What are tropical rainforests?

By Rhett A. Butler
April 1, 2019

Part I:

TROPICAL RAINFORESTS OF THE WORLD

Tropical rainforests are a world like none other; and their importance to the global ecosystem and human existence is paramount. Unparalleled in terms of their biological diversity, tropical rainforests are a natural reservoir of genetic diversity which offers a rich source of medicinal plants, high-yield foods, and a myriad of other useful forest products. They are an important habitat for migratory animals and sustain as much as 50 percent of the species on Earth, as well as a number of diverse and unique Indigenous cultures. Tropical rainforests play an elemental role in regulating global weather in addition to maintaining regular rainfall, while buffering against floods, droughts, and erosion. They store vast quantities of carbon, while producing a significant amount of the world's oxygen.

Despite their monumental role, tropical forests are restricted to the small land area between the latitudes 23.5° North and 23.5° South of the equator, or in other words between the Tropic of Capricorn and the Tropic of Cancer. Since the majority of Earth's land is located north of the tropics, rainforests are naturally limited to a relatively small area.

Tropical rainforests, like so many other natural places, are a scarce resource in the 21st century. The vast swaths of forest, swamp, desert, and savanna that carpeted Earth's land surface a mere five generations ago have been reduced to scattered fragments; today, more than two-thirds of the world's tropical rainforests exist as fragmented remnants. Today around 13 percent of Earth's land is covered with tropical forests (about 2 billion hectares or 7.7 million square miles), of which much consists of tropical rainforest. The largest unbroken stretch of rainforest is found in the Amazon river basin of South America. Nearly two-thirds of this forest lies in Brazil, which holds about one-third of the world's remaining tropical rainforests. Another 20 percent of the world's remaining rainforest exists in Indonesia and Congo Basin, while the balance of the world's rainforests are scattered around the globe in tropical regions.

The global distribution of tropical rainforests can be broken up into four biogeographical realms based roughly on four forested continental regions: the Ethiopian or Afrotropical, the Australiasian or Australian, the Oriental or Indomalayan/Asian, and the Neotropical. Just over half the world's rainforests lie in the Neotropical realm, roughly a quarter are in Africa, and a fifth in Asia. The remaining five percent or so are scattered across Australia, New Guinea, and various Pacific Islands.

Part II:

WHERE ARE TROPICAL RAINFORESTS LOCATED?

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.

Mesoamerica, the Caribbean, and South America

Part III:

TYPES OF TROPICAL FORESTS

Rainforests are found throughout the world, not only in tropical regions, but also in temperate regions like Canada, the United States, and the former Soviet Union. These forests, like tropical rainforests, receive abundant, year-round rainfall, and are characterized by an enclosed canopy and high species diversity, but lack the year-round warmth and sunlight associated with tropical rainforests. However this section focuses on tropical rainforests, and these are the only forest forms discussed here.

Tropical rainforests merge into other types of forest depending on the altitude, latitude, and various soil, flooding, and climate conditions. These forest types form a mosaic of vegetation types which contribute to the overwhelming diversity of the tropics.

EQUATORIAL EVERGREEN RAINFOREST VS. MOIST FOREST

There are two major types of wet tropical forests: equatorial evergreen rainforests and moist forests, which includes monsoon forests and montane/cloud forests. Equatorial rainforests, often considered the "real rainforest," are characterized by more than 80 inches (2,000 mm) of rain annually spread evenly throughout the year. These forests have the highest biological diversity and have a well-developed canopy "tier" form of vegetation. Roughly two-thirds of the world's tropical wet forests can be considered the equatorial type. These forests are near the equator where there is very little seasonal variation and the solar day is a constant length all year round. The greatest expanses of equatorial rainforest are found in lowland Amazonia, the Congo Basin, the Southeast Asian islands of Indonesia, and Papua New Guinea.

Tropical moist forests are found at a greater distance from the equator where rainfall and day length vary seasonally. These forests get "only" 50 inches (1,270 mm) of rain annually and are markedly distinguished from equatorial rainforests by a cooler dry season. During this dry season, many trees shed some or even all their leaves, creating a seasonal reduction of canopy cover and allowing more sunlight to reach the forest floor. The increased sunlight reaching the forest floor allows the growth of vigorous understory vegetation not found in lowland equatorial forest. Such moist forest is found in parts of South America, the Caribbean, West Africa, and Southeast Asia, especially Thailand, Burma, Vietnam, and Sri Lanka.

