The world has many thousands of living coral reefs, located in the tropics and sub-tropics between approximately 30" N and 30r S, where the minimum sea surface temperature rarely falls below 18°C (FigurelS.l). Collectively, they cover an area in excess of 6 x 105 km2 (Smith 1978) and encompass a wide range of forms, biological composition, diversity and structural organization. This reflects disparate bio-geological origins, ages, biogeographic settings and environments (Figure 15.2). The largest coral reefs are oceanic atolls on top of submerged volcanoes, often measuring up to tens of kilometers across (Figure 15.2). The largest continuous tracts of coral reefs occur on shallow (<100 m deep) continental shelves (Figure 15.2a). Reef forms include both coastal and island fringing reefs, and autonomous platforms located from a few to tens of kilometers from the nearest land, and hundreds of meters to tens of kilometres from each other. For example, in eastern Australia, the Great Barrier Reef occupies a region approximately 2000 km long and 50-150 km wide, and contains almost 3000 fringing and platform reefs ranging in length from less than 1 km to about 30 km (Hopley 1982). Continental shelf reef systems with similar diversity of form can be found along the eastern coasts of Africa, Asia and Central America.

Biodiversity and the products of ecosystem function are both very apparent on coral reefs. Through geological time scales, their ecosystem processes produce, accumulate and cement limestone skeletons of a diversity of taxa into wave-resistant structures which can dwarf the tallest forests. Through evolutionary, ecological and human time scales, they provide foci for speciation and habitats for a spectacular variety and a substantial biomass of other biota. Today, they provide important ecosystem services to

Functional Roles of Biodiversity: A Global Perspective rftfrs

Edited bv H.A. Mooney, J.H.'Cushman, E. Medina, O.E. Sala and E.-D. Sehulze © 1996 SCOPE Published in 1996 by John Wiley & Sons Ltd unep

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