Species diversity in tropical seagrass beds and environmental fluctuation
Hiroshi Mukai
Akkeshi Marine Biological Station, Hokkaido University, Japan
mukaih@hucc.hokudai.ac.jp
Seagrass bed is a fundamental component of tropical coastal ecosystems with coral reef and mangrove swamp. Seagrasses have been evolved in Tethys shallow seas in cretaceous, and with continental drift they were expanded the distribution to Atlantic and the Pacific. In the Pacific area, Malaysiana, area enclosed by a triangle of Indonesia, Phillipines and New Guinea Island, has been believed as the most closed area to the origin of seagrass speciation (den Hartog, 1970). Species diversity of seagrasses is highest in this area. Seagrasses have expanded the range of distribution with decrease of species diversity along ocean currents to the central Pacific, Ryukyu Archipelago, Indian Ocean, and the east coast of Australia in current tens of millions years (Mukai, 1993). Species diversity of island is balanced between colonization and extinction rates (McArthur & Wilson, 1967). The fact that the Eastern Pacific islands have not seagrass flora even now, means that the extinction rate is very high in small island habitats. It suggests the seagrass bed ecosystems are vulnerable to environmental fluctuation of habitats. An example, sudden disappear of a small seagrass bed in Thailand which has supported a dugong, can be presented.
Environmental fluctuation concerned with human activities includes global warming, abnormal weather conditions, increase of CO2 level, eutrophication, etc. Eutrophication and strong storms caused by abnormal weather are affecting on seagrass diversity in tropical islands.
