World Rainforests

WHERE RAINFORESTS ARE LOCATED: Biogeographical Tropical Forest Realms

The majority of tropical rainforests are found in four biogeographic realms: the Afrotropical (mainland Africa, Madagascar, and scattered islands), the Australian (Australia, New Guinea, and the Pacific Islands), the Indomalayan (India, Sri Lanka, mainland Asia, and Southeast Asia), and the Neotropical (South America, Central America, and the Caribbean islands).

AFROTROPICAL REALM

Most of the tropical rainforests of Africa exist in the Congo (formerly Zaire) River Basin, although an unbroken forest once stretched from Senegal on the Atlantic coast to the Rift Valley. West Africa has suffered heavy deforestation from logging and agriculture and only a small portion of the original cover remains. Since the 1990s, timber from Central Africa, especially Gabon, Cameroon, and the Congos, is increasingly used to fill the void created by the departure from the market of West African timber exporters. Consequently, deforestation is accelerating in Central Africa. Large areas of forest are also being concessions for industrial-scale agriculture, like oil palm plantations.

Around the turn of the century, West Africa had some 193,000 sq. miles (500,000 sq. km) of coastal rainforest. However, the tropical forests of West Africa, mostly lowland formations easily accessible from the coast, have been largely depleted by commercial exploitation, namely logging, and conversion for agriculture. Now, according to the Food and Agriculture Organization of the United Nations, less than 20 percent of West Africa's moist forests remain, of which much is degraded. In more populous states, notably Nigeria, human population pressures have put a tremendous strain on forests, while other countries like Côte d'Ivoire have suffered extensive forest loss as a result of commercial logging and agriculture. The effects from forest loss are yet to be fully understood, though erosion has greatly increased as has the incidence of drought in the interior countries of Mali and Niger. These coastal forests appear to play a substantial role in maintaining rainfall in these interior countries.

The rainforests of Central Africa still cover a substantial area. 75 percent of Africa's remaining rainforest is located in Central Africa, covering about 540,000 square miles (1.4 million square km). The bulk of this region's forests are found in the Congo Basin in the Democratic Republic of Congo and Congo. These forests are mostly threatened by logging, industrial agriculture, and subsistence activities, especially fuelwood collection and smallholder agriculture.

African rainforest is notably drier and more seasonal than sister rainforests of Asia and Latin America. Pollen studies suggest that during the past ice ages much of the African rainforest was savanna. There is concern that future climate change could again bring drought to parts of tropical Africa

AUSTRALIAN AND PACIFIC REALMS

The bulk of the forest in this region lies on the world's second largest island, New Guinea. Australia has small sections of forest on the Cape York peninsula in the extreme northeastern part of the continent near the coast. New evidence suggests that Australian rainforest covers more of Australia today than it did in the past 18,000 years. Under the cooler and drier conditions (rainfall decreased by as much as 80 percent) of the past glacial period, Australia's rainforest retreated and was replaced by dry, fire-loving eucalyptus. When the ice ages ended, small pockets of rainforest (10-20 percent of the coverage that exists today) that survived served as refuges to recolonize the surrounding land.

The plant and animal species of New Guinea and Australia, including the original Australoid dark-skinned, frizzy-haired human inhabitants, are similar because during the ice ages, when the sea levels dropped, these two land bodies were linked. As a result, both land masses have an unusually high diversity of marsupials which have filled the niches left vacant by the absence of cats, monkeys, civets, and other mammal groups. Also part of the ancient land mass were the Aru islands, a group of small closely-packed islands of the western coast of New Guinea. The strip of water between these Aru islands and the Kei islands to the west, is the dividing line between the Australian realm and the set of islands connected to neither realm during the recent ice ages. These islands, including Lombock, Flores, Timor, Sulawesi (Celebes), Ceram, Halmahera, are today part of Indonesia and house their own unique species, many of which are characteristic of neither the Indomalayan nor Australian realm. On Sulawesi (Celebes), when bats are excluded, mammal endemism is 100 percent, meaning none of the island's mammal species are found elsewhere.

Although technically not part of any realm, oceanic Pacific Islands will be mentioned here. These islands, many of which are volcanic, have never been part of a mainland mass. These islands also have forest cover, although these forests only make up a tiny portion of the world's total.

INDOMALAYAN/ ASIAN REALM

The majority of the remaining tropical rainforest in Asia lies in Indonesia (on scattered islands), the Malay peninsula (Malaysia, Thailand, Myanmar), and Laos and Cambodia. Forest once covered a much greater area in Asia, but centuries of tremendous population pressure has significantly reduced the natural extent, and today only scattered fragments remain.

Southeast Asia's rainforests are some of the oldest on Earth. Studies suggest that some forests in present day Malaysia may have existed over 100 million years ago. However, these ancient forests did not much resemble the ones of today. These early rainforests had far fewer flowering plants, so species today associated with flowering plants, including many birds, insects, and mammals, had yet to come into existence. Borneo, Sumatra, Java, and other Southeast Asian islands lacked many of the familiar large mammals they have today. When the ice ages caused a drop in sea level these animals migrated from greater Asia to Southeast Asia.

Ice ages lock up ocean waters in polar ice and cause ocean waters to condense, causing the existing sea levels to fall. These events meant profound changes for Southeast Asia since much of the shallow South China Sea became dry land. As the ocean levels dropped, Sumatra, Java, Borneo, the Malay Peninsula, and Indochina were connected, allowing mainland and island species to cross over. As global temperatures cooled and glaciation expanded, the tropical rainforest retreated to small pockets and in many areas was replaced by deciduous forest, savanna, or montane forest. The more extensive montane habitats and savanna habitats enabled mountain and savanna plants and animals like guar (a relative of the domestic cow) and the tiger to disperse into the tropics. As the ice age ended, glaciers retreated, and the climate warmed, the tropical rainforest surviving in Sumatra, Borneo, and Malay Peninsula served as a reservoir from which species could recolonize surrounding areas as they returned to forest. This could explain why today the pockets of remaining montane forest like that of Mount Kinabalu in Sabah (Malaysian Borneo) have flora that more closely resembles the plants of the Himalayas and New Zealand.

