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. Over time, extensive deforestation due to logging, agricultural expansion, and infrastructure development has fragmented this once-continuous expanse. West Africa, in particular, has experienced significant forest loss, with only a small portion of the original cover remaining. Since the 1990s, as West African timber resources became depleted, timber production shifted to Central Africa, particularly in Gabon, Cameroon, and the Congos, leading to an acceleration of deforestation in the region. Additionally, industrial-scale agricultural concessions, such as oil palm plantations, have further contributed to forest loss and degradation.

Around the turn of the century, West Africa had some 193,000 sq. miles (500,000 sq. km) of coastal rainforest. However, these forests, primarily lowland formations with easy access from the coast, have been extensively exploited for commercial logging and converted to agriculture. According to the Food and Agriculture Organization of the United Nations, less than 20 percent of West Africa's moist forests remain, much of which is degraded. In heavily populated countries like Nigeria, human population pressures have intensified deforestation, while Côte d'Ivoire has experienced extensive forest loss due to both commercial logging and agricultural expansion. The full environmental consequences of this deforestation are still being studied, but increased soil erosion and changes in rainfall patterns are evident. Some research suggests that these coastal forests play a key role in maintaining rainfall levels in interior countries like Mali and Niger, which have seen rising drought incidence.

Despite extensive forest loss elsewhere, the rainforests of Central Africa still cover a vast expanse, accounting for about 75 percent of Africa's remaining rainforest. These forests span approximately 540,000 square miles (1.4 million square km), with the bulk of them located in the Congo Basin, primarily within the Democratic Republic of Congo and the Republic of Congo. Although these forests remain relatively intact compared to other tropical regions, they face increasing threats from industrial logging, agricultural expansion, and subsistence activities such as fuelwood collection and smallholder farming. Infrastructure projects, including road networks that facilitate logging and agricultural access, have also contributed to deforestation.

African rainforests are notably drier and more seasonal than their counterparts in Asia and Latin America. Pollen studies suggest that during past ice ages, much of the African rainforest was replaced by savanna. As concerns about climate change intensify, scientists worry that shifting weather patterns and rising temperatures could again bring prolonged droughts to parts of tropical Africa. Some studies indicate that rainforest resilience depends on maintaining intact forest cover and limiting human-induced fragmentation, as deforestation exacerbates climate variability by reducing the landscape’s ability to retain moisture. Additionally, there is growing interest in community-led conservation efforts and sustainable land-use practices that could mitigate some of the pressures on these ecosystems.

AUSTRALIAN AND PACIFIC REALMS

The majority of the rainforest in this region is found on New Guinea, the world's second-largest island. Australia has smaller sections of tropical forest, primarily on the Cape York Peninsula in the extreme northeastern part of the continent near the coast. Recent evidence suggests that Australia's rainforest cover is more extensive today than it was during the past 18,000 years. During the last glacial period, cooler and drier conditions prevailed, reducing rainfall by as much as 80 percent. As a result, Australia's rainforest retreated, replaced by fire-adapted eucalyptus woodlands. When the ice age ended, small rainforest refugia—remnants making up only 10-20 percent of today's coverage—enabled recolonization of the surrounding landscape.

New Guinea and Australia share many plant and animal species due to their historical connection during past ice ages, when lowered sea levels created a land bridge between the two landmasses. This shared history explains the high diversity of marsupials in both regions, filling ecological niches that, in other continents, are occupied by placental mammals like cats and primates. Also part of this ancient landmass were the Aru Islands, a small, closely packed group off the western coast of New Guinea. The waterway separating these islands from the Kei Islands marks the boundary between the Australian realm and a group of islands that were never connected to either major landmass during the ice ages. These islands—including Lombok, Flores, Timor, Sulawesi (Celebes), Ceram, and Halmahera—are now part of Indonesia and contain many endemic species that are not characteristic of either the Indomalayan or Australian realms. For example, Sulawesi's mammal fauna, excluding bats, is nearly 100 percent endemic, meaning none of its terrestrial mammal species are found elsewhere.

Although technically outside the boundaries of any particular biogeographic realm, the Pacific Islands are worth mentioning here. Many of these islands are volcanic in origin and have never been connected to a mainland. Despite their isolation, they host unique ecosystems, including rainforest-covered landscapes. However, these forests represent only a small fraction of global rainforest coverage.

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 have significantly reduced the natural extent, leaving only scattered fragments today.

Southeast Asia's rainforests are among the oldest on Earth. Studies suggest that some forests in present-day Malaysia may have existed for over 100 million years. However, these ancient forests did not resemble today's ecosystems. Early rainforests contained far fewer flowering plants, meaning many species that depend on them—such as pollinators and fruit-eating mammals—had not yet evolved. During past ice ages, sea levels dropped, allowing terrestrial species to migrate between mainland Asia and Southeast Asian islands like Sumatra, Java, and Borneo.

The ice ages profoundly impacted Southeast Asia’s rainforests. As ice sheets grew, global sea levels dropped, exposing vast land bridges that connected Sumatra, Java, Borneo, and the Malay Peninsula to mainland Asia. This allowed species to migrate freely between the regions. However, as the climate cooled, tropical rainforests retreated to isolated pockets, while much of the land was overtaken by deciduous forests, savannas, or montane habitats. As temperatures warmed again, the surviving rainforests in Sumatra, Borneo, and the Malay Peninsula became vital reservoirs from which species recolonized their surroundings. This legacy is evident today in montane forests, such as those on Mount Kinabalu in Sabah (Malaysian Borneo), where plant species more closely resemble those from the Himalayas and even New Zealand.

