2.1: Sub-Saharan Africa’s Natural Environment
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Much of the African continent encompasses the Afrotropical ecoregion, which is separated from other ecoregions by the Indian Ocean to the East, the Atlantic Ocean to the West, and the Saharan Desert to the North. These major geographic features have acted as barriers to movement since the African continent first took its current shape, enabling species and ecosystems characteristic of the region to evolve in relative isolation from those of other ecoregions. The Afrotropical ecoregion can be further subdivided into eight terrestrial biomes (Figure 2.1), each with its own distinct climate, geology, and biota (Burgess et al., 2004):
- Tropical and subtropical savannahs and grasslands: Sub-Saharan Africa’s largest biome is a mosaic of grasslands, woodlands, bushlands, thickets, and semi-arid drylands that are maintained by fire and grazing. East and Southern Africa’s miombo and mopane savannah-woodland ecosystems are included in this ecosystem.
- Deserts and arid scrublands: A biome of areas where evaporation exceeds precipitation, generally with rainfall < 250 mm/year. Generally associated with searing daytime temperatures and wind-swept sand dunes, this biome contains scrub deserts rich in succulent plants, rocky mountain deserts, and arid grassland-savannah mosaics, such as the Sahel region located just south of the Sahara.
- Tropical moist forests: Lowland broadleaf ecosystems with near-continuous canopies that run as a broad band across equatorial Africa. This biome is characterized by high rainfall ( 2 m/year), low variability in temperatures, and very high species diversity.
- Montane grasslands and scrublands: A patchily distributed biome that occurs at altitudes 800 m and has enough rainfall that a variety of grasses can thrive. Generally lacking trees except along some rivers and streams, it includes high altitude heathlands and other Afro-alpine areas.
- Mediterranean scrub: A scrubland ecosystem of limited extent, better known as the Fynbos or Cape Floristic Region, that is situated at Africa’s southwestern tip. Characterised by hot dry summers and cool moist winters, it contains one of Earth’s richest concentrations of endemic plant species.
- Flooded grasslands and savannahs: Grasslands, marshes, and shallow lakes that are periodically flooded by water that can be fresh, brackish, or hypersaline. When flooded, these areas host some of the largest water bird congregations in the region.
- Tropical dry forests: A highly restricted forest type that can be found in western Zambia and adjacent Angola, as well as on Cabo Verde. While these areas may receive high rainfall, they are characterized by seasonal droughts that can last several months.
- Mangroves: Coastal wetlands of tropical climates characterized by distinctive woody plants with aerial roots that can tolerate saltwater. Typically associated with intertidal zones and muddy bottoms, mangroves provide nursery grounds for many aquatic animal species.
In addition to these terrestrial biomes, Sub-Saharan Africa also contains several aquatic biomes. Prominent freshwater biomes include several large rivers along with their headwaters and deltas, numerous small rivers, multiple large and small lakes, as well as a variety of wetland ecosystems such as swamps, bogs, and salt marshes (WWF/TNC, 2013). Prominent marine biomes include tropical coral reefs along Africa’s east coast, as well as temperate continental shelves and seas along South Africa and Namibia (Spalding et al., 2007). There are also several important oceanic upwellings—areas of high productivity where surface waters are fertilised by nutrient-rich waters that “wells up” from below; these include the tropical Gulf of Guinea upwelling along West Africa, and the Benguela upwelling ecosystem along Africa’s southwest coast.
The variety of biomes present in Sub-Saharan Africa is the result of variable geology and a long history of changes in climate and ecological communities. For example, when Earth’s climate was warmer, tropical moist forests were more widely distributed. As the planet cooled during glacial periods, forests contracted and became fragmented while grasslands expanded; some new biomes developed as the climate changed and species moved around. Even today, biome boundaries are still shifting: for example, over the last few decades the boundary between the Sahara Desert and Sahel has shifted by hundreds of kilometres southward (Foley et al., 2003). The development, fragmentation, and movement of these and other biomes, as well as the influence of major dispersal barriers, such as large rivers and mountain ranges, have stimulated speciation, as different populations became specialised to conditions that were restricted to their particular elevations or on certain sides (wet or dry, sunny or shady) of mountain ranges.
Sub-Saharan Africa boasts tremendous species richness, the result of a complex geological and environmental history.
