I. Distribution of the World’s Tropical Rainforests

Tropical rainforests are found across the world between North and South of the Equator, covering regions in America, Asia, Africa and Oceania.

Rainforests usually occur in regions with a hot climate, abundant rainfall all year round, and very little seasonal change. Most of the world’s tropical rainforests are distributed in areas where the average annual temperature is above 26°C, the average monthly temperature is over 18°C, and the total annual rainfall is more than 2,000 mm. The two most important factors in the distribution of rainforest are temperature (vertically) and precipitation (horizontally).

Tropical rainforests are geographically located between the latitudes 5-15°N and 10-25°S of the equator as described below.

The figure shows that tropical rainforests of the world are situated between the Tropic of Capricorn and the Tropic of Cancer, and divided by the equator into some unequal parts. However, the northern and southern borders of tropical rainforests do not coincide exactly with any latitudinal limits. One of the major reasons is that the changes in the distance between the Earth and the sun, a result of the Earth’s rotation and its revolution around the sun, create differences in climate around the world at different times of the year. Another important reason is that the regional climate is influenced by the surface structure of the Earth, such as mountains, plains and oceans. As a result, the division of the tropics is not based on definite boundaries of latitude, but on the altitude of the 20°C isotherm.

P.W. Richards, a botanist from the United Kingdom, is the author of a profound and far-reaching book, The Tropical Rain Forest: An Ecological Study, in which he was the first to make a comprehensive and systematic analysis of rainforests. The division in this book leaves tropical rainforests located in three major regions of the world: Neotropics (America), Afrotropics (Africa) and Indo-Malay. The 2010 data (Source: mongabay.com, 2014) indicate that the area of world’s tropical rainforests reached 2.4 billion hectares, accounting for 44% of the total global forest area. Of the total, the tropical rainforests in Americas take up the largest share (53%). The rainforest along the Amazon River basin is bordered by the Andes Mountains to the west, Guyana to the east, Bolivia and Paraguay to the south, and southern Mexico and the Antilles to the north. Brazil has 481 million hectares of tropical rainforest, the most of any country in the world. African tropical rainforests account for 27% of the world, which are mainly found in the Democratic Republic of Congo (177 million hectares), Gabon, Congo, Cameroon, Ghana, etc. The Indo-Malay rainforest, also known as the Asian rainforest, accounts for only 20% of the world’s rainforests, the smallest when compared to other rainforest systems. It encompasses areas from Indonesia to Malaysia, Papua New Guinea, Thailand and India. In Indonesia alone, the area of tropical rainforests amounts to 127 million hectares, while the rest of the Asian countries have tropical rainforests of less than 100 million hectares, only tens of millions or even millions of hectares. The area of tropical rainforests in China is estimated at 2-3 million hectares, accounting for less than 2‰ of the world’s total.

Ⅱ. Difference between World’s Three Rainforest Biomes

The tree tropical rainforest biomes are very similar in appearance and structure, but their species diversity varies from one region to another.

(1) Leguminosae is the dominant plant family in the American rainforest ecosystem. The arborous layer is dominated by Mora and Eperua, and there is also diverse species in the family of Bromeliaceae, Cactacea, Araceae and Palmae. Vine plants and epiphytes are especially abundant. Cash crops such as Hevea brasiliensis and cacao trees are native plants here. The unique species Victoria regia has leaves up to 1.5m in diameter. The American rainforest also provides a habitat for animals including capuchins, spider monkeys, sloths, tamandua, vampire bats, hummingbirds and anaconda.

(2) The number of plant species found in the African rainforest is small, but it is home to many endemic species, especially Palmae plants such as palms and oil palms. There are 20 species of Coffea in Africa alone (35 species in the world). The dominant plant family in West Africa is Meliaceae (Khaya senegalensis, etc.), which is followed by Leguminosae. Major animals living in the African rainforest range from gorillas to chimpanzees, long-tailed pangolin, hippos and sunbirds.

(3) The Asian rainforest or the Indo-Malay rainforest biome is dominated by Borneaceae, and the arborous layer is represented by Borneaceae plants. Plants with large and beautiful flowers, particularly tall palms, are not found in this area, but it is home to a tall woody plant called Alsophila spinulosa as well as Calamus and epiphytic Orchidaceae plants. Animals inhabiting the Asian rainforest include orangutans, gibbons, cobras, macaques, pythons, tree frogs, green peacocks, monitor lizards, slender loris, giant squirrels and so forth.

Ⅲ. Threats to the Rainforest

Forest covered about 50% of the Earth’s land area 8000 years ago. Just a few thousand years ago, tropical rainforests covered as much as 12% of the Earth’s land surface. The expansion and shrinkage of the tropical rainforest is a testimony to the changes of natural environment and, to a certain extent, the men’s understanding and utilization of the rainforest.

Humans pose the greatest threat to the tropical rainforests. Mankind’s greed for interests, industrial logging, and the fast expansion of cash crops, such as rubber, to the original tropical rainforest only for immediate economic benefits has led to great rainforest loss. The rate of tropical rainforest destruction is escalating worldwide to 12 million hectares a year, of which tropical rainforests in Asia are disappearing more rapidly than any others on Earth. Half the world’s rainforests have been razed and cover only less than 7% of the Earth’s land surface. Today, fragmented rainforests have replaced continuous tracks of rainforest in two-thirds of the rainforest ecosystems. The destruction of a large area of tropical rainforests has resulted in the loss and degradation of wildlife habitats. In tropical Africa, 65% of the wildlife habitat has been lost. Likewise, an alarming 67% decline is reported in tropical Asia, among which 31% of some primates in Southeast Asia have lost their homes, while only 1.2%-22.9% are under protection. The hot, damp conditions in tropical areas allow for the rapid decomposition of organic matter and increasing circulation of materials. The shrinking of the tropical rainforest could have significant impacts on the ecosystem, ranging from soil erosion to habitat degradation, droughts, and the mass extinction of species, especially species with extremely small populations.

The large-scale tropical deforestation, the spread of genomic isolation, and the fragmentation of tropical rainforests have become the most urgent ecological concerns in the international community, as well as the hot-spot issues in the Convention on Biological Diversity.