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Tropical rainforests mainly occur in the World's equatorial regions. Tropical forests are restricted to the small land area between the latitudes 22.5° North and 22.5° South of the equator - between the Tropic of Capricorn and the Tropic of Cancer (see map). They are also located on major separate continental forests which preserve them as independent, non-contiguous realms.
Rhett Butler, on his excellent site Mongabay, refers to these four regions as the Afrotropical, the Australian, the Indomalayan and the Neotropical rainforest realms.
The Afrotropical Rainforest Realm
Most of the tropical rainforests of Africa exist in the Congo (Zaire) River Basin. Remnants also exist throughout Western Africa which is in a sorry state due to the plight of poverty which encourages subsistence agriculture and firewood harvesting. This realm is increasingly dry and seasonal when compared to the other realms. The outlying portions of this rainforest region are steadily becoming desert. FAO suggests this realm "lost the highest percentage of rainforests during the 1980s, 1990s, and early 2000s of any biogeographical realm".
The Australian Oceanic Pacific Rainforest Realm
Very little of the rainforest is located on the Australian continent. Most of this rainforest is located in Pacific New Guinea with a very small portion of the forest in the Northeast of Australia. Actually, the Australian forest has expanded over the last 18,000 years and remains relatively untouched. The Wallace Line separates this realm from the Indomalayan realm. Biogeographer Alfred Wallace marked the channel between Bali and Lombok as the divide between two great zoogeographic regions, the Oriental and Australian.
The Indomalayan Rainforest Realm
Asia's remaining tropical rainforest is in Indonesia (on scattered islands), the Malay peninsula and Laos and Cambodia. Population pressures have dramatically decreased the original forest to scattered fragments. Southeast Asia's rainforests are some of the oldest in the World. Studies have indicated that several have existed for over 100 million years. The Wallace Line separates this realm from the Australian realm.
The Neotropical Rainforest Realm
The Amazon River Basin covers some 40% of the South American continent and dwarfs all other forests in Central and South America. The Amazon rainforest is roughly the size of the forty-eight contiguous United States. It is the largest continuous rainforest on Earth.
The good news is, four-fifths of the Amazon is still intact and healthy. Logging is heavy in certain areas but there is still debate over the adverse effects but governments are involved in new pro-rainforest legislation. Oil and gas, cattle and agriculture are major causes of neotropical deforestation.