PRIMARY VS. SECONDARY FOREST

Throughout this site, other books, and discussions about tropical rainforest, the term "primary forest" is used. Primary forest [data table] refers to untouched, pristine forest that exists in its original condition. This forest has been relatively unaffected by human activities. Primary rainforest is often characterized by a full ceiling canopy and usually several layers of understory. The ground floor is generally clear of heavy vegetation because the full canopy allows very little light, necessary for plant growth, to penetrate. Occasionally, when a canopy tree falls, a temporary "light gap" is opened in the canopy, allowing growth of floor and understory species. Primary forest is the most biologically diverse type of forest.

Secondary forest is rainforest that has been disturbed in some way, naturally or unnaturally. Secondary forest [data table] can be created in a number of ways, from degraded forest recovering from selective logging, to areas cleared by slash-and-burn agriculture that have been reclaimed by forest. Generally, secondary forest is characterized (depending on its level of degradation) by a less developed canopy structure, smaller trees, and less diversity. Due to the lack of a full canopy, more light will reach the floor, supporting vigorous ground vegetation. "Jungle" is the term often applied to secondary forest with dense ground growth, but it is also applied to some tropical moist forests where seasonal variations permit thick ground growth.

This data is from Global Forest Watch 2019 using a 30 percent tree cover thresh hold as the definition of "forest". All figures are hectares. The data includes tropical forest cover ranging from tropical dry forests to tropical rainforests.

LOWLAND VS. MONTANE FOREST

Lowland tropical rainforest refers to the majority of tropical rainforest, that is, forest which grows on flat lands at elevations generally less than 3,300 feet (1,000 m)—although elevation may vary. Lowland primary forest, often characterized by more than five forest tier levels, is usually taller and more diverse than montane forest. It has a greater diversity of fruiting trees; hence more animals specially adapted to feed on their fruits and more large mammals. Lowland rainforest is far more threatened than montane forest because of its accessibility, more suitable soils for agriculture, and more hardwoods valuable as timber. In many countries, virtually all lowland primary forest is gone, while montane forest still remains.

Tropical montane rainforest is forest that grows on mountains and above an altitude of 3,300 feet. High montane forest, above 6,600-10,000 feet (2,500-3,000 meters) in elevation, is often manifested as "cloud forest," forest that receives the majority of its precipitation from mist or fog that passes up from the moist, humid lowlands. The trees of cloud forests are typically shorter than those of lowland forest resulting in a less-developed canopy. Nevertheless, cloud forest trees are heavily burdened with epiphytes that thrive with the abundance of moisture from the passing fog. Trees in places like the lower elevations of the Andes in Ecuador, Peru, Colombia, and Venezuela; Central America (Monteverde in Costa Rica in particular); Borneo (Mount Kinabalu); and Africa (Ethiopia, Kenya, Rwanda, Zaire, Uganda), are frequently green with dense moss and beautiful, often rare, orchids.

Patches of cloud forests tend to have many endemic species, because they are often isolated from other sections of cloud forest by valleys and ridges. These species are prevented from migrating to other forest areas by these obstacles to the sides, by the lowland forest below, and by steep cliffs above. Cloud forests are home to an abundance of hummingbirds, frogs, and epiphytes like orchids, bromeliads, and mosses. Many of these species are endemic to a single locality, like the Golden toad of Monteverde, Costa Rica, a species which is now believed to be extinct. Cloud forests generally lack an abundance of large-bodied mammals due to the small number of fruiting trees.

Tropical montane forests are especially in the South American Andean region, where much of the forest has been cleared for agriculture. Of the continent's endangered species, a disproportionate number of those are found in yungas, the regional name for tropical montane forests in the Andes. These forests have also been little studied.

Above 10,000 feet (3,300 m), cloud forest may give way to sub-alpine and alpine forest. These habitats have less rain, fewer trees, and reduced biodiversity compared with lower elevation forests.

OTHER TYPES OF FOREST

SEASONAL OR MONSOON FOREST
Monsoon forests are tropical moist or seasonal rainforests found primarily in Asia (India/Sri Lanka to China), West and East Africa, Northern Australia, and Eastern Brazil. In this type of forest there is a distinct cooler dry season and a distinct wet season. These forests tend to be less diverse and more dwarfed in terms of tree size in comparison to typical equatorial rainforests.

Monsoon forests are highly threatened worldwide by clearing for cultivation, especially in West Africa, where over 90 percent of the coastal rainforests and the monsoon forests have been cleared.

IGAPÒ FOREST
Igapò forest is rainforest that is regularly inundated for extended periods during the flood season (sometimes considered permanently flooded rainforest). The best known of such forests are found in the Amazon Basin where they make up about 2 percent of the total rainforest. Igapò forest trees are shorter than those of non-flooded forest because of the instability caused by the wet, poorly drained soils (hence it is sometimes known as "swamp forest") and characterized by certain tree species like Cecropia, Ceiba, and Mauritia palms (also known as the aguaje palm). Many igapò tree species have stilt roots and flying buttresses to lend structural support. Igapò forest is flooded (4-10 months of the year) and flooding is usually predictable. Fish play an important role in seed dispersal in this forest system.

