In corals, diversity of coral growth form, life-history strategies, demographic performance, palatability, protective symbionts and strength of attachment have both phenotypic and genotypic bases (Veron 1986, 1995; Knowlton and Jackson 1994). Such diversity maximizes the chance that long-term accretion of framework will be achieved in the face of periodic disturbance (Connell 1978; Rogers 1993). The "intermediate disturbance hypothesis" (Connell 1978) holds that a low rate of disturbance allows competitively dominant corals to monopolize areas, a high rate allows only the most rapid colonizers to dominate, and an intermediate rate favors coexistence of many species. At the population level, the resilience of massive corals killed or injured by crown-of-thorns starfish (Done 1987) or hurricane waves (Massel and Done 1993) is largely a function of the relationship between intensity and frequency characteristics of the disturbance and key life-history parameters of the corals.
Depauperate coral communities at reefs with weak or infrequent connections to source areas may lack both resistance and resilience (Preece and
Johnson 1993; Johnson and Preece 1993). For example, a combination of catastrophic events at very depauperate eastern Pacific Reefs decimated their two main framework builders (Glynn and de Weerdt 3991). The coral community lacked resistance because these corals happened to be vulnerable to high temperature associated with the 1982 El Nino (Glynn 1988, 1990), and to storm waves, which easily dislodged the decaying skeletons. It lacked short-term resilience because there were insufficient larvae of any coral species establishing colonies at the site. Restoration of its functionality in terms of framework accretion awaits a chancc colonisation event by the reef-building biota, an event which may be extremely rare in human time scales.
15.6 SUMMARY, RECOMMENDATIONS AND CONCLUSIONS 15.6.1 Summary
The coral reef has traditionally been portrayed as an oasis of diversity and biomass in the oceanic desert. This chapter emphasizes the there is also a great diversity among coral reefs, due to differences in their biogeographic and environmental settings, and their geological histories. Nevertheless, all build and maintain substantial wave-resistant structures and accumulate biomass per hectare well above that of the surrounding ocean. It has been proposed that if they are "healthy", they perform the critical ecosystem-level functions of photosynthesis, respiration and calcification within predictable bounds (Kinsey 1988, 1991), regardless of the specific details of biological composition and diversity. Reefs performing up to these standards (Table 15.1) possess a capacity for net accretion of limestone which is an essential, but not sufficient, condition for maintenance of the coral reef as an entity.
Net limestone accretion could conceivably be achieved solely by encrusting and erect coralline algae, foraminifera. molhjsks and echinoderms. However, in the absence of a framework, these groups are incapable of building the porous, three dimensional, wave-resisting reef structures that provide habitats for a myraid other forms. Reef-building corals provide that dimensionality.
Corals can be severely depleted or entirely destroyed over large areas by any number of natural and anthropogenic disturbances, events which in many circumstances lead to an undergrazed reef covered by macroalgae, or a bare, overgrazed reef being actively eroded. Diversity of growth form, palatability, tenacity and physiological tolerances at the levels of genotypes and species - provide corals collectively with the capacity to occupy gaps and resist disturbance in a wide range of environments. Diversity of life-history strategies and abundant parental stocks in strategic places provide coral communities with a capacity for resilience. Reefs or patches of reef may rely mostly on their near neighbours for year-to-year recruitment, but the global distribution of coral reefs also reflects a remarkable capacity for reef species to disperse across oceans. Managers and commercial users of coral reef resources need to be mindful of the connectivities among reef populations and of the range of time and spacc scales over which they operate, and scientists need to provide managers with a more quantitative understanding of these issues (Done 1994).
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