2.2 Ecosystems of the World

An ecosystem is a community of organisms and their physical and chemical environment interacting as an ecological unit. It represents all biological and abiotic components, including man, within a defined and delimited biotope, and is characterized by distinct ecological biocoenotic features of structure and functioning. The ecosystem relates to many different scales. At the lowest end of the scale, the ecosystem may comprise a rock, a fallen tree, or a certain layer in a plant community (pico-ecosystem, as described by Ellenberg, 1973). At the largest end of the scale it relates to life support media and covers oceanic, terrestrial, limnic, or urban ecosystem categories (mega-ecosystem). The present description is restricted to terrestrial ecosystems and focuses in particular on potential natural vegetation. Centred on the plant formation, it classes natural forests and woodlands in hot and cold climates at macro-ecosystem level. The description is structured by classifying the forests and woodlands at microecosystem (formation group) level according to major physiognomic, phenological, and growth features in relation to the site and environmental conditions. These units correspond roughly to the bioclimatic life zones of Holdridge (1967). The description is patterned on the serial structure of the physiognomicecological classification of plant formations of the earth (Miiller-Dombois and Ellenberg, 1974, pp. 466-488) commonly quoted as the UNESCO Classification.