**1. Introduction**

Africa houses the largest area of savanna, covering about 65% of the continent [1]. However, the natural savanna ecosystems in Africa are undergoing a serious transformation, which is due to the conversion of land parcels into wide-ranging

livestock production foraging arenas, slash and burn for the production of crops, and creation of municipal cities [2–4]. Despite these changes having enormous impacts on the capability of the ecological system to exchange and store carbon [4, 5] as well as on species diversity and composition, the factors affecting the species diversity and composition are still poorly understood. Furthermore, poor grazing management, linked with livestock overstocking, results in extreme treading, which has a detrimental impact on the vegetation [6–8]. This also leads to an increase in the bulk density of the soil and reduces its water permeation [8, 9]. Hence, it results in alterations in the composition of the plant species, reducing the rates of photosynthetic activities in the leaf and reducing the storage of the carbon stock in the ecosystem [7, 10].

On the contrary, the aboveground biomass may be stimulated by light grazing via increased cultivation of grasses [8, 11, 12]. Indeed, increased germination of young shoots from the cultivators leads to a better and improved generation of fresh energetically photosynthesizing green vegetation. This ultimately, when combined with reduced accrual of dead biomass, leads to an improvement in the penetration of light into the canopy and intensifies the uptake of carbon dioxide and storage of carbon by the grasslands [10, 12].

Moreover, degradation of habitats, excessive utilization, harmful non-native species, contamination, and alteration in the climate have affected and still impact the biosphere's ecological systems [13, 14]. Sixty percent (60%) of the globe's ecosystems are estimated by Refs. [15, 16] to be utilized in an unsustainable way. Seventy-five percent (75%) of the stocks of fish resources are being used excessively, which may lead to their depletion. Moreover, about 13 million hectares of tropical forest ecosystems are cleared annually [15, 16], significantly affecting the biotic diversity and composition, especially in African savannas.

The rate at which the loss of biological diversity proceeds if continues, we may face a mass extinction [13]. The decline in biological diversity represents a planet's irreversible loss. It poses a serious menace to the life support system of humanity, and some authors describe it as the amenities that are delivered by natural systems representing the entirety from the food we consume to the air we breathe (e.g., [17–19]). This chapter, therefore, contextualizes the factors affecting species diversity and composition in an African savanna. Understanding these factors will provide policymakers with a strong basis for planning and managing the forest ecosystems in an African savanna.

#### **1.1 Concepts of species diversity and composition and ecosystem functioning**

The term "biodiversity" was first used in its long version (biological diversity) by Ref, [20] and is most commonly used to describe the number of species. It came into effect in the mid-1980s, prefigured by a convention in 1986, followed by the book called biodiversity [21]. These proceedings, in most cases, are construed as the inauguration of the story of biodiversity.

Faith [22] defines biodiversity as a variation in living things, from genetically modified characters to species and, ultimately, ecological systems. The term "biodiversity" energized some essential concepts formulated over the preceding decade. Moreover, the concept of the variation in living things itself has current value since it avails the chance for humanity to benefit. The International Union for the Conservation of Nature [23], summed up these initial concepts about diversity as affording both "insurance" as well as "investment" benefits. The convention on biological diversity (CBD) and intergovernmental platform on biodiversity and

#### *Contextualizing the Factors Affecting Species Diversity and Composition in the African Savanna DOI: http://dx.doi.org/10.5772/intechopen.108413*

ecosystem services (IPBES) echoed the focus on biodiversity. The IPBES conceptual framework describes nature's contribution to humanity [24], which encompassed maintaining future generations' choices availed by biotic diversity as variety. This significance of diversity in living things supplements the recognized worthiness of singular species and solidifies the concept that biotic diversity may denote a singular assemblage of species (or other units) and the quantity of disparity as a property of that assemblage.

The other prominent concept with regard to species diversity, composition, and ecosystem functioning is that modern energy and water have a direct influence on the richness of plant species. This is due to the fact that they control the primary productivity of an ecosystem, which helps in the formation of the food chain and drives other indirect effects (i.e., primary productivity hypothesis) and also by the degree to which an organism can tolerate one or more environmental factors that control the distribution of species (i.e., physiological tolerance hypothesis) [25–28].

Therefore, the theory of the connection of water and energy dynamics postulates that there is an impact of the environment on the distribution of abiotic resources (e.g., water, temperature, and ultraviolet-B (UV-B)) among various species and that this is what principally controls regional biological diversity patterns [29]. Its basic view is that water and energy control the extensive species diversity patterns [30, 31].

