A biogeographic realm or ecozone is the broadest biogeographic division of Earth's land surface, based on distributional patterns of terrestrial organisms. They are subdivided into ecoregions, which are classified based on their biomes or habitat types.
The realms delineate the large areas of 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 realm 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,[ citation not found] Hedberg 1971[ citation not found]). In Canadian literature, the term was used by Wiken  in macro level land classification, with geographic criteria (see Ecozones of Canada).   Later, Schültz  would use it with ecological and physiognomical criteria, in a way similar to the concept of biome.
The World Wildlife Fund scheme    is broadly similar to Miklos Udvardy's system,  the chief difference being the delineation of the Australasian realm relative to the Antarctic, Oceanic, and Indomalayan realms. In the WWF system, the Australasia realm includes Australia, Tasmania, the islands of Wallacea, New Guinea, the East Melanesian Islands, New Caledonia, and New Zealand. Udvardy's Australian realm includes only Australia and Tasmania; he places Wallacea in the Indomalayan Realm, New Guinea, New Caledonia, and East Melanesia in the Oceanian Realm, and New Zealand in the Antarctic Realm.
|million square kilometres||million square miles|
|Palearctic||54.1||20.9||Including the bulk of Eurasia and North Africa.|
|Nearctic||22.9||8.8||Including most of North America.|
|Afrotropic||22.1||8.5||Including Trans-Saharan Africa and Arabia.|
|Neotropic||19.0||7.3||Including South America, Central America, and the Caribbean.|
|Australasia||7.6||2.9||Including Australia, New Guinea, New Zealand, and the neighbouring islands. The northern boundary of this zone is known as the Wallace Line.|
|Indomalaya||7.5||2.9||Including the Indian subcontinent, Southeast Asia, and southern China.|
|Oceania||1.0||0.39||Including Polynesia (except New Zealand), Micronesia, and the Fijian Islands.|
- Holarctic kingdom Heilprin (1887)
- Nearctic region Sclater (1858)
- Palearctic region Sclater (1858)
- Holotropical kingdom Rapoport (1968)
- Neotropical region Sclater (1858)
- Ethiopian region Sclater (1858)
- Oriental region Wallace (1876)
- Austral kingdom Engler (1899)
- Cape region Grisebach (1872)
- Andean region Engler (1882)
- Australian region Sclater (1858)
- Antarctic region Grisebach (1872)
- Transition zones:
- Mexican transition zone (Nearctic–Neotropical transition)
- Saharo-Arabian transition zone (Palearctic–Ethiopian transition)
- Chinese transition zone (Palearctic–Oriental transition zone transition)
- Indo-Malayan, Indonesian or Wallace's transition zone (Oriental–Australian transition)
- South American transition zone (Neotropical–Andean transition)
- Indo-West Pacific region
- Eastern Pacific region
- Western Atlantic region
- Eastern Atlantic region
- Southern Australian region
- Northern New Zealand region
- Western South America region
- Eastern South America region
- Southern Africa region
- Mediterranean–Atlantic region
- Carolina region
- California region
- Japan region
- Tasmanian region
- Southern New Zealand region
- Antipodean region
- Subantarctic region
- Magellan region
- Eastern Pacific Boreal region
- Western Atlantic Boreal region
- Eastern Atlantic Boreal region
- Antarctic region
- Arctic region
According to the WWF scheme: 
- Arctic realm
- Temperate Northern Atlantic realm
- Temperate Northern Pacific realm
- Tropical Atlantic realm
- Western Indo-Pacific realm
- Central Indo-Pacific realm
- Eastern Indo-Pacific realm
- Tropical Eastern Pacific realm
- Temperate South America realm
- Temperate Southern Africa realm
- Temperate Australasia realm
- Southern Ocean realm
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- Cosmopolitan distribution
- Phytochorion and World Geographical Scheme for Recording Plant Distributions, used in botany
- Udvardy, Miklos D.F. (1975). "A classification of the biogeographical provinces of the world". IUCN Occasional Paper. Morges: International Union for Conservation of Nature and natural resources (IUCN) (18).
- Wicken, E. B. 1986. Terrestrial ecozones of Canada / Écozones terrestres du Canada. Environment Canada. Ecological Land Classification Series No. 19. Lands Directorate, Ottawa. 26 pp.
- Scott, Geoffrey A.J. (1995). Canada's vegetation: a world perspective. McGill-Queen's Press. p. 13.
- Schültz, J. Die Ökozonen der Erde, 1st ed., Ulmer, Stuttgart, Germany, 1988, 488 pp.; 2nd ed., 1995, 535 pp.; 3rd ed., 2002.
Translated in English as: Schültz, Jürgen (2005). The Ecozones of the World: The Ecological Divisions of the Geosphere (2nd ed.). Berlin: Springer. ISBN 9783540285274.
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