Biogeographic Realms and Regions

• Biogeographic realms and regions are large areas of the Earth’s surface, each with a distinct assemblage of plants and animals shaped over millions of years by historical (evolutionary, tectonic) and ecological processes.
• They explain why similar climates on different continents carry quite different species: the rainforests of the Amazon, the Congo and Borneo look alike but hold sloths, okapis and orangutans respectively. This puzzle founded biogeography as a discipline.
• Realms are drawn on shared evolutionary history, not on climate. Climate-based units are biomes, which cut across realms.

The Concept of Realms and Regions

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Evolution of the concept

• Philip Lutley Sclater (1858) divided the land into six regions on the basis of bird distribution: Palaearctic, Aethiopian, Indian, Australian, Nearctic and Neotropical.
• Alfred Russel Wallace (1876), in The Geographical Distribution of Animals, extended Sclater’s six regions to all animal groups and gave each four sub-regions. His scheme of faunal regions remains the classical framework.
  • Thomas Henry Huxley (1868) grouped such regions into larger units, Arctogaea, Neogaea and Notogaea, stressing the uniqueness of Southern Hemisphere faunas.
• Later revisions came from Philip Jackson Darlington Jr. (1957), Samuel Charles Kendeigh (1961), Wilma George (1962), Gustaf de Lattin (1967) and Joachim Illies (1974). They redrew boundaries but kept Wallace’s broad pattern.
• Miklos D. F. Udvardy (1975), for IUCN and UNESCO, produced the first scheme for conservation: 8 realms, 14 biome types and 193 biogeographic provinces. It guided the selection of biosphere reserves under the Man and the Biosphere Programme.
• David Olson, Eric Dinerstein and colleagues (2001), for WWF, mapped 867 terrestrial ecoregions nested in 14 biomes and 8 realms. This is today’s standard map; its 2017 update by Dinerstein and others has 846 ecoregions.
• Ben Holt and colleagues (2013) updated Wallace’s map using the distributions and evolutionary relationships (phylogenies) of 21,037 species of amphibians, birds and mammals.
  • They recognised 11 realms and 20 regions, adding Panamanian, Saharo-Arabian, Sino-Japanese, Madagascan and Oceanian realms.
• Water has its own schemes: the Marine Ecoregions of the World (Mark Spalding and others, 2007) has 12 realms, 62 provinces and 232 ecoregions; the Freshwater Ecoregions of the World (Robin Abell and others, 2008) has 426 ecoregions based on fish.

Realm, region, biome and ecoregion

• The units nest: each ecoregion belongs to one realm and one biome.
  • The Western Ghats moist forests are an ecoregion of the tropical moist forest biome in the Indomalayan realm.
  • The same biome in the Neotropic realm forms the Amazon ecoregions; the structure is similar, but the species are different.
• Realm boundaries follow history; biome boundaries follow climate. A realm therefore contains several biomes, from tundra to desert in the Palearctic.

What draws the boundaries

• Barriers: oceans, deserts (the Sahara), and high mountains and plateaus (the Himalaya and Tibet) have blocked the exchange of species for millions of years.
• Plate tectonics: the break-up of Pangaea and Gondwana isolated Australia, South America, Africa, Madagascar and India, each carrying its own lineages.
  • Land connections later mixed them: the Bering land bridge joined the Nearctic and Palearctic (together the Holarctic), and the Isthmus of Panama (about 3 million years ago) set off the Great American Biotic Interchange.
• Glaciation: the Pleistocene ice ages pushed northern faunas south and back, and lowered sea level to expose land bridges such as the Sunda Shelf.
• Transition zones: realms rarely meet along a sharp line.
  • Wallacea, between Wallace’s line and Weber’s line, mixes Asian and Australian faunas.
  • The Himalaya, Tibet and the mountains of China mark the transition between the Palearctic and the Oriental (Indomalayan) faunas. Mexico and the Sahara–Arabian belt are other transition zones.
• Oceanic islands that were never joined to a continent hold peculiar, unbalanced faunas. Hawaii has no native reptiles, amphibians, freshwater fishes or land mammals except one bat.
  • Some authors treat Madagascar, New Zealand, Hawaii and the Greater Antilles as minor regions of their own.

