The Earth can be divided into distinct regions based on the presence or absence of various organisms. These regions are known as realms. Over time, different scientists have proposed various classifications of these realms.
In 1857, P. L. Sclater categorized the Earth’s geographical areas into six regions based on bird distribution. Later, in 1876, Alfred Russel Wallace expanded upon Sclater’s concept by considering the distribution of all terrestrial vertebrates and invertebrates. While he retained the six-region model, he renamed Sclater’s “Indian region” as the “Oriental region.”
Wallace’s proposed realms are separated by natural barriers, distinguishing them from one another. His classification is depicted on a world map, with the boundaries between realms marked by dotted lines, which are referred to as Wallace’s Line.
Types of Zoogeographical Realms/Regions
1. Palaearctic Realm.
2. Nearctic Realm.
3. Neo-tropical Realm.
4. Ethiopian Realm.
5. Oriental Realm.
6. Australian Realm.
1/ Palaearctic Realm
A. Geographical Boundaries:The Palaearctic Realm encompasses Europe, the northern part of Africa, the Asian Himalayas, and the Nanling range of China.
B. Subdivisions:Wallace further divided this realm into four sub-regions:
1. European Sub-region: Includes Northern and Central Europe, as well as the Black Caucasus.
2. Mediterranean Sub-region: Covers parts of Africa, Asia, Europe, the Arabian Peninsula, Afghanistan, and Baluchistan.
3. Siberian Sub-region: Consists of Northern Asia, including the areas beyond the Himalayas.
4. Manchurian Sub-region: Encompasses Mongolia, Korea, Manchuria, and Japan.
C. Climatic Conditions:The region experiences extreme climatic variations, with severe cold in Siberia and intense heat in the Sahara Desert.
D. Ecological Conditions:: The Palaearctic Realm is characterized by diverse ecosystems, including deciduous forests, vast grasslands, coniferous forests, and mixed forests. Tundra regions are also present.
E. Characteristic Vertebrate Fauna:
• Fish: Common species include carp, salmon, pike, and sticklebacks.
• Amphibians: European salamander, Proteus, Hynobius, Bombinator, Alytes, and Didocus.
• Reptiles: Sand boa, Trigonophis lizard, and alligators.
• Birds: Arctic tern, pheasants, wrens, finches, warblers, and geese.
• Mammals: The realm is home to 39 mammalian families, with Seluinidae and Ailuropodie being endemic. Other notable species include porcupines, wild dogs, wild asses, European bison, polar cats, and deer.
2/ Nearctic Realm
A. Geographical Boundaries:The Nearctic Realm includes all of North America to the north, extends southward to Mexico, reaches eastward to Greenland, and stretches westward to the Aleutian Islands.
B. Subdivisions:This realm is divided into four sub-regions:
1. Californian Sub-region: Covers Vancouver Island, parts of British Columbia, Nevada, and some areas of the Cascade Hill region. This sub-region is known for its relatively low biodiversity.
2. Rocky Mountain Sub-region: Located east of California, this region features a high and rugged mountain range. It is one of the richest areas in terms of faunal diversity within the Nearctic Realm.
3. Allegheny Sub-region: Situated east of the Rocky Mountain sub-region, it is bordered by the Great Lakes in the north and has a moderate level of biodiversity.
4. Canadian Sub-region: Includes North America and Greenland, though it is not particularly known for its rich wildlife diversity.
C. Climatic Conditions:Similar to the Palaearctic Realm, this region experiences both extreme cold and intense heat, depending on the location.
D. Ecological Conditions:The Nearctic Realm consists of diverse ecological zones, including deciduous forests, vast grasslands, coniferous forests, dry lands, and tundra regions.
E. Characteristic Vertebrate Fauna:
• Fish: Common species include Lepisosteus, Polydon, Acipenser, and various types of perches.
• Amphibians: Notable species include Siren, Amphiuma, Cryptobranchus, Ambystoma, Ascaphys, and Axolotl larvae, most of which belong to the order Caudata.
• Reptiles: Prominent reptiles include snakes such as Conophis, Chilomeniscus, Pituophis, and Farancia; lizards like Phrynosoma and Uta; and turtles including Aromochelys and Chelydra.
• Birds: The region is home to numerous migratory birds, including turkeys, pelicans, crows, cuckoos, pigeons, sarus cranes, swans, kites, rails, owls, and hawks.
• Mammals: The Nearctic Realm hosts a variety of mammals, including Didelphis (opossum), armadillos, caribou, pronghorns, shrews, moles, bears, wolves, monkeys, deer, bats, goats, musk oxen, and bison. Endemic mammalian families include Aplodontidae and Pronghorn.