The Ecology of Coral Reefs

I n the simplest sense, coral reefs are wave-resistant piles of limestone and calcareous sediments built by a thin veneer of living organisms (Hubbard 1997). But these piles are of great ecological and resource significance for their massiveness, extremely high biodiversity, and distinct trophic structure and primary production. Reefs grow most prolifically in clear, warm, shallow, and nutrient-poor waters of the western tropical Atlantic and Indo-West Pacific, and more poorly in the eastem Pacific and eastern Atlantic where upwelling and high planktonic productivity inhibit reef development (Adutuv and Dubinsky 1990). Scleractinian corals, hydrocorals, and coralline red algae are the primary framework constructors of coral reefs, although a host of other algae and invertebrate phyla also contribute to reef mass. The scale of reefs is enormous, and they are the largest durable bioconstruction projects on earth. The Panama Canal is still one of the most significant human construction projects, but it is paltry by comparison with the unexceptional coral reefs along the Caribbean coast of Panama. Moreover, modem reefs are only youngsters-less than 10,000 years old-because older reefs were drowned by the rapid Holocene rise in sea level (Hubbard 1997). Over the longer term of millions of years, even small atolls like Enewetak have accumulated 2to 3-km-thick piles of limestone, just as Darwin (1842) predicted. There are two aspects of the scale of reef bioconstruction that define the ecological setting for everything else in this chapter. First, reefs determine the physical structure of the coastline and that of adjacent environments and ecosystems (Ogden 1997). Wherever they co-occur, reefs are the protective barrier against the sea for seagrass beds (see Williams and Heck, Chapter 12, this volume) and mangroves (see Ellison and Farnsworth, Chapter 16, this volume), just as mangroves and seagrasses trap and stabilize runoff from the land and thereby prevent reefs from being drowned in sediments. These linkages can also have negative effects; for example, contaminated sediments continued to kill offshore reef corals for years after a large oil spill killed the mangroves along the central Caribbean shoreline of Panama (Jackson et al. 1989; GuzmAn et al. 1994). Thus, in a very real sense the actual habitats as well as all their inhabitants are alive. The second major point about bioconstruction is that the apparent physical stability of reefs belies an underlying natural turmoil of growth, death, and destruction of calcareous organisms (Glynn 1997; Hallock 1997; Hubbard 1997). Much like a modern city, reefs are constantly being rebuilt and tom down at the same time. Corals are the bricks, broken pieces of plant and animal skeletons the sand, and algal crusts and chemical cements the mortar. The production, accumulation, and cementation of all this calcareous material into solid limestone determine reef growth. Destruction is due to storm damage and even more to pervasive grazing and excavation by organisms. The key point is that even small changes in rates of construction or destruction may cause big increases or decreases in reef mass. For example, eastern Pacific reefs virtually disappeared after the strong El Nifio of 1983 due to coral death and intense bioerosion (Glynn and Colgan 1992). This fragility also scales up in geological time; the history of reefs over the past half billion years is punctuated by long episodes of little or very different kinds of reef development that were modulated by shifts in oceanographic regimes (Hallock 1997; Wood 1999). Coral reefs are the most taxonomically diverse of all marine ecosystems, but the nature and extent of this diversity is known only in the broadest outlines for most groups (Paulay 1997). Coral reefs probably contain at least a million species,