There is a great amount of biological diversity on Earth, with millions of different species living in all kinds of ecosystems. This biodiversity is not distributed equally across the globe, with a larger amount of species living in the warmer areas near the tropics. The study of how species and ecosystems are distributed in the world is called biogeography.
Biogeography developed since the 18th century as explorers, mostly from Europe, traveled around the world and described the different species they saw. One of the main contributors to the development of this science was Carl Linnaeus, a Swedish biologist who created a system to name living organisms that is still used to this day. He noticed that species were not equally distributed across different climates, setting the basis for the study of the differences between ecosystems. Shortly after, French naturalist Georges-Louis Leclerc, Comte de Buffon, observed that different species across various regions had some similarities, and hypotesized that the continents were initially connected and were later separated by the seas.
Numerous scientists contributed to the study of biogeography in the 19th century, such as Charles Darwin, who introduced the ideas of natural selection and evolution. These new concepts influenced the development of theories on the geographical distribution of species, which may change over time.
The most influential contributor to this science in the 19th century was English naturalist Alfred Russel Wallace, so much so that he is now nicknamed “the father of biogeography”. Wallace studied the distribution of species in the Amazon basin and the Malay Archipelago, where he identified the so-called Wallace Line, a boundary that separates the islands of Indonesia into a western portion with a fauna related to that of Asia, and an eastern portion hosting species linked with those of Australasia. Wallace also noticed the stark difference between species of North and South America.
Map of the Wallace Line and other proposed boundaries between the fauna of South East Asia (Sunda) and Australasia (Sahul), with their ancient coastlines. The transitional zone between the two regions is also called Wallacea (Kanguole, Wikimedia Commons, CC BY-SA 4.0).
In the 20th century, the theory of continental drift and the concept of Pangaea, a supercontinent that united every landmass in the world, revolutionized biogeography. This theory led to a new understanding of how species evolve after being separated by geographical obstacles. Now we know that several factors influence the distribution of species, such as climate, latitude, elevation, isolation, and geographical features.
Modern biogeography divides the world into biogeographical realms, also called ecozones. Ecozones are based on the distribution of terrestrial species, and are general groupings of organisms that have evolved in relative isolation from other regions. This division was first identified by English ornithologist Philip Sclater, and later expanded by Wallace, who separated the world into six biogeographic regions. The current system adopted by the World Wildlife Fund (WWF) defines eight ecozones similar to those described by Hungarian-Canadian biologist Miklos Udvardy. The WWF also divides ecozones into bioregions, which are further subdivided into ecoregions.
The map of the zoogeographical regions published by Alfred Russel Wallace in 1876.
According to the WWF system, the eight ecozones are: Nearctic, Neotropic, Palearctic, Afrotropic, Indomalaya, Australasia, Oceania, and Antarctic. The Nearctic realm includes all of North America and Greenland, extending south to the highlands of Mexico, and excluding southern Florida. Most of coastal Mexico, southern Mexico, southern Florida, Central America, and the Caribbean islands are grouped with South America in the Neotropical realm. The Neotropical realm is characterized by the huge tropical rainforest which extends from southern Mexico to Brazil and includes the Amazon rainforest, one of the regions with the highest amount of biodiversity in the world.
North America was previously joined with Eurasia in the supercontinent of Laurasia, while South America was part of Gondwana. Only a few millions of years ago the Isthmus of Panama connected North and South America, allowing for an exchange of species known as the Great American Interchange, which resulted in the migration of many species both southward and northward. Before this event, South America was isolated for millions of years, and this allowed its fauna to evolve into numerous unique species, which are now mostly extinct. Meanwhile, North America has been connected to Asia through a land bridge over what is now the Bering Strait, and shares many species with the Palearctic realm. For this reason the Nearctic and Palearctic realm are sometimes grouped together as the Holarctic realm.
