The classification of the Earth's immense variety of ecosystems into a manageable system is a major scientific challenge important for management and conservation of the biosphere. At the global level, most classification systems work between the complexities of community ecology and the simplified terms of a general habitat classification. These systems use a habitat type definition with a climatic descriptor; for example, tropical moist forest, or temperate grassland. Some systems incorporate global biogeography to account for differences in biota between regions of the world which may have very similar climate and physical characteristics. The measurement of ecosystem diversity is still in its infancy. Nevertheless, ecosystem diversity is an essential element of total biodiversity and should be reflected in any biodiversity assessment.