Due to this dynamic geological, climatic, and environmental history, as well as all the factors that have promoted speciation, Sub-Saharan Africa boasts tremendous species richness. The region is particularly well known for its mammals, particularly its charismatic terrestrial megafauna and other large mammals that attract millions of tourists from all around the world each year (Figure 2.2). Among the most famous are the Big Five animals—lions (Panthera leo, VU), savannah elephants (Loxodonta africana, VU), African buffalo (Syncerus caffer, NT), African leopards (P. pardus, VU), and black rhinoceros (Diceros bicornis, CR). Other notable mammals include cheetahs (Acinonyx jubatus, VU), the fastest mammal on Earth; Maasai giraffes (Giraffa camelopardalis tippelskirchii, VU), the world’s tallest mammal; the giant eland (Tragelaphus derbianus, VU), the world’s largest antelope; and Africa’s four species of great apes. Many small mammals are also noteworthy. For example, East Africa’s naked mole-rat (Heterocephalus glaber, LC) is the world’s only mammalian thermoconformer—meaning it is almost entirely cold-blooded; like reptiles their body temperature tracks ambient temperatures (Buffenstein and Yahav, 1991). The naked mole-rat and Southern Africa’s Damaraland mole-rat (Fukomys damarensis, LC) are the only known eusocial mammals; like some ants and bees, only one female (the queen) reproduces with one to three breeding males, while all the other colony members are sterile workers (Jarvis et al., 1994).
While Africa’s large mammals are a major tourist drawcard, the region hosts many other rich and noteworthy wildlife assemblages. With more than 2,100 bird species, 1,400 of them found nowhere else on Earth (Sinclair and Ryan, 2011), the Afrotropics may be the most taxonomically diverse bird region on Earth (Lotz et al., 2013). Among the many bird species that call Africa home is the world’s largest extant species of bird, the red-necked ostrich (Struthio camelus camelus); standing up to 2.74 m tall, it is in dire need of conservation attention (Miller et al., 2011). Africa is also home to the world’s heaviest extant flying animal, the kori bustard (Ardeotis kori, NT), which can weigh over 20 kg (Dunning, 2008). Over 100,000 insects have been described in Sub-Saharan Africa (Miller and Rogo, 2001), which include the world’s smallest butterfly, the dwarf blue (Oraidium barberae, LC) of Southern Africa, and the aptly named goliath beetles (Goliathus spp.), which can be found throughout much of tropical Africa. The region also hosts a great number of noteworthy endemic amphibians and reptiles, which include the world’s largest frog, the Goliath frog (Conraua goliath, EN) of Cameroon and Equatorial Guinea, and the black mamba (Dendroaspis polylepis, LC), arguably the world’s most feared snake, which is widespread across Africa’s savannahs. Lastly, Africa is home to Jonathan the Aldabra giant tortoise (Aldabrachelys gigantea, VU); having hatched in 1832, he is considered the oldest living terrestrial animal in the world.
Species that have survived previous mass extinction events are unable to withstand the current onslaught of human activities.
The region’s plant richness, estimated at over 45,000 species (Klopper et al., 2007), is also important from a global perspective. Many plant species have high economic value, particularly those that have been domesticated in the region, and are now important crops across the world. Primary among these are coffee—second only to tea in worldwide popularity as a beverage—which is native to West and Central Africa (Coffea robusta) and Ethiopia (Coffea arabica). Other important crops that originated in in the Afrotropics include okra, black-eyed peas, watermelon, and African oil palm. Conserving the wild genetic diversity of these domesticated plants in their native ranges is important because they may serve as “insurance” for today’s crops that may be less productive in future due to anthropogenic climate change (Davis et al., 2012). Others, such as the wide variety of plants utilised in traditional medicine to treat malaria, may one day lead to new antimalarial drugs (Chinsembu, 2015). Similarly, many plant species also have high evolutionary value. These include relict species that survived previous mass extinction events, such as cycads (Encephalartos spp.) (unfortunately several cycad species are now Extinct in the Wild), and Lazarus species that were once believed to be extinct, such as the unique jellyfish tree (Medusagyne oppositifolia, CR) of the Seychelles.
A few small and isolated African ecosystems are particularly rich in species. Particularly noteworthy is the Rift Valley lakes, such as Lake Victoria, Lake Malawi, and Lake Tanganyika, which hold the richest freshwater fish diversity in the world. For example, nearly 14% of the world’s freshwater fish species occur in Lake Malawi (also known as Lake Nyasa). Moreover, over 90% of Lake Malawi’s 500–1,000 (numbers vary by source) fish species (Figure 2.3) are endemic, and thus found nowhere else on Earth. The Cape Floristic Region is home to the greatest concentration of non-tropical endemic species in the world, including speciose well-known plant genera like Protea and Erica. The Succulent Karoo, directly north of the Cape Floristic Region, may be the most floristically rich desert in the world (Mittermeier et al., 2004). Africa has deservedly received international acclaim for these and many other natural wonders. Prominently, more than 37 sites in Sub-Saharan Africa have already been recognized as natural World Heritage Sites. One such site is also Africa’s oldest national park, Virunga National Park in eastern DRC, which contains at least 218 mammal and 706 bird species (WHC, 2007).