ABSTRACT
Biodiversity, distribution, and ecosystem services Tropical coral reefs are extremely complex ecosystems and true diversity hot spots in the oceans. Only a small fraction of the diversity is made up by stony corals and coralline algae, which build the reefs. The bulk is provided by fish and invertebrate animals dwelling in and around the reefs. The combined species richness of coral reefs has been tabulated to be close to 100,000 (Reaka-Kudla, 1997), but true diversity could well be ten times as great. That 4-5 per cent of all described living species are associated with coral reefs is astounding: reefs occupy only a small fraction of the oceanic habitats (less than 0.1 per cent) let alone the entire Earth. Also, reefs lack insects that make up the bulk of biodiversity in tropical rainforests, the terrestrial equivalents of coral reefs. Reef diversity varies substantially with geographic region. Highest diversity is found in the ‘coral triangle’ in the tropical west Pacific, lowest diversity is observed in the Caribbean. Tropical coral reefs only occur between 34°N and 32°S in well-lit shallow
water. Their ecological requirements can be well-established by looking at a global distribution map. Reefs are heavily concentrated in ocean interiors and at the western margins of oceans. This is due to cool and nutrient-rich currents moving along the western margins of north-south oriented continents. The ability of tropical reef corals to thrive in oligotrophic settings is clearly linked to the coral-algal symbiosis. Corals take up most of their food from the photosynthetic products of algae that they host in their tissues, whereas the algae receive carbon dioxide and inorganic nutrients from the coral animal. Symbiosis also enhances growth rates of corals by the removal of carbon dioxide for algal photosynthesis. A completely different type of coral reefs gained attention only in the past
two decades or so. These are cold-water reefs known from the Polar Circle to
the Southern Ocean. The corals building these reefs do not harbour symbionts and depend entirely on external feeding on zooplankton and particulate organic matter. As they do not require light, they thrive in deeper waters of 200-1000m. The diversity of corals in these reefs is much lower than even in moderately diverse tropical settings. Nevertheless, cold-water coral ecosystems are biodiversity hot spots in the deep sea with hundreds of associated species, including fish and invertebrate animals (Roberts et al, 2006). Coral reefs provide many important ecosystem services. Thanks to very
efficient nutrient recycling in reefs, a large biomass of fish and other animals can be maintained in regions that are otherwise oceanic deserts. Fisheries in coral reefs provide a major food source. The aesthetic and recreational value of coral reefs attracts tourists. Reefs protect coastlines from wave action and erosion and provide building materials. Due to the intense competition, reef organisms developed a large suite of chemical compounds for defence and attack. Many of these substances are expected to be useful for new drugs. Snails and sponges have already delivered medicines for the treatment of cancer and melanoma but the potential of reefs for pharmacy is still underexplored. The genetic treasure of reefs is perhaps the most direct service that is offered by biodiversity. The direct economic service of coral reefs has been estimated to be US$30 billion per year (Cesar et al, 2003) but this number is very conservative as it excludes cold-water reefs and uncovered medical treasures.