The Eight Biogeographic Realms

• The WWF scheme recognises eight terrestrial realms. It descends from Wallace’s six regions, with Oceania and Antarctica added as separate realms.

01. Palearctic realm

• Europe, North Africa, Arabia outside the south, and Asia north of the Himalaya. The largest realm, spanning Mediterranean scrub and Saharan desert, temperate forest, steppe, boreal taiga and Siberian tundra.
• Glaciation shaped it most: repeated ice advances pushed species into southern refugia (Iberia, Italy, the Balkans, the Caucasus) and back.
  • Vicariance (splitting of populations by barriers) and dispersal together explain its varied character; centuries of human change have since modified its landscapes.
• Key fauna: European bison, Siberian (Amur) tiger, saiga, snow leopard, dormice and mole rats, great flocks of migratory birds.
• Key biomes: boreal forest, tundra, temperate forest, steppe, Mediterranean scrub and desert.

02. Nearctic realm

• North America, Greenland, the Canadian Arctic Archipelago and the Mexican highlands. A gradient runs from Arctic tundra and boreal forest through temperate deciduous forest, prairies and deserts to Mediterranean woodland in California.
• Glaciation was the dominant force: the Laurentide ice sheet repeatedly compressed and released populations.
• It shares many genera with the Palearctic through the Bering land bridge; the two together form the Holarctic. Their species differ, as with the American and European bisons.
• Key fauna: American bison, grey wolf, grizzly bear, bald eagle, pronghorn (the only surviving member of an endemic family), pocket gophers, wild turkey.
• Key flora: coniferous and deciduous forests, prairie grasses and desert scrub.

03. Neotropic realm

• South and Central America, the Caribbean and lowland southern Mexico. It holds the highest biodiversity of any realm: the Amazon, the world’s largest tropical rainforest, and the Andes, whose relief creates countless microhabitats.
• It was an island continent for most of the Cenozoic, which explains its endemic groups: sloths, anteaters and armadillos, New World monkeys, hummingbirds and many plants.
• Key fauna: jaguar, sloth, tapir, macaws, poison dart frogs.
• It is culturally diverse, with many indigenous communities living close to the forest. Chief threats: deforestation, habitat loss and climate change.

04. Afrotropic realm

• Sub-Saharan Africa, Madagascar and south-western Arabia. Its ecosystems range from savannas and tropical rainforests to deserts and mountains.
• Key fauna: the richest surviving large-mammal fauna on Earth (elephants, lions, rhinoceroses, giraffes, gorillas), with hippopotamus, aardvark and ostrich characteristic.
  • Madagascar is so distinct, with lemurs, tenrecs and fossa, that the 2013 scheme makes it a separate realm.
• Sub-Saharan Africa is equally rich in cultures closely tied to the land, so conservation is a socio-economic as well as an ecological task. Chief threats: habitat destruction, poaching and climate change.

05. Indomalayan (Oriental) realm

• The Indian subcontinent, southern China, mainland Southeast Asia, the Philippines and western Indonesia (Sumatra, Java, Borneo). Wallace’s line separates it from Australasia.
• Tropical rainforests and extensive mangroves sustain tigers, Asian elephants, rhinoceroses, gibbons, orangutans, tree shrews and hornbills, with a high level of endemism.
  • Its broad sub-units are the Indian subcontinent, Indochina and the Sunda shelf (Malaysia and western Indonesia).
• Chief threats: deforestation, fragmentation and climate change, which endanger both species and forest-dependent people.

06. Australasian realm

• Australia, New Guinea, New Zealand, New Caledonia, the Solomon Islands and eastern Indonesia east of Wallace’s line.
• Long isolation after the break-up of Gondwana produced its marsupials (kangaroos, koalas, wombats) and the egg-laying monotremes (platypus and echidna), a classic case of divergence under geographic isolation.
• Ecosystems: arid outback, savannas, eucalyptus woodland, rainforests, and the Great Barrier Reef, the largest coral reef system offshore.
  • New Guinea has rich rainforests with birds of paradise; New Caledonia has the flightless kagu; New Zealand has kiwis and the tuatara.
• Chief threats: habitat loss, invasive species (cats, foxes, rabbits) and climate change.