One of the most important biogeographical boundaries in the region is the "Wallace Line," named after the 19th-century biogeographer Alfred Wallace. This line separates the Indomalayan realm from the Australian realm and runs between the islands of Bali and Lombok. Despite their proximity, these islands have vastly different species compositions due to a deep ocean trench that has long acted as a barrier to migration. As a result, Sulawesi (Celebes) and Lombok host distinct flora and fauna that differ significantly from those of Borneo and Java, which are part of the Indomalayan realm. Many species that evolved east of the Wallace Line exhibit closer affinities with Australian wildlife than with their Asian counterparts.

NEOTROPICAL REALM

The Amazon River Basin is roughly the size of the contiguous United States and covers about 40 percent of the South American continent. Shaped by both environmental conditions and historical human influence, the Amazon is a mosaic of ecosystems, including rainforests, seasonal forests, deciduous forests, flooded forests, and savannas. The basin is drained by the Amazon River, the largest river in the world by discharge and the second longest after the Nile. With over 1,100 tributaries—17 extending more than 1,000 miles—the river serves as a vital artery for the region. Two of these tributaries, the Negro and the Madeira, surpass even the Congo River in volume, reinforcing the Amazon's role as the lifeblood of the rainforest.

The geological history of the Amazon reveals a dynamic past. At one time, the Amazon River flowed westward, potentially as part of a proto-Congo River system before the breakup of the supercontinent Gondwana. Around 15 million years ago, tectonic activity caused the Andes to rise, altering the Amazon's course and forming an immense inland sea. Over time, this body of water transitioned into a vast, swampy freshwater lake, where marine species adapted to a freshwater existence. Today, over 20 species of stingrays—closely related to their Pacific Ocean counterparts—inhabit the Amazon's freshwater systems, highlighting the region's deep evolutionary connections.

Roughly ten million years ago, erosion gradually breached the western sandstone barriers, allowing the Amazon to reverse its flow and drain toward the Atlantic. This transition marked the birth of the modern Amazon rainforest. During the ice ages, fluctuating sea levels further shaped the basin, transforming the great Amazon lake into a river network. Three million years ago, the formation of the Central American Isthmus facilitated species migration between North and South America, profoundly influencing the region's biodiversity.

Throughout Earth's history, global climate shifts have repeatedly altered the Amazon rainforest. Ice age periods led to significant rainforest contraction, with large sections reverting to savanna and montane forests. These fragmented forest "islands" created isolated habitats, fostering evolutionary divergence among species. Similar patterns have been observed in Africa, where core samples suggest that even the mighty Congo Basin once experienced a drastic reduction in rainforest coverage. As the ice ages ended, these isolated populations reunited, increasing genetic diversity and contributing to the Amazon’s unparalleled richness in species. Approximately 6,000 years ago, rising sea levels once again inundated portions of the basin, forming vast floodplains.

While the Amazon often dominates discussions of the Neotropical realm, the region hosts other significant river systems and forests. The Orinoco River, stretching over 1,600 miles through Venezuela and Colombia, is an essential component of South America’s hydrology. Uniquely, the Orinoco connects with the Amazon via the Casiquiare canal, a rare natural waterway linking two of the world's major river systems. South of the Amazon, the Tocantins River supports an expanse of forest, though much of this area has been affected by human activity. Along Brazil’s Atlantic coast, remnants of the once vast Atlantic Forest persist, though only a fraction—less than 5 percent—of the original cover remains due to centuries of deforestation.

In the northwest, the Chocó rainforest of Colombia is one of the most biodiverse yet least studied ecosystems in the world. Meanwhile, the Pacific coastal rainforests stretch from Ecuador into Central America, providing habitat for numerous endemic species. Much of Central America and the Caribbean was once blanketed in tropical rainforest, but widespread land conversion has significantly reduced these ecosystems. Today, only a few Caribbean islands retain primary forest, while some rainforest persists in protected areas across Central America. Between 1990 and 2005, the region suffered the highest percentage loss of forest among all tropical areas, with nearly 30 percent of its forests disappearing.

South America continues to experience the highest total deforestation rates globally. Between 2000 and 2005, an estimated 4.3 million hectares of forest were cleared annually, primarily in the Amazon. The primary drivers of deforestation include large-scale cattle ranching and industrial agriculture, particularly soybean cultivation. Additionally, road networks, hydroelectric projects, and mining operations contribute to habitat loss and fragmentation. In recent years, researchers have raised concerns about intensifying droughts in the Amazon, which may be exacerbated by deforestation and global climate change. The extreme drought of 2010, one of the worst on record, left major rivers dry, stranded communities, and fueled tens of thousands of wildfires. Such events underscore the fragile balance of the Amazon and the broader Neotropical realm.

Despite these challenges, conservation initiatives and sustainable management strategies continue to evolve. Indigenous-led conservation efforts, protected area expansions, and innovative agroforestry practices offer promising approaches to balancing ecological health with human development. Scientific research and international collaboration remain crucial in ensuring the long-term resilience of the Neotropical rainforest and its unparalleled biodiversity.

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?

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