Definition
A biogeographic realm (also called a zoogeographic or eco‑zone) is one of the largest spatial units used to describe the distribution of terrestrial organisms on Earth. Realms are defined by the distinctive assemblages of flora and fauna that have evolved relatively independently due to long‑term geographic isolation, plate tectonic history, and climatic regimes.
Historical development
The concept of dividing the world into major biogeographic regions dates to the 19th‑century work of naturalists such as Alfred Russel Wallace and Philip Sclater. Modern realm delineations incorporate phylogenetic, paleontological, and species‑distribution data, refining earlier schemes that were based primarily on visible faunal differences.
Current consensus classification
The most widely accepted global framework, presented by the World Wildlife Fund (WWF) and supported by subsequent biogeographical literature, recognizes eight terrestrial realms:
| Realm | Geographic scope | Representative region(s) |
|---|---|---|
| Nearctic | North America (including Greenland) and high‑latitude Arctic islands | United States, Canada |
| Palearctic | Europe, northern Africa, most of Asia north of the Himalayas, and the Arabian Peninsula | Europe, Siberia |
| Neotropical | South America, Central America, Caribbean, and southernmost North America | Amazon Basin, Andes |
| Afrotropical (formerly Ethiopian) | Sub‑Saharan Africa, Madagascar, parts of the Arabian Peninsula | Congo Basin, East African savannas |
| Indomalayan (formerly Oriental) | South and Southeast Asia, including Indian subcontinent and the Malay Archipelago west of Wallace’s Line | Indian subcontinent, Borneo |
| Australasian | Australia, New Guinea, New Zealand, and neighboring islands east of Wallace’s Line | Australian outback, New Guinea highlands |
| Oceanian | Remote Pacific islands not included in the Australasian realm, such as Polynesia, Micronesia, and Fiji | Hawaiian archipelago, Samoa |
| Antarctic | Continental Antarctica and nearby islands | Antarctic Peninsula |
These realms are further subdivided into biogeographic provinces (also called ecoregions) that capture finer‑scale ecological and evolutionary patterns.
Criteria used for delineation
- Endemic higher taxa – Presence of plant or animal families, orders, or sub‑orders largely confined to a single realm.
- Historical isolation – Continental drift, oceanic barriers, and mountain ranges that limited gene flow over millions of years.
- Phylogenetic divergence – Molecular clock estimates indicating distinct evolutionary lineages corresponding to realm boundaries.
- Climatic and habitat continuity – Broad similarity in climate regimes (e.g., temperate vs. tropical) that support comparable community structures.
Applications
- Conservation planning – Realms help prioritize regions for biodiversity protection, ensuring representation of globally distinct lineages.
- Biogeographical research – Analyses of species richness, endemism, and evolutionary history frequently reference realm boundaries.
- Ecological forecasting – Climate‑change impact models often project shifts in realm‑level community composition.
Limitations
- Transition zones (e.g., the Mexican highlands between Nearctic and Neotropical) can blur boundaries, leading to mixed assemblages.
- Marine and freshwater systems are not encompassed by the terrestrial realm framework; separate marine biogeographic provinces exist.
- Anthropogenic changes (e.g., invasive species, habitat alteration) may erode the historical distinctiveness that realms reflect.
See also
- Biogeographic province
- Ecoregion
- Zoogeography
- Phytogeography
References
- Olson, D. M., et al. (2001). Terrestrial Ecoregions of the World: A New Map of Life on Earth. BioScience, 51(11), 933–938.
- Wallace, A. R. (1876). The Geographical Distribution of Animals. Macmillan.
- Holt, B. G., et al. (2013). An update of Wallace’s zoogeographic regions of the world. Science, 339(6115), 74–78.
- WWF (2022). Global 200: Terrestrial Ecoregions. Retrieved from https://www.worldwildlife.org/biomes.
