The Indian Ocean is the third-largest of the world's five oceanic divisions, covering 70,560,000 km2 or 19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by the Southern Ocean or Antarctica, depending on the definition in use. Along its core, the Indian Ocean has some large marginal or regional seas such as the Arabian Sea, the Laccadive Sea, the Somali Sea, Bay of Bengal, and the Andaman Sea.
Of Earth's 36 biodiversity hotspot nine are located on the margins of the Indian Ocean.
The origin of this diversity is debated; the break-up of Gondwana can explain vicariance older than 100 mya, but the diversity on the younger, smaller islands must have required a Cenozoic dispersal from the rims of the Indian Ocean to the islands. A 'reverse colonisation', from islands to continents, apparently occurred more recently; the chameleons, for example, first diversified on Madagascar and then colonised Africa. Several species on the islands of the Indian Ocean are textbook cases of evolutionary processes; the dung beetles, day geckos, and lemurs are all examples of adaptive radiation.Many bones of recently extinct vertebrates have been found in the Mare aux Songes swamp in Mauritius, including bones of the Dodo bird and Cylindraspis giant tortoise. An analysis of these remains suggests a process of aridification began in the southwest Indian Ocean began around 4,000 years ago.
Mammalian megafauna once widespread in the MPA was driven to near extinction in the early 20th century. Some species have been successfully recovered since then — the population of white rhinoceros increased from less than 20 individuals in 1895 to more than 17,000 as of 2013. Other species are still dependent of fenced areas and management programs, including black rhinoceros, African wild dog, cheetah, elephant, and lion,
This biodiversity hotspot is a patchwork of small forested areas, often with a unique assemblage of species within each, located within 200 km from the coast and covering a total area of c. 6,200 km2, It also encompasses coastal islands, including Zanzibar and Pemba, and Mafia.
This area, one of the only two hotspots that are entirely arid, includes the Ethiopian Highlands, the East African Rift valley, the Socotra islands, as well as some small islands in the Red Sea and areas on the southern Arabic Peninsula. Endemic and threatened mammals include the dibatag and Speke's gazelle the Somali wild ass and hamadryas baboon, It also contains many reptiles.In Somalia, the centre of the 1,500,000 km2 hotspot, the landscape is dominated by Acacia-Commiphora deciduous bushland, but also includes the Yeheb nut and species discovered more recently such as the Somali cyclamen, the only cyclamen outside the Mediterranean. Warsangli linnet is an endemic bird found only in northern Somalia. An unstable political regime has resulted in overgrazing which has produced one of the most degraded hotspots where only c. 5 % of the original habitat remains.
Encompassing the west coast of India and Sri Lanka, until c. 10,000 years ago a landbridge connected Sri Lanka to the Indian Subcontinent, hence this region shares a common community of species.
Indo-Burma encompasses a series of mountain ranges, five of Asia's largest river systems, and a wide range of habitats. The region has a long and complex geological history, and long periods rising sea levels and glaciations have isolated ecosystems and thus promoted a high degree of endemism and speciation. The region includes two centres of endemism: the Annamite Mountains and the northern highlands on the China-Vietnam border.Several distinct floristic regions, the Indian, Malesian, Sino-Himalayan, and Indochinese regions, meet in a unique way in Indo-Burma and the hotspot contains an estimated 15,000–25,000 species of vascular plants, many of them endemic.
Sundaland encompasses 17,000 islands of which Borneo and Sumatra are the largest. Endangered mammals include the Bornean and Sumatran orangutans, the proboscis monkey, and the Javan and Sumatran rhinoceroses.
Stretching from Shark Bay to Israelite Bay and isolated by the arid Nullarbor Plain, the southwestern corner of Australia is a floristic region with a stable climate in which one of the world's largest floral biodiversity and an 80% endemism has evolved. From June to September it is an explosion of colours and the Wildflower Festival in Perth in September attracts more than half a million visitors.