dead but intact coral skeletons) varies among species (Öhman et al. 1998) and the importance of physical structure in settlement is not yet clear. Where the habitat is complex, the influence of predation may be reduced, increasing the survival of smaller, motile reef organisms (Dulvy et al. 2002; Jones and McCormick 2002; Hixon and Jones 2005; Lee 2006). It is likely, therefore, that competition for refugia could be intense (Hixon and Jones 2005), resulting in less aggressive individuals occupy- ing unfavourable territories and being more susceptible to predation (Holbrook and Schmitt 2002). Reef fishes tend to use refuge holes most appropriate for their body size (Hixon and Beets 1993; Beukers and Jones 1997; Friedlander and Parish 1998). Thus the scale of structural complexity, or the reduction of it, will likely influence the size of fishes that are able to escape predation. Almany (2004) dem- onstrated that there is a complex interplay between habitat complexity, predation, and competition in determining the early post-settlement survival of common damselfishes, but any decline in structural complexity of reef habitats is likely to be detrimental for the abundance and diversity of motile reef organisms. 9.3.3 Phase-Shifts to Macroalgal-Dominated Systems Coral mortality following bleaching events provides space on the reef for rapid colonisation by turf-forming algae (Diaz-Pulido and McCook 2002). On reefs where herbivorous fishes and/or urchins are abundant, algal assemblages may remain as cropped turf forms for several years after the bleaching event (Aronson et al. 2002; Arthur et al. 2005). However, if grazing pressure is low, fleshy macroalgae can develop within months of coral depletion (Ostrander et al. 2000; McClanahan et al. 2001; Diaz-Pulido and McCook 2002), limiting the future recovery of coral assem- blages (Hughes et al. 2007) and potentially bringing about a phase shift from coral- to macroalgal-dominated reefs. Excess nutrients will further increase the likelihood of
9 Coral Bleaching and Consequences for Motile Reef Organisms 149 habitat degradation by enhancing algal growth, suppressing coral recruitment, and supporting increased densities of bio-eroding organisms (Fabricius 2005). Shifts from coral- to macroalgal-dominated states will have broad consequences for motile reef fauna. Species that feed, shelter or recruit to live corals will not occur on algal-dominated reefs (Jones et al. 2004). Some species, such as the spiny lobster ( Panulirus argus ) and Nassau grouper ( Epinephalus striatus ), shelter within macroalgal habitats as juveniles (Marx and Herrnkind 1985; Dahlgren and Eggleston 2000) and their abundance may increase when macroalgae proliferate. However, the adult habitat on coral reefs must be preserved if the densities of these animals are to be maintained. Algae also provide a habitat for invertebrate crypto-fauna, with areas of greater algal biomass harbouring a greater abundance of invertebrates (Zeller 1988; Dulvy et al. 2002), which may attract invertebrate predators. Similarly, some fishes, such as Naso unicornis and Kyphosus vaigiensis , feed primarily on macroalgae (Choat et al. 2002) and may be attracted to reefs where macroalgae are prominent. Macroalgae do not, however, provide structural complexity and reef
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