(e.g., Jackson, 1991). Because this diversity is expressed in a spectacular array of form and color, and because of increasing threats to the survival of this diversity (Hallock et al., 1993), coral reefs have attracted considerable tourism and public interest. Moreover, modern coral reefs, representing approximately 6,000 years of growth during the most recent period of sea-level rise, are the oldest and largest biogenic structures in nature bearing detailed paleoecological and climate records (Dunbar and Cole, 1993; S.V. Smith and Buddemeier, 1992). Whereas global climate change, and the potential associated effects of sea-level rise, increasing water temperature, and increasing ultraviolet (UV-B) radiation, are possible long-term threats to reefs, there is an immediate need to manage the relentless impact of explosive human population growth on reef habitats (D'Elia et al., 1991; Norse, 1993).

Reefs provide good examples of the importance of linkages between habitats—reef biodiversity is dependent on adjacent ecosystems for feeding areas and nursery grounds and as buffers against land runoff of sediments and nutrients (Kuhlmann, 1988; Ogden, 1988). Runoff from terrestrial environments is one of the most severe problems affecting reefs around the world. Sediment load from agriculture or forest clear-cutting, freshwater, and industrial activity are known to be damaging reefs (Grigg, 1984, 1994; Hodgson, 1989; Richmond, 1993; Sebens, 1994) either by killing colonies, preventing settlement of new recruits, interfering with sexual reproduction, or all three. Eutrophication from agricultural fertilizers and human sewage is a particular problem, first because organic enrichment causes faster growth in weedy macroalgae than in corals, overgrowing and killing them (S.V. Smith et al., 1981), and second because corals are adapted to live in a nutrient-poor environment, and thus overfertilizing alters the productive but very fragile relationship between the corals and their algal symbionts (Falkowski et al., 1993). In addition, reefs hold a significant portion of the fisheries resources of developing tropical countries, and they are very sensitive to overfishing, especially the removal of large predators and herbivorous fishes (see Boxes 2 and 6).

Biotic and abiotic disturbances on reefs that shift reef composition from framework builders (corals) to nonframework builders (algae) have particularly dramatic effects on biodiversity. One example is provided in Figure 5 that illustrates how the delicate natural balance (a) between fishes, the ratio of coral to large algae, and the herbivorous sea urchin Diadema antillarum in Caribbean reefs has been tipped first one way (b) by overfishing and then another way (c) by mass mortality of the urchin due to an unknown pathogen (Jackson, 1994).

Excellent opportunities exist in coral reef systems to look directly at the dynamic interface between the natural patterns, processes, and consequences of biodiversity, and the increasing pressures from human activities. A broad range of observed transitions between different reef communities exhibiting differing levels of impacts and thus threshold effects—effects that may be irreversible over the scale of a human lifetime (Knowlton, 1992)—offers an irresistible comparative menu for study.

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