Global climate variations could also impact the allocation of species, composition of communities, and structure of an ecosystem [32–34]. Global climate change is not anymore a debatable matter, though various scientists may disagree on the precise forecasts from several models [35], and coming up with viable solutions to the connections between vegetation and climate dynamics is an exceptionally precarious issue of current research [36, 37]. Ref. [38] predicts a rise in the average temperature of between 0.3 and 4.5°C by the end of the twenty-first century, with the Arctic having experienced speedy warming over the current decades.

#### **1.2 The distribution of the African savannas**

Worldwide, savanna is the second largest biome, covering about 33 million km<sup>2</sup> or nearly 20% of the earth's land surface [39–41]. Africa inhabits the largest savanna, which occupies about half of the continent with approximately 15.1 million km<sup>2</sup> [42]. Besides, Ref. [1] note Africa to encompass undoubtedly the largest area of savanna, covering 65% of the African content. Tropical savannas are found in the move between the deserts and the tropical rainforests, where there is limited rainfall to support the forest.

Savanna ecosystems predominantly are comprised of a combination of open grasslands, closed coppices with closed thickets consisting of trees and shrubs, which have broad leaves and woodlands that have dispersed trees [43]. This according to Ref. [44] can have implications on human livelihood globally in an even to any alteration to the ecosystem.

The western central rainforest is surrounded by tropical savannas. To the north and south, it is surrounded by deserts. For African savannas, systematic numerical classifications based on climate and physiognomy have been used. The bioclimatic categorization separates four physiognomies of savanna (**Table 1**) [46] cited in [45]).

Despite it being challenging to describe the confines of the African savanna accurately, Okigbo (1985) estimates that it occupies over 12 million km2 and covers approximately 60% of tropical Africa. It encompasses all or parts of closely all the 45 countries of tropical Africa (**Figure 1**).


#### **Table 1.**

*Bioclimatic zones of the African savanna.*

#### **Figure 1.**

*The distribution of African savannas (in color) around the Congo Basin with tropical moist forests, in dark green. (adapted from White, 1983; Geldenhuys and Golding, 2008). The symbols indicate the following vegetation units (phytochoria): I = Guineo-Congolian regional centre of endemism (RCE); II = Zambezian RCE; III = Sudanian RCE; IV = Somalia-Masai RCE; X = Guinea-Congolia/Zambezian regional transition zone (RTZ); XIO = Guinea-Congolia/Sudanian RTZ; XII = Lake Victoria regional mosaic (RM); XIII = Zanzivar-Inhambane RM; XIV = Kalahari-highveld transtion zone; XVI = Sahel RTZ (adapted from Geldenhuys & Golding, 2008).*

*Contextualizing the Factors Affecting Species Diversity and Composition in the African Savanna DOI: http://dx.doi.org/10.5772/intechopen.108413*

#### *1.2.1 Arid savanna zone*

The arid savanna zone includes a large portion of central Mali, northern Burkina Faso, southern Niger, northeast Nigeria, Chad, Sudan, and Ethiopia. It also includes a large portion of northern Senegal from Dakar to south of the Senegal river. This region reaches southern Ethiopia, central Tanzania, and Somalia via a thin strip that passes into Kenya. Along with eastern Zambia and southwest Angola, it is also common in southern Mozambique, Zimbabwe, and eastern and northern Botswana. The predominant plant species in this region include *Acacia* spp., including *Acacia* Senegal (gum Arabic), *Acacia vaddiana*, *Leptadenia pyrotechnica*, *Salvadora* spp., *Grewia* spp., and *Acacia seyal* in low places susceptible to flooding, as well as Sahelian grasses, such *Aristida* and *Chloris* spp.

This zone is characterized by low rainfall (300–600 mm/year), a brief rainy season (2–3 months), and frequent droughts. The leguminous Faidherbia albida trees, which shed their leaves in the wet season and manure the soil to provide feed for cattle in the dry season, have historically been preserved by the Serer people of Senegal.

Additionally, significant in eastern Africa are *Commiphora* species. Nearly every tree in the Sahel seems to have a purpose, whether it is for fruit (*Balanites aegyptiiaca*, *Phoenix dactylifera*), fodder (Acacia spp.), or both (Okigbo, 1986). During the protracted dry season, when grazing is scarce, a large number of trees and bushes are severely pruned for food and subsistence.

Dune sands are common in this area, especially in West Africa, and it has historically been subject to wind erosion to a large extent. However, other soils with marginally higher productivity are also present. Although there is little or inconsistent rainfall, this region is crucial for agriculture. A sizeable portion of it is cultivated in West Africa, where pearl millet is mostly grown along with big herds of domestic cattle. As a result, the long rainy season in the northern half of the zone fits the early mature types, known as Souna in Senegal (Bilquez, 1975).

