The Indo-Pacific is a vast biogeographic region of Earth. In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two. The term is especially useful in marine biology, ichthyology, and similar fields, since many marine habitats are continuously connected from Madagascar to Japan and Oceania, and a number of species occur over that range, but are not found in the Atlantic Ocean. As a distinct marine realm, the region has an exceptionally high species richness, with the world's highest species richness being found in at its heart in the Coral Triangle, and a remarkable gradient of decreasing species richness radiating outward in all directions. The region includes over 3,000 species of fish, compared with around 1,200 in the next richest marine region, the Western Atlantic, and around 500 species of reef building corals, compared with about 50 species in the Western Atlantic. The term first appeared in academic use in oceanography and geopolitics. Scholarship has shown that the "Indo-Pacific" concept circulated in Weimar Germany, and spread to interwar Japan. German political oceanographers envisioned an "Indo-Pacific" comprising anticolonial India and republican China, as German allies, against "Euro-America". Since the late 2010s, the term "Indo-Pacific" has been increasingly used in geopolitical discourse. It is seen by some as a replacement for the "Asia-Pacific" regional concept which was pioneered by Australia and Japan in the 1970s and 1980s. Australia officially transitioned its strategic focus from the 'Asia-Pacific' to the 'Indo-Pacific' in 2013, becoming the first country to formally adopt the Indo-Pacific as its official strategic framework. As a geopolitical concept, the Indo-Pacific has a "symbiotic link" with the Quadrilateral Security Dialogue, or "Quad", an informal grouping between Australia, Japan, India, and the United States. It has been argued that the concept may lead to a change in popular "mental maps" of how the world is understood in strategic terms. According to the political scientist Amitav Acharya, the "Indo-Pacific" was a concept built by strategists. The Indo-Pacific started to gain ground in international relations literature as a geopolitical challenge by the U.S. toward China, which still prefers the regional term Asia-Pacific. The companion term "Euro-Atlantic" is often used by European leaders to denote a vaguely defined geopolitical space which is understood to include Europe and North America. The Euro-Atlantic term is also popular with Chinese leaders. In its widest sense, the Indo-Pacific geopolitically covers all nations and islands surrounding either the Indian Ocean or the Pacific Ocean. This definition encompasses mainland African and Asian nations who border these oceans, such as India and South Africa, Indian Ocean territories such as the Kerguelen Islands and Seychelles, Indonesia (which is within the bounds of both the Indian Ocean and the Pacific), Philippines, Taiwan, China, Korea, Japan, Russia and other Far East nations bordering the Pacific, as well as Australia and all the Pacific Islands east of them. The widest geopolitical definition also encompasses Pacific nations of the Americas such as Canada or Mexico, and the subarctic areas of the North Pacific, like Alaska's Bering Sea islands. ASEAN countries (defined as those in Southeast Asia) are considered to be geographically at the centre of the political Indo-Pacific.
Subdivisions
The WWF and Nature Conservancy divide the Indo-Pacific into 3 realms (or subrealms), and each of these into 25 marine provinces and 77 ecoregions (Marine Ecoregions of the World; MEOW) based on data-driven expert opinion. Other schemes for subdivision of the Indo-Pacific have included: 5 provinces, based on endemism in fishes; 3 regions split into 10 provinces based on dissimilarity of fish assemblages, 11 provinces based on range boundaries in corals, 12 divisions split into 124 ecoregions based on biogeographic clustering from coral distributions and finally 8 realms from distributions of 65,000 marine species. All but the last of these schemes were tested against one another by an international consortium of marine scientists using genetic data from 56 Indo-Pacific species, with the reasoning that genetic data should reflect the evolutionary processes that structure the Indo-Pacific. While there was no clear winning scheme, and all schemes were supported by data from at least one species, the genetic data in general favored schemes with few subdivisions, supporting the Indo-Pacific as relatively unstructured biogeographic realm - possibly the world's largest. Below are briefly described the 3 MEOW realms of the Indo-Pacific:
Central Indo-Pacific
The Central Indo-Pacific includes the numerous seas and straits connecting the Indian and Pacific oceans, including the seas surrounding the Indonesian archipelago (with the exception of Sumatra's northwest coast, which is part of the Western Indo-Pacific), the South China Sea, the Philippine Sea, the north coast of Australia, and the seas surrounding New Guinea, western and central Micronesia, New Caledonia, the Solomon Islands, Vanuatu, Fiji, and Tonga. The Central Indo-Pacific, due in part to its central location at the meeting of two oceans, has the greatest richness and diversity of marine organisms, specifically located within the Coral Triangle, which contains 76% of all known coral species in the world.
Eastern Indo-Pacific
The Eastern Indo-Pacific surrounds the mostly volcanic islands of the central Pacific Ocean, extending from the Marshall Islands in the west through central and southeastern Polynesia to Hawaii, to the west coast of Chile. The World Wide Fund for Nature believe the region ends at Chile's Easter Island and Isla Salas y Gómez, although it is sometimes extended even further to include Chile's Desventuradas Islands and Juan Fernández Islands.
Western Indo-Pacific
The Western Indo-Pacific covers the western and central portion of the Indian Ocean, including Africa's east coast and the Mozambique Channel, surrounding Madagascar, the Seychelles, the Comoros, the Mascarene Islands, Maldives, and the Chagos Archipelago; as well as the Red Sea, the Gulf of Aden, the Persian Gulf, the Arabian Sea, the Bay of Bengal, and the Andaman Sea.
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