VÀRZEA FOREST
Vàrzea forests are floodplain forests which flood seasonally. Unlike swamp forests, varzeà forests have relatively rich soils from the annual replenishment of nutrients from whitewater rivers. Because these forests are more suitable for agriculture than typical rainforest, they are some of the most threatened. Even in the Amazon where vast majority of such forests are found, vàrzea are disappearing rapidly for development.

Floodplain forests, especially those located on river banks and islands, are often short-lived due to the meandering nature of tropical lowland rivers which eat away at the forests' base. According to Amazon Headwaters, a book by Michael Goulding and his colleagues, research in Peru suggests that most floodplain forests are rarely older than 200 years and may have turnover rates exceeding 1.6 percent, implying an average tree life of 63 years. For this reason, floodplain forests are nearly always in some stage of succession with pioneer species like Cecropia being replaced with Kapok (Ceiba) and fig trees further away from the river.

HEATH FOREST
Heath forests are found on well-drained, sandy soils that are extremely nutrient-poor. These forests are characterized by certain tree species tolerant of the poor, acidic soil conditions and are considerably "stunted" in comparison with typical rainforests. More light reaches the forest floor making for dense tree growth. Heath forests, also known as blackwater or caatinga forests, are drained by blackwater rivers and are found primarily in the Amazon Basin (the Rio Negro drainage), but also in parts of Asia.

PEAT FOREST  [news and information on peatlands]
Peat forest is found in small parts of Africa, northeastern South America, and large areas in southeast Asia (especially Borneo and Sumatra). These swamp forests appear in places where dead vegetation becomes waterlogged and accumulates as peat. The peat acts as a sort of sponge withholding moisture at times of little rainfall and absorbing monsoon rains. When peat swamp forests are drained for agricultural projects, they become highly susceptible to combustion. Under the dry el Niño conditions of 1997-98, thousands of fires raged in the peat swamps of Indonesia. Fires in peat swamps are extraordinarily difficult to extinguish because the conflagration continues in the deeper layers of peat.

TERRA FIRME FOREST
Terra Firme literally means "firm earth" and refers to rainforest that is not inundated by flooded rivers. This forest is noticeably taller and more diverse (>400 species/hectare in some areas) than igapò or flooded forest. It is found only on dry, well-drained soils and is characterized by such species as Brazil nut trees, Rubber trees, and many tropical hardwood trees.

MANGROVE FOREST [news and information on mangroves]
Mangrove forest is found in silt-rich, saline (brackish water) habitats worldwide, generally along large river deltas, estuaries, and coastal areas. It is characterized by low tree diversity, almost exclusively mangroves, with a low broken canopy. Mangroves are evergreen trees and shrubs that are well adapted to their salty and swampy habitat by having breathing roots (pneumatophores) that emerge from the oxygen-deficient mud to absorb oxygen.

Mangrove swamps are home to numerous bizarre amphibious fish species like the mudskippers of eastern Africa to Australia and Anableps, the so-called four-eyed fish of the New World. Mudskippers are renowned for their preference for terrestrial haunts over aquatic realms. These fish spend more time on floating debris, tree toots, and plants than they do in the water where they only go to escape predators. Watching a group of Mudskippers reminds the observer of what our ancestors must have looked like when they first left the ocean for life on land. Mudskippers are highly intelligent fish that feed primarily on insects and crustacean.

A second amphibious species found in mangrove forests is the Anableps. a species widespread in the New World from Central America to northern South America. Most notable about its physical features is its double-lobed eyes which allow it to see both above and below the water line as it swims along the water surface. The Anableps, too, regularly leaves the water to perch on tree roots and rocks.

Mangrove forests are some of the most threatened ecosystems on the planet because of their proximity to the ocean (prime resort/development property) and the tendency for local people and governments to undervalue the services they provide. A recent study by the Food and Agriculture Organization of the United Nations found that 20 percent of the world's mangrove forests have disappeared since 1980, mostly due to farming, harvesting for timber and charcoal, freshwater diversion, real estate development, and conversion for tourism.

According to the Environmental Justice Foundation, about 38 percent of global mangrove deforestation is linked to shrimp farm development. Mangrove clearing for commercial shrimp and prawn hatcheries is particularly prevalent in Southeast Asia. Ironically this form of aquaculture has come at the expense of the natural fish and shrimp hatchery.

The destruction of mangrove forest has dire implications for the fisheries industry, since these forests provide an important spawning ground and serve as a nursery for many commercially important species. In addition, mangrove forest protects coastal regions against storm damage and erosion. Research conducted following the 2004 tsunami in Asia found that areas forested with mangroves suffered less damage than areas without tree vegetation.

