bommies’’), promontories with enhanced current velocities and reduced sedimentation. A demographic study revealed that growth and survival of juvenile Pocillopora verrucosa and Acropora retusa, the two most common branching species of each taxon, were affected by predation and competition with vermetid gastropods. By 24 months of age, 20–60% of juvenile corals suffered partial predation by corallivorous fishes, and injured corals experienced reduced growth and survival. A field experiment confirmed that partial predation by corallivorous fishes is an important, but habitat-modulated, constraint for branching corals. Competition with vermetid gastropods reduced growth of both branching species but unexpectedly also provided an associational defense against corallivory. Overall, the impact of abiotic constraints was habitat-specific and similar for Acropora and Pocillopora, but biotic interactions, especially corallivory, had a greater negative effect on Acropora than Pocillopora, which may explain the local shift in coral community composition. Key words: Acropora retusa; associational defense; coral; corallivory; demography; disturbance; habitat structure; Moorea lagoons, French Polynesia; Pocillopora verrucosa; recovery; resilience; sedimentation. INTRODUCTION Coral reefs are species-rich and functionally complex communities that are impacted worldwide by both natural and human disturbances (Connell et al. 1997, Steneck 2009). Community shifts are a common recent phenomenon across reefs in the Caribbean and Indo- Pacific, and have been related in time and space to catastrophic natural disturbances, especially storms (Done et al. 1991), crown-of-thorns starfish (COTS) outbreaks (Bruno and Selig 2007), bleaching events (Lenihan et al. 2008), and disease epidemics (Aronson and Precht 2001). Anthropogenic disturbances have also led to coral loss and community shifts. For instance, trophic cascades, caused by the fishing down of herbivores, may result in increases in macroalgae, which, in turn, can outcompete corals for space, light, and nutrients (Hughes 1994, Mumby et al. 2007; but see Aronson and Precht 2006, Bruno et al. 2009). Coral community shifts have followed various trajec- tories, including loss of live coral cover (Aronson and Precht 2006), shifts from coral- to macroalgae-dominat- ed reefs (Hughes 1994), and sometimes shifts back to coral dominance (Edmunds and Carpenter 2001). In other cases, transitions from hard scleractinian corals to soft corals occur (Bruno and Selig 2007). Regardless of the trajectory, transitions can occur gradually, through the sequential loss of herbivore species (Hughes 1994), or disease-related declines of dominant corals, for example (Aronson and Precht 2001). Shifts may also occur more abruptly following natural catastrophes, such as storms and COTS outbreaks. Many shifts have been characterized by the loss of branching acroporid corals, and their replacement by pocilloporids, which have opportunistic life history traits, and/or massive poritids, which are slow-growing and generally resistant to disturbance (Bruno and Selig 2007, Edmunds and Lenihan 2010, Lenihan and Edmunds 2010). However, Manuscript received 20 January 2011; revised 27 April 2011; accepted 29 April. Corresponding Editor: R. B. Aronson. 4 E-mail: lenihan@bren.ucsb.edu 1959