Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia or SEA, is the geographical southeastern subregion of Asia, consisting of the regions that are south of China, south-east of the Indian subcontinent and north-west of Australia. Southeast Asia is bordered to the north by East Asia, to the west by South Asia and the Bay of Bengal, to the east by Oceania and the Pacific Ocean, and to the south by Australia and the Indian Ocean. Apart from the British Indian Ocean Territory and two out of 26 atolls of Maldives in South Asia, Southeast Asia is the only other subregion of Asia that lies partly within the Southern Hemisphere. The majority of the subregion is still in the Northern Hemisphere. East Timor and the southern portion of Indonesia are the only parts that are south of the Equator.
The vast majority of Southeast Asia falls within the warm, humid tropics, and its climate generally can be characterised as monsoonal.The animals of Southeast Asia are diverse; on the islands of Borneo and Sumatra, the orangutan, the Asian elephant, the Malayan tapir, the Sumatran rhinoceros and the Bornean clouded leopard can also be found. Six subspecies of the binturong or bearcat exist in the region, though the one endemic to the island of Palawan is now classed as vulnerable.
Tigers of three different subspecies are found on the island of Sumatra, in peninsular Malaysia, and in Indochina all of which are endangered species.
The Komodo dragon is the largest living species of lizard and inhabits the islands of Komodo, Rinca, Flores, and Gili Motang in Indonesia.
The Philippine eagle is the national bird of the Philippines. It is considered by scientists as the largest eagle in the world, and is endemic to the Philippines' forests.
The wild Asian water buffalo, and on various islands related dwarf species of Bubalus such as anoa were once widespread in Southeast Asia; nowadays the domestic Asian water buffalo is common across the region, but its remaining relatives are rare and endangered.
The mouse deer, a small tusked deer as large as a toy dog or cat, mostly can be found on Sumatra, Borneo, and in Palawan Islands, The gaur, a gigantic wild ox larger than even wild water buffalo, is found mainly in Indochina. There is very little scientific information available regarding Southeast Asian amphibians.
Birds such as the green peafowl and drongo live in this subregion as far east as Indonesia. The babirusa, a four-tusked pig, can be found in Indonesia as well. The hornbill was prized for its beak and used in trade with China. The horn of the rhinoceros, not part of its skull, was prized in China as well.
The Indonesian Archipelago is split by the Wallace Line. This line runs along what is now known to be a tectonic plate boundary, and separates Asian species from Australasian species. The islands between Java/Borneo and Papua form a mixed zone, where both types occur, known as Wallacea. As the pace of development accelerates and populations continue to expand in Southeast Asia, concern has increased regarding the impact of human activity on the region's environment. A significant portion of Southeast Asia, however, has not changed greatly and remains an unaltered home to wildlife. The nations of the region, with only a few exceptions, have become aware of the need to maintain forest cover not only to prevent soil erosion but to preserve the diversity of flora and fauna. Indonesia, for example, has created an extensive system of national parks and preserves for this purpose. Even so, such species as the Javan rhinoceros face extinction, with only a handful of the animals remaining in western Java.
The shallow waters of the Southeast Asian coral reefs have the highest levels of biodiversity for the world's marine ecosystems, where coral, fish, and molluscs abound. According to Conservation International, marine surveys suggest that the marine life diversity in the Raja Ampat is the highest recorded on Earth. Diversity is considerably greater than any other area sampled in the Coral Triangle composed of Indonesia, the Philippines, and Papua New Guinea. The Coral Triangle is the heart of the world's coral reef biodiversity, the Verde Passage is dubbed by Conservation International as the world's 'center of the center of marine shorefish biodiversity'. The whale shark, the world's largest species of fish and 6 species of sea turtles can also be found in the South China Sea and the Pacific Ocean territories of the Philippines.
The trees and other plants of the region are tropical; in some countries where the mountains are tall enough, temperate-climate vegetation can be found. These rainforest areas are currently being logged-over, especially in Borneo.
While Southeast Asia is rich in flora and fauna, Southeast Asia is facing severe deforestation which causes habitat loss for various endangered species such as orangutan and the Sumatran tiger. Predictions have been made that more than 40% of the animal and plant species in Southeast Asia could be wiped out in the 21st century. At the same time, haze has been a regular occurrence. The two worst regional hazes were in 1997 and 2006 in which multiple countries were covered with thick haze, mostly caused by 'slash and burn' activities in Sumatra and Borneo. In reaction, several countries in Southeast Asia signed the ASEAN Agreement on Transboundary Haze Pollution to combat haze pollution.
The 2013 Southeast Asian Haze saw API levels reach a hazardous level in some countries. Muar experienced the highest API level of 746 on 23 June 2013 at around 7 am.