Sahelian Acacia savanna

Category: Grassland, Tundra, and Human Biomes.

Geographic Location: Africa.

Summary: This enormous band of semiarid habitat in subtropical Africa abuts the Sahara Desert; it is known for the periodic occurrence of severe droughts and famine.

An enormous band of arid and semiarid habitat, composed of three characteristic ecoregions, crosses the width of Africa. The Sahelian Acacia Savanna biome occupies the middle third, spanning the continent from the Atlantic coast of Senegal in the east to the Red Sea coast of Sudan. This biome has pronounced seasonality in precipitation, with the rains falling from May to September, followed by a six- to eight-month period of drought. Precipitation varies from 79 inches (2,000 millimeters) per year in the north of the Sahelian acacia savanna, where it transitions into the Sahara Desert, to about 236 inches (6,000 millimeters) per year in the south, where it borders the wetter Sudanian savanna. In fact, the name of the Sahelian region as a whole derives from an Arabic word for shore or coast, referring to how this sahel is a transition in type from the sandier, less vegetated Sahara Desert.

Historical and current human activity in the region is strongly tied to the availability of water, with rain-fed agriculture uncommon in the more arid northern regions, but more viable in the wetter southern regions. Typical crops include millet and sorghum, which may be abandoned in drought years. Under these conditions, abandoned land is degraded as drought intensifies the effects of the loss of native vegetation. Transhumance, the seasonal migration of nomadic people and their livestock from the drier north to the wetter south, has been by far the most important traditional pastoral use of the Sahel.

Vegetation

Native vegetation here varies with latitude and topography and includes both wooded grassland and deciduous bushland. Among the most common woody species are Acacia tortilis and A. laeta. Most trees and shrubs are deciduous, dropping their leaves for much of the yearly dry period. In addition to providing forage for goats and cattle, many native savanna trees provide for other human uses. Gum arabic, a derivative of the sap of two acacia species, especially A. senegal, has a variety of traditional and commercial uses that make this species of great human importance.

Other tree species serve as valuable sources of fuel, fence wood, and traditional medicines. By their very presence, savanna trees also provide services such as soil stabilization and the creation of fertile sites for small-scale crop production. Annual grasses such as Cenchrus biflorus, Schoenefeldia gracilis, and others are also common.

Fauna

The Sahelian acacia savanna is sparsely populated, with somewhat higher human population densities in its wetter, southern portions. This vast ecoregion has been greatly altered by a combination of drought and human-caused alterations in the landscape, particularly grazing and browsing by livestock. Its once large and diverse herds of native ungulates and their associated predators have been significantly diminished by these changes, and also by over-hunting for sport and food, made possible by modern vehicles and firearms. However, many of these species continue to persist within scattered protected areas established in the region. Some of the countries in this region are among the world’s poorest, and resources for conservation are limited.

Compared with other tropical parts of Africa and other continents, the Sahel ecoregion today does not support a high diversity of species. However, it does contain a few dozen endemic (found only here) vertebrate species, including 10 reptile and four gerbil species (genus Gerbillus). Diverse and abundant communities of native ungulates, including gazelle, oryx, and hartebeest species, were once widespread but now are mostly confined to a few protected areas. Associated large predators including the African wild dog (Lycaon pictus), cheetah (Acinonyx jubatus), and lion (Panthera leo) have suffered similar fates, as has the West African giraffe (Giraffa camelopardus).

Two species of ungulate, the scimitar-horned oryx (Oryx dammah) and the bubal hartebeest (Alcelaphus busephalus busephalus), a subspecies of the common hartebeest, are now thought to be extinct in the wild. The region’s wetlands, both permanent ones like those in the inner Niger delta and ephemeral ones that form elsewhere here in the wet season, provide a critical link for annual bird migrations along the Afrotropical-Palearctic flyway.

Across this region, vertebrate biomass has clearly shifted from native ungulates to cattle and goats. In the east, camels are important livestock. The effects of livestock and grazing on native vegetation have been most pronounced near the permanent waterholes.

Environmental Threats

The future prospects of the Sahelian acacia savanna are strongly tied both to the actions of its human population and to changes linked to the global climate. Though population densities are comparatively low, from one to five people per 0.4 square mile (1 square kilometer) in the north to 50 to 100 people per 0.4 square mile (1 square kilometer) in the south, the human population here continues to grow at around 3 percent per year, a rate that will more than double in size by 2050.

In a part of the world where poverty levels and rates of land degradation are high, a warming and drying climate is likely to mean that the chance of famine in the Sahel, a region already known for this problem, will remain high. Further degradation of the land and disruption of various habitats will go hand in hand with this crisis. Climate change estimates suggest temperatures will increase across the region by 5.4 to 9 degrees Fahrenheit (3 to 5 degrees Celsius) by 2050, decreasing agricultural production in some places by 13 percent to 50 percent.

The history of other parts of the developing world show that programs to improve education—especially of women—and to alleviate poverty are effective long-term solutions to slowing population growth. Continued technical and financial support for agriculture from the developed world will also be needed as this growing population moves further into the 21st century. Restoration of its degraded lands and protection for its native megafauna—now mostly restricted to several national parks scattered throughout the region—will also require more effort. Increased attention to all three areas would pay dividends to its burgeoning human population and help increase the political stability of the region.

Bibliography

Bourliere, F., ed. Ecosystems of the World, Tropical Savannas. Paris: United Nations Educational, Scientific, and Cultural Organization (UNESCO), 1983.

“Demographic Challenges of the Sahel.” Population Reference Bureau, 14 Jan. 2015, www.prb.org/resources/demographic-challenges-of-the-sahel/. Accessed 1 Sept. 2022.

Raynaut, C. and E. Gregoire et al. Societies and Nature in the Sahel. London: Routledge, 1997 .

Salgado, S. Sahel: The End of the Road. Berkeley: University of California Press, 2004.

Stewart, R. “Desertification in the Sahel.” http://oceanworld.tamu.edu/resources/environment-book/desertificationinsahel.html.