A biogeographic realm or ecozone is the broadest biogeographic division of the Earth's land surface, based on distributional patterns of terrestrial organisms. They are subdivided in ecoregions, which are classified in biomes or habitat types.
The realms delineate large areas of the Earth's surface within which organisms have been evolving in relative isolation over long periods of time, separated from one another by geographic features, such as oceans, broad deserts, or high mountain ranges, that constitute barriers to migration. As such, biogeographic realms designations are used to indicate general groupings of organisms based on their shared biogeography. Biogeographic realms correspond to the floristic kingdoms of botany or zoogeographic regions of zoology.
Biogeographic realms are characterized by the evolutionary history of the organisms they contain. They are distinct from biomes, also known as major habitat types, which are divisions of the Earth's surface based on life form, or the adaptation of animals, fungi, micro-organisms and plants to climatic, soil, and other conditions. Biomes are characterized by similar climax vegetation. Each realm may include a number of different biomes. A tropical moist broadleaf forest in Central America, for example, may be similar to one in New Guinea in its vegetation type and structure, climate, soils, etc., but these forests are inhabited by animals, fungi, micro-organisms and plants with very different evolutionary histories.
The patterns of distribution of living organisms in the world's biogeographic realms were shaped by the process of plate tectonics, which has redistributed the world's land masses over geological history.
The usage of the term "ecozone" is more variable. It was used originally in stratigraphy (Vella, 1962, Hedberg, 1971). In Canadian literature, the term was used by Wiken (1986) in macro level land classification, with geographic criteria (see Ecozones of Canada). Later, Schültz (1988) would use it with ecological and physiognomical criteria, in a way similar to the concept of biome.
In the Global 200/WWF scheme (Olson & Dinerstein, 1998), originally the term "biogeographic realm" in Udvardy sense was used. However, in a scheme of BBC, it was replaced by the term "ecozone".