The Palearctic realm is the largest ecozone, and includes all of Europe, North Africa, and most of Asia. The southern border of this realm runs across the Sahara desert, the Arabian desert, the mountain ranges of South Asia, such as the Himalayas, and southern China. The highlands of eastern Taiwan and the Japanese archipelago (excluding the Ryukyu Islands) are also part of the Palearctic ecozone. This realm includes widely different ecosystems, with the most notable geographic features being the Mediterranean Sea, the Saharan and Arabian deserts, the mountains of Western and Central Asia, the vast Eurasian Steppe, and Siberia.
All of Sub-Saharan Africa is part of the Afrotropical realm, together with the southern Arabian Peninsula, the southeastern coast of Iran, Madagascar, and the islands of the western Indian Ocean. The Afrotropical realm was formerly known as the Ethiopian Region. This ecozone includes the vast rainforests of the Congo basin and West Africa, which are characterized by a high level of biodiversity. Savannas are widespread in Africa and cover a large portion of this realm, which also features the deserts of Southern Africa. A notable area of this ecozone is Madagascar, another region with a large biodiversity and many endemic species.
The Indian subcontinent, southern China, and South East Asia are part of the Indomalayan realm, which includes the Philippines, western Taiwan, and the Ryukyu Islands of Japan. This ecozone is sometimes called Oriental realm. The Wallace Line separates the Indomalayan realm from the Australasian realm. The islands of Sumatra, Borneo, Java, and Bali are part of the Indomalayan realm, while Sulawesi, the Moluccas, Lombok, and the other islands in the eastern portion of the archipelago are in the Australasian realm. This realm also includes New Guinea, Australia, New Zealand, the Solomon Islands, Vanuatu, and New Caledonia.
The Indomalayan realm is formed by parts of both Laurasia and Gondwana, and features a blend of elements from both former supercontinents. This realm notably hosts the dense forests of South East Asia, which offer a great amount of biodiversity. The Australasian realm was instead mostly part of Gondwana, and has been isolated for millions of years, leading to the evolution of many unique species of animals and plants. The exchange of species across the Wallace Line is relatively recent and asymmetric, with more Indomalayan species spreading to Australasia than the opposite.
The Oceanian realm is the only one that does not include a continental landmass, and consists of the smaller islands of the Pacific Ocean, such as Micronesia, Fiji, and Polynesia, excluding New Zealand but including Hawaii and Easter Island. The unique geography of this region has led to many instances of island gigantism, dwarfism, and other peculiar adaptations, and few indigenous land animals compared to other realms.
Finally, the Antarctic realm includes Antarctica and several sub-Antarctic islands such as South Georgia and the South Sandwich Islands, Bouvet Island, the Crozet Islands, the Prince Edward Islands, Heard Island and McDonald Islands, and the Kerguelen Islands. As part of Gondwana, Antarctica was previously much warmer, but most of its species went extinct as the continent became colder. Now very few species of plants and animals live here, mostly along the coast and on the islands that surround Antarctica.
Map of the ecozones according to the WWF (edited from a work by carol, Wikimedia Commons, CC BY-SA 3.0).
Similarly to the terrestrial ecozones, the WWF has also defined twelve marine biogeographic realms: Arctic (Arctic Ocean and adjacent seas), Temperate Northern Atlantic (which includes the Mediterranean Sea, Black Sea, and northern Gulf of Mexico), Tropical Atlantic (central Atlantic Ocean and southern Gulf of Mexico), Temperate South America (part of both Atlantic and Pacific coasts of South America), Temperate Southern Africa (waters around Southern Africa), Temperate Australasia (waters around southern Australia and New Zealand), Temperate Northern Pacific (northern Pacific Ocean), Western Indo-Pacific (most of the Indian Ocean), Central Indo-Pacific (seas surrounding Indonesia, the South China Sea, and the Pacific Ocean until the islands of Melanesia), Eastern Indo-Pacific (Pacific Ocean between Micronesia and Polynesia), Tropical Eastern Pacific (Pacific Ocean near the coast of America), and Southern Ocean (waters around Antarctica).