Africa

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Soil-Forming Processes

Hot, humid tropical climates provide ideal conditions for chemical weathering, which gradually brings about the disintegration of parent rock material into soil particles. In drier climates, physical weathering—the breakup of parent material by the force of moving water and wind—is more important than chemical weathering.

Soils in humid tropical regions are especially subject to leaching, the process in which smaller particles and minerals are carried by groundwater downward through the soil. When the upper layers of soil have been leached of critical nutrients, they are left primarily with iron and aluminum compounds, accounting for both their infertility and their typical brick-red or yellow color.

Salinization and calcification are important processes in the development of soils in arid and semiarid regions. Salinization occurs when salts, dissolved in soil water, are carried upward through the soil and deposited on or near the surface as a result of evaporation. Few species of plants are adapted to survive in soils with high salt concentrations. Calcification is prevalent in moderately dry to semiarid environments, generally in grassland areas. Calcification involves development of a subsurface layer of calcium carbonate, carried downward from the upper soil layers.

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Major Soil Types

Despite the richness of vegetation, soils in the tropical forests of Central and West Africa are poor. The heavy rainfall of these areas is acidified as it passes through organic material on the forest floor and leaches most of the mineral content from the upper soil layers. The resulting soils—classified as oxisols—are quite infertile, forcing plants to gain most of their nutrient needs from decaying vegetation. Oxisols are reddish or yellowish in color, reflecting the high concentrations of iron and aluminum compounds in them.

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The soils of the moist boundary areas between the forests and savanna are subject to high levels of chemical weathering. The resulting ultisols are highly leached (but less so than oxisols), have low to moderate organic content, and are generally infertile.

In savanna regions where there are moderate levels of seasonal rainfall, chemical weathering is less pronounced than in moister environments. Calcification occurs where calcium carbonate levels are high. The resulting soils are called alfisols. The organic content of alfisols is relatively high, and they are generally quite fertile.

In Africa’s arid and semiarid regions, low rainfall and sparse vegetation gives rise to aridisols. These soils are poorly developed, with little chemical weathering, infrequent leaching, and low organic content. Salinization is widespread, resulting in the concentration of mineral salts at or near the surface. In areas with less saline soils, irrigated agriculture is possible. Without proper management, however, irrigated aridisols may become infertile as a result of salinization or waterlogging.

Soils in certain seasonally dry areas are heavy and have a high clay content. The clay in these vertisols expands when it is wet, then shrinks and cracks when dried. These soils are associated with river floodplains and former lake bottoms, notably the middle Nile Valley and Lake Chad Basin. Vertisols are quite fertile due to their high mineral content, and are used extensively to grow cotton and grain crops.

Younger, less developed soils are classified as inceptisols. Included are the thin, rocky soils derived from younger volcanic deposits on the Ethiopian Plateau, young beach deposits in coastal regions, and active sand dunes in semiarid areas. Inceptisols are generally infertile.

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Animal Life

Africa teems with animals of all shapes and sizes. The continent has thousands upon thousands of species of mammals, fish, reptiles, amphibians, birds, and insects. Many of these animals are linked in an intricate food web. For example, hippopotamuses deposit large amounts of nutrients in bodies of water where they rest and defecate; these nutrients support abundant growth of plants, insects, and other smaller creatures that in turn provide food for species higher in the food chain. Other animals are linked in symbiotic relationships, such as between big game animals and birds known as oxpeckers. These birds eat the ticks that pester the large animals.

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Continental Forest Animals

African tropical forests offer many niches and habitats for different species. With a wide array of different types of food resources available year-round, they allow large numbers of species to coexist.

Different habitats occur at different heights of the forest. Each of these habitats harbors a distinct set of animal species. The ground layer is strewn with nutrient-rich organic litter, providing a rich environment for many arthropods and insects, as well as creatures that feed on them, such as moles and rats. The forest canopy supports numerous species of mammals, including many kinds of monkeys and flying squirrels. Species of birds, reptiles, bats, and insects also abound. The forest floor tends to be less diverse, although its wildlife includes some of the largest and most fascinating species in Africa, including elephants, lowland and mountain gorillas, and okapis, giraffe-like animals found in the Congo Basin. Other species include duikers, bushbucks, forest pigs, giant pangolins (a type of armored anteater), and drills and mandrills (two species of baboon).

Different areas of the African tropical forest vary in species diversity: For example, the relatively undisturbed forests of Gabon are more diverse than those in Nigeria, where there have been major human impacts on forest ecosystems. Overall, Africa’s forests appear to support less biodiversity than the larger, more heterogeneous forests of the Amazon and Southeast Asia.

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Animal Life on Madagascar

Despite its proximity to the continental mainland, Madagascar’s animal life developed in isolation after the island broke away from the rest of Africa about 135 million years ago. It is estimated that 90 percent of the species inhabiting its tropical forests are endemic, meaning that they are found nowhere else in the world. Madagascar has some 25 species of lemurs and 30 species of tenrecs, a type of insectivore. Other native mammal species include several civets and the fossa, a member of the cat family measuring almost 1.5 m (5 ft) in length. Madagascar’s bird and insect populations are equally rich and are also largely endemic.

Because more than 90 percent of Madagascar’s forests have been affected by human activity, many species on the island face extinction because of loss of habitat. Many other notable species have already vanished, including the giant lemur, giant tortoise, and elephant bird.

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