The "Wallace line," named in honor of the nineteenth century biogeographer Alfred Wallace, separates the Indomalayan realm from the Australian realm. Wallace first documented the odd discontinuity of fauna between Bali and Lombock and is credited, along with the renowned British naturalist Charles Darwin, in formulating the theory of evolution. The Indomalayan realm extends east to Borneo and south to Bali. Sulawesi (Celebes) and Lombock, despite their proximity to Borneo and Java (respectively), are not included because they are separated by a deep channel and were not linked to the Indomalayan land mass formed when the ocean receded during the ice ages. Land-based species, and many flying species deterred by the winds, were not able to cross over and the flora and fauna are quite different and distinct in these adjacent areas.

NEOTROPICAL REALM

The Amazon River Basin is roughly the size of the forty-eight contiguous United States and covers some 40 percent of the South American continent. Reflecting environmental conditions as well as past human influence, the Amazon is made up of a mosaic of ecosystems and vegetation types including rainforests, seasonal forests, deciduous forests, flooded forests, and savannas, among others. The basin is drained by the Amazon River, the world's largest river in terms of discharge, and the second longest river in the world after the Nile. The river is made up of over 1,100 tributaries, 17 of which are longer than 1000 miles, and two of which (the Negro and the Madeira) are larger, in terms of volume, than the Congo (Zaire) river. The river system is the lifeline of the forest and its history plays an important part in the development of its rainforests.

At one time Amazon River flowed westward, perhaps as part of a proto-Congo (Zaire) river system from the interior of present day Africa when the continents were joined as one as part of Gondwana. Fifteen million years ago (an eye-blink in geologic time), the Andes were formed when they were forced up by the collision of the South American plate with the Nazca plate. The rise of the Andes and the linkage of the Brazilian and Guyana bedrock shields blocked the river and caused the Amazon to become a vast inland sea. Gradually this inland sea became a massive swampy, freshwater lake and the marine inhabitants adapted to life in freshwater. For example, over 20 species of stingray, most closely related to those found in the Pacific Ocean, can be found today in the freshwaters of the Amazon.

About ten million years ago, waters worked through the sandstone to the west and the Amazon began to flow eastward. At this time the Amazon rainforest was born. During the ice age, sea levels dropped and the great Amazon lake rapidly drained and became a river. Three million years later, the ocean level receded enough to expose the Central American isthmus and allow mass migration of mammal species between the Americas.

The ice ages caused tropical rainforests around the world to retreat. Although debated, it is believed that much of the Amazon reverted to savanna and montane forest (see Ice Ages and Glaciation). Savanna divided patches of rainforest into "islands" and separated existing species for periods long enough to allow genetic differentiation (a similar rainforest retreat took place in Africa. Delta core samples suggest that even the mighty Congo watershed was void of rainforest at this time). When the ice ages ended, the forest was again joined and the species that were once one had diverged significantly enough to be constitute designation as separate species, adding to the tremendous diversity of the region. About 6,000 years ago, sea levels rose about 130 meters, once again causing the river to be inundated like a long, giant freshwater lake.

The massive size of the Amazon and its tributaries make it easy to overlook the other great rivers and forest ecosystems of the Neotropical realm. For example, the Orinoco River flows over 1,600 miles through Venezuela. Interestingly, the Orinoco River system is linked to the Amazon River basin through a unique natural river system called the Casiquiare canal. The Casiquiare canal is the only river on the planet which links two major river systems. To the south of the Amazon is an expanse of forest that lies in the Tocantins river system. A small area of forest, greatly reduced by human activity to less than 5 percent of its original cover, is found along the Atlantic seaboard in Brazil. The highly threatened Chocò rainforest is found along the northwestern coast of the continent in Colombia, while the Pacific coast rainforest runs from Ecuador into Central America.

Much of Central America and many of the Caribbean islands were once forested with tropical rainforest, although these have been greatly reduced. Few Caribbean islands still retain any primary forest cover, while rainforest continues to persist in some parks and reserves in Central America. Central America suffered the highest percentage loss of forest of any tropical region between 1990-2005, losing almost 30 percent of its forests.

Today South America suffers the highest total loss of forest—around 4.3 million hectares were cleared per year between 2000 and 2005. Most of the forest loss has occurred in the Amazon rainforest where large tracts of land are being cleared for cattle ranches, and to a lesser degree, other forms of agriculture like industrial soy farms. Scientists are concerned that forest loss could escalate in the Amazon due to increasingly dry conditions. In 2010, the Amazon suffered the most severe drought on record, leaving rivers dry and communities stranded. Tens of thousands of fires burned.

Review questions:

• Rainforests are generally broken into how many biogeographical realms?
• The largest expanse of rainforest is located on what continent?
• Most of the rainforest in Africa is found in what basin?
• How is African rainforest generally different from rainforests of Asia and South America?
• What is the world's second largest island?
• Does Australia naturally have monkeys?
• What is the Wallace Line?
• How did the Ice Ages affect islands and forests in southeast Asia?
• True or False—The Amazon River Basin is roughly the size of the forty-eight contiguous United States.
• Is the Amazon River the largest river, in terms of volume, in the world?
• Is the Amazon River the longest river in the world?
• What continent loses the most area of forest each year?

Other versions of this page

Continued / Next: Types of Rainforest

[an error occurred while processing this directive]