07. Oceanian realm

• The smallest realm: the scattered islands of Polynesia, Micronesia and Fiji, including Hawaii. Isolation makes their endemism very high.
• Native land mammals are limited to bats, such as the giant fruit bats (flying foxes) of Fiji, Samoa and Tonga. Isolation also produced the Hawaiian honeycreepers and endemic palms and ferns.
• Coral reefs, for example around Fiji, support numerous fish, corals and other invertebrates.
• Chief threats: habitat destruction, invasive species, overfishing and climate change, which raises sea level and bleaches the reefs.

08. Antarctic realm

• Antarctica and the sub-Antarctic islands, such as South Georgia and the South Sandwich Islands. Extreme cold and ice allow only specialised life.
  • On land: mosses, lichens and just two native flowering plants, Antarctic hair grass and Antarctic pearlwort.
• Key fauna: Emperor and Adélie penguins, seals and seabirds. Krill in the Southern Ocean form the base of the food web for whales, seals and birds.
• The sub-Antarctic islands are key breeding grounds for seabirds and seals. The realm is also a climate-change sentinel for science. Chief threats: overfishing, tourism and warming.

Comparative snapshot of the eight realms

Biogeographic Zones of India

India among the realms

• India lies almost wholly in the Indomalayan realm, but Ladakh and the high Himalaya belong to the Palearctic, and the north-west borders the Saharo-Arabian belt.
• Its fauna is therefore a meeting of elements:
  • Indo-Malayan: hornbills, the hoolock gibbon and the clouded leopard in the North-East, and the lion-tailed macaque in the Western Ghats;
  • Palearctic: snow leopard, Tibetan wild ass (kiang) and yak in the Trans-Himalaya;
  • Afro-Arabian: Asiatic lion, chinkara, caracal and striped hyena in the dry north-west and peninsula;
  • Gondwanan relicts, such as the purple frog of the Western Ghats, and Indian endemics, such as the blackbuck, nilgai and four-horned antelope.
• This mix and India’s relief give it 4 of the world’s 36 biodiversity hotspots.

The ten biogeographic zones

• W. A. Rodgers and H. S. Panwar (1988), of the Wildlife Institute of India, divided India into 10 biogeographic zones, in Planning a Wildlife Protected Area Network in India.
  • Their 2002 revision, with V. B. Mathur, recognises 26 biotic provinces.
• The zones combine physiography, climate, vegetation and fauna. They are India’s equivalent of realms and ecoregions, and the basis for planning a representative protected-area network.

• The Deccan Peninsula covers about 42% of the country; the Islands cover only 0.3% but are among the richest in endemics.
• The two Himalayan zones differ in kind. The Trans-Himalaya, in the rain shadow, is a cold desert with Palearctic fauna. The Himalaya proper is a forested mountain zone, with natural vegetation stacked by altitude.
• The Western Ghats and the North-East are the two most endemic-rich mainland zones. They fall in the Western Ghats–Sri Lanka and the Himalaya and Indo-Burma hotspots respectively.

Applications in Biogeography

• Understanding endemism: realms explain why certain species are confined to particular regions, as a direct result of long isolation.
• Tracing speciation: comparing related groups across realm boundaries shows how vicariance and dispersal have produced new species.
• Revealing past connections: shared plant and animal families across the former Gondwana fragments (southern beeches in South America, Australia and New Zealand; ratite birds) are direct evidence of past continental positions.
• Guiding conservation:
  • Udvardy’s provinces guided the global network of biosphere reserves.
  • WWF ecoregions underlie the Global 200 priority ecoregions (1998). They also measure how ecologically representative protected areas are under Target 3 (30 by 30) of the Kunming–Montreal Global Biodiversity Framework (2022).
  • In India, the Rodgers–Panwar zones guide the planning of protected areas and biosphere reserves so that each zone is represented.
• Tracking change: realm and ecoregion maps provide the baseline for measuring range shifts under climate change and the spread of invasive species across realm boundaries.

Illustrative example: the Australasian marsupials

• The marsupial fauna of the Australasian realm (kangaroos, koalas, wombats) is the signature of that realm’s long isolation after the break-up of Gondwana.
• It stands in sharp contrast to the placental-dominated faunas of every other realm. The example shows how realm-level biogeography records deep evolutionary history in present-day distributions.