Mangrove forests are slow to recover from clearing and degradation. For example, seismic lines only a few meters wide in the mangrove forest of Nigeria were still visible by air a decade after they were cut.

REVIEW QUESTIONS

Review questions - Part I

• Where are rainforests located?
• How much land area rainforests do cover?
• What percentage of Earth is covered by rainforests?
• How many rainforest biogeographical realms are there?
• What biogeographical realm has the most rainforest?
• True or false - less than 5% of Earth's land is covered with rainforests.

Review questions - Part II

• 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?

Review questions: - Part III

• What is the difference between primary and secondary forest?
• True or false—Cloud forest is found in mountainous areas.
• True or false—flooding is common in the Amazon rainforest.
• Why are mangrove forests important?
• Why are mangrove forests being destroyed?

CITATIONS

Citations - Part I

• D.W. Orr Earth in Mind: On Education, Environment, and the Human Prospect, Washington, D.C.: Island Press, 1994.
• David Quammen Wild Thoughts from Wild Places (New York: Scribner, 1998).
• M. McKloskey in "Note on the Fragmentation of Primary Rainforest," Ambio 22 (4), June: 250-51 1993 using analysis of satellite images.
• Deforestation rates and tropical forest cover are taken from the latest State of the World's Forests 2011 (SOFO) published by the United Nations Food and Agriculture Organization (FAO).

Citations - Part II

• Australia's rainforest coverage during the most recent ice ages is discussed in M. Hopkins and P. Reddell (Australia's CSIRO 1998) and van Osterzee (Where Worlds Collide, New York: Cornell University Press. 1997). T.F. Flannery (The Future Eaters, New York: Braziller 1995) also discusses vegetation shifts wrought by climate change and human influences.
• Van Osterzee (Where Worlds Collide, New York: Cornell University Press, 1997), Quammen (The Song of the Dodo, New York: Scribner 1996.), and Browne (The Secular Ark: Studies in the History of Biogeography, New Haven: Yale University Press 1983) provide an easily understandable review of the Wallace line biogeography including the current distribution of flora and fauna in the region and the impact of changing sea levels. Rubeli (Tropical Rainforest in South-East Asia, Kuala Lumpur: Tropical Press Sdn. Bhd., 1986.) discusses the link between flora of New Zealand, the Himalayas, and Borneo.
• The history of the Amazon River Basin is covered engagingly in Goulding (Amazon-The Flooded Forest, New York: Sterling Publishing Co., Inc. 1990).

Citations - Part III

• The mechanism responsible for the worldwide decline in amphibian populations is debated by Lips ("Decline of a montane amphibian fauna," Conservation Biology Vol. 12 No. 1 (106-117), Feb. 1998.), Sessions et. al. (Sessions, S.K. Franssen, R.A., Horner, V.L., "Morphological Clues from Multilegged Frogs: Are Retinoids to Blame?" Science 284 (5415) 1999), Tangley ("The Silence of the Frogs," U.S. World and News Report 8/3/98), and Tuxill ("The Latest News on the Missing Frogs," World Watch May/June 1998). For alternative commentary from an unlikely source see M. Fumento ("With Frog Scare Debunked, It Isn't Easy Being Green," The Wall Street Journal 5/12/99).
• The "Primary Cover versus Total Forest Cover" table is taken from Myers, N., "Tropical forests: present status and future outlook," Climactic Change 19 (3-32), 1991.
• Pearce correlates forest clearing in West Africa to falling precipitation in the African interior in "Lost Forests Leave West Africa Dry," The New Scientist 1-18-97.
• The Amazonian igapò is the subject of Goulding's Amazon-The Flooded Forest, New York: Sterling Publishing Co.,Inc. 1990.
• Brookfield, H., Potter, L., and Byron, Y. provide a short description of Indonesian peat forests in In Place of the Forest: Environmental and Socio-economic Transformation in Borneo and the Eastern Malay Peninsula (New York: United Nations University Press, 1995), while T. Nishizawa and J. I. Uitto, eds. (The Fragile Tropics of Latin America: Sustainable Management of Changing Environments, New York: United Nations University Press, 1995) review Latin American forest types.
• Threats to mangrove forest from shrimp aquaculture and oil activities are examined in Moffat, D. and Lindén, O., "Perception and Reality: Assessing Priorities for Sustainable Development in the Niger River Delta," Ambio Vol. 24 No. 7-8 (527-538), Dec. 1995; and Boyd, C.E. and Clay, J.W., "Shrimp Aquaculture and the Environment" Scientific American. Vol. 278, No. 6 June 1998, respectively.