#### *1.2.2 Subarid savanna zone*

From Dakar, the subarid savanna zone extends through the majority of northern Nigeria, central Chad, southern Burkina Faso, central Mali, and southern Burkina Faso. In Sudan, it broadens to encompass a sizable portion of the central rain lands as well as a few blue Nile irrigation projects. The region includes a significant portion of the Rift Valley in the south of Addis Ababa in Ethiopia, the Karamoja district in northeastern Uganda, a small portion of the Rift Valley in Kenya to the east of Nairobi, and a portion of central Tanzania. This region also includes the majority of western Zimbabwe, southern Zambia, and southern Angola.

Although alfisols are the most common upland soils in this region, there is significant regional diversity, including dune sands in some regions of West Africa, vertisols around lake Chad and in Ethiopia, Somalia, and Sudan, and oxisols and entisols in southern Africa. F. albida and Hyphaene thebiaca are indicator species, and mixed combretaceous and acacia tree savanna make up the majority of the vegetation in this zone. These species are protected and used as browse, together with Parkia spp. Mango trees are grown in areas with shallow groundwater. Cenchrus ciliaris, Cenchrus biflorus, Eragrostis tremula, and Pennisetum pedicillatum are examples of grasses (Okigbo, 1986).

With significant populations concentrated throughout most of both western and eastern Africa, this region is crucial for agriculture. An essential component of Africa's primary belt for producing cereals is the subarid savanna region.

#### *1.2.3 Subhumid savanna zone*

Another significant agricultural region in the African savanna is the subhumid zone, which has the greatest potential for the production of annual crops, particularly cereals.

It stretches from southern Senegal, Gambia, and Guineas through southern Mali and Burkina Faso, northern Cote d'Ivore, Ghana, Togo, Benin, central Nigeria, Chad, and the Central African Republic, before entering southern Sudan, the majority of Uganda, and western Kenya.

There are also significant portions of Tanzania, Mozambique, northern Zambia, southern Zaire, and Angola.

The soils in a large portion of this zone are classified as alfisols, despite the fact that the soil associations in this zone are complex and there are numerous diverse catenary sequences, similar to the subarid zone. However, there are sizable areas of ultisols in Guinea and Uganda and oxisols in Angola, the Central African Republic, and Zaire. Isoberlinia spp., Burkea Africana, and Afzekia Africana are prevalent in West Africa, while in the miombo woodlands, Brachystegia spp. and Julbernardia spp. define the natural vegetation in this zone. Parkia clappertoniana and Butyrospermum spp., sometimes known as the shea butter but, which is a popular source of fat and oil, are protected species. Milicia (previously Chlorophora) excels and Entandrophragma spp. are examples of timber trees. Andropogon gayanus, Hyparrhenia spp., and Pennisetum spp. are examples of tall grasses (Okigbo, 1986).

#### *1.2.4 Humid savanna zone*

The humid savanna zone, which begins in West Africa, includes Guinea Bissau, most of Guinea, a portion of southern Mali, Cote d'Ivoire, Ghana, Togo, Benin, a sizable portion of central Nigeria, Cameroon, the majority of the Central African Republic, a small portion of southwest Sudan, the majority of Uganda, the Kenyan highlands, the majority of Rwanda, Burundi, a portion of western and southeastern Tanzania. Although the soils of this region are diverse, a significant portion of the West African coast, as well as Burundi, Rwanda, Uganda, and some areas of Zaire, are classified as utisols. Other large areas are classified as alfsols in much of Benin, Cote d'Ivoire, Ghana, Nigeria, Tanzania, and Togo, and oxisols in Cameroon, Central African Republic, and Zaire.

The majority of the zone's natural climax vegetation would be light forest, with some open woodland in the drier regions and on the less fertile soils, but due to human activity, the majority of the forest has been transformed into derived savanna. This is a transition zone between savanna and forest, and between a unimodal rainfall distribution in roughly 190–200 days in the savanna part and a bimodal distribution in two rainy seasons, each lasting 2–4 months and adding up to 210–230 days in the area derived from the forest.

Some areas of the zone, especially those with light or shallow soils, can occasionally make it difficult to plant annual crops because, even if it is sometimes possible to grow two crops per year, there may not be enough rain in season to result in an ideal crop. The humid savanna is well adapted to a variety of crops, both annual and perennial because it is a transition zone between unimodal and bimodal rainfall distribution, as well as between savanna and forest.
