Chapter 1
Savanna landscapes
What are savannas?
The term ‘savanna’ frequently conjures up images of vast grassy plains interspersed with umbrella-topped trees and populated by herds of large grazing animals. These images, characteristic of Africa, offer only a partial insight into the extraordinarily rich and varied landscapes that make up world savannas. Such landscapes are amongst the most dynamic and diverse biomes (or biological life zones) that cover the Earth’s surface. They represent a fascinating, colourful, and abundant resource that has performed a key role in the evolution of human beings and continues to tax our ingenuity in balancing resource use against conservation in a world of rapidly growing population.
Savannas are the most widespread form of vegetation in the tropics and sub-tropics, making up around half of the surface area. They cover well over 20 per cent of Australia depending upon how they are defined, around 45 per cent of South America, and more than 50 per cent of Africa. There are patches across India, South-east Asia, and the Pacific islands, and savannas extend up through Central America and the Caribbean towards North America. They are most simply described as areas covered by a nearly continuous grassy layer, interspersed with trees and shrubs of varying densities and heights. The nature of savannas will be explored later, but attempts to give a precise definition are fraught with difficulty because savanna vegetation is frequently a mosaic of trees, grasses, and shrubs. The proportions of these not only change rapidly over time but also include patches that are not strictly categorized as savanna. Consequently generalizations that seem appropriate at a global scale are not necessarily apposite at a regional or local level.
The notion of a savanna landscape is a conceptual image as well as a visual panorama. Despite widespread appreciation of what constitutes such a landscape, there have been several different approaches to interpreting its meaning. Most people have an idea of a mapable unit where land, people, and history combine to give a unique image and memory of place. Landscape comprehends scenery, wildlife, and human activities. It includes vegetation that is unarguably recognized as savanna, as well as tracts of different formations and their constituent biological organisms. Issues of scale and the level of technicality also impinge upon definition but, whichever interpretation is appropriate, it has been estimated that around one-fifth of the world’s population lives in or near savanna regions. Because savannas are typically located between the consistently wet and consistently dry regions of the tropics, they inevitably overlap, at a broad scale, with the peripheries of forest on the one hand and arid regions on the other. They merge with temperate grasslands and woodlands, and may also abut aquatic ecosystems (smaller-scale ecological units) associated with river valleys, lake margins, wetlands, and coastal areas. Savannas are also often criss-crossed by rivers, giving rise to gallery forests and shallow damp depressions forming seasonally flooded and more marsh-like environments such as the Pantanal in South America or the Okavango Delta in southern Africa. They may be interrupted by influential geological features, such as volcanic deposits or limestone outcrops that shape the nature of soils supporting different types of vegetation. In this way savanna landscapes typically include tracts of differing habitats, which nonetheless make up the total multifaceted picture that characterizes a rich and intriguing biome.
Where are the savannas?
The global distribution of savannas (see Figure 1) illustrates the broad pattern to either side of the lines of the tropics. However, such a large-scale view conceals both the diversity and the far-reaching character of the plant and animal life. Most savannas reflect strong seasonal climates but their boundaries have shifted over time with climatic changes and, more dramatically, since the onset of human disturbance. Many savannas are today better termed derived savannas, where the vegetation resembles true savanna superficially but may relatively quickly revert to some other form given respite from disturbance. In this way we may refer to the true savanna as climax vegetation (in other words, the type of vegetation that may theoretically occur given equilibrium with climate and other determining factors over a long period of time). This attractive supposition is hardly ever achieved, since savannas are constantly changing and there is debate as to whether savannas are simply transitional landscapes perpetually in the process of transformation.
1. The world distribution of savannas.
The world picture does nevertheless show several characteristic features. Savannas are clearly widespread in tropical and sub-tropical latitudes and comparisons with maps of world climate demonstrate that they occur over areas with highly seasonal and often irregular rainfall. Savannas are found predominantly in the southern hemisphere, with the major realms located in Africa, large stretches of South America, and Australia. Africa holds the largest areas of savanna in the northern hemisphere but savanna landscapes extend into Central America and the Caribbean with scattered tracts throughout India, South-east Asia, and numerous other less extensive localities in the sub-tropics. At a local level, differing plant associations make up a mosaic of vegetation formations whose features can vary over a few metres. Each of these scales has its own distinctive character and contributes to the overall diversity of the savanna landscape.
Although savannas result mainly from similar temperature and rainfall conditions throughout the world, they show distinct differences in fauna and flora. The similarities between savanna and neighbouring biomes within each of the continental ranges can be greater than the similarities between different world savanna regions. For example, the vegetation of the Brazilian savannas (or cerrados) shows greater affinity with the flora of the Amazonian forest than to the savannas of Africa or Australia.
Marked seasonal variation is characterized by unpredictably long wet or dry spells. The dryness tends to restrict tree growth and as a result there is a range of vegetation that stretches from sparse grassy and shrubby formations near the drier margins to nearly continuous tree canopies at the wetter transitions (see Figure 2). Furthermore although savannas are typically found over the continental plateaus of the southern hemisphere, they are also found over varied tropical environments from sea level to altitudes over 1,000 metres (m). In addition, they are predominantly underlain by coarse-textured, weathered, and infertile soils that have accounted for their lack of exploitation for cultivation in the past. At greater altitudes and at latitudes further north and south of the tropics, they may merge into more typically temperate grasslands and woodlands, which, however, lack the same characteristic grass species.
2. Characteristic forms of savanna vegetation and the typical composition of the Brazilian cerrados.
Savanna landscapes
Landscape captures the total picture that can be observed at any one place at a given period of time. This is usually underpinned by a combination of topography and vegetation. The structure and general composition of the plant cover is often the feature that catches the eye, and is at first sight similar in appearance and function throughout the world’s savannas. However, a closer look reveals changes in physiognomy or appearance, and phytosociology or plant associations. These result in part from different evolutionary pathways determined by factors such as continental drift and the present-day positioning of the savanna zones within seasonal climates. More spectacular differences are seen in the variety and numbers of animals, and in the movement and settlement of humans.
Landscape reflects the sweep of topography, influenced by the underlying rock composition and structure. In savannas the topography can change from rolling or steep-sided hills to smooth undulating plateaus and level plains. The steepness of the relief also controls the flow of water over the surface and the drainage through the soil, and in turn these processes shape the form and variety of plants. The vegetation is made up of perennial plants with permanent above-ground structures; perennials with seasonal above-ground structures but often with underground perennating structures; and annuals, with a marked lack of ephemeral species (see Chapter 3).
The frequency and intensity of fire complicates a smooth succession of plant colonization and all savannas have adjusted to burning at regular, frequent, or periodic fire intervals. They have been termed ‘fire-adapted landscapes’ because of the constant pattern of burning and recovery. With frequent burning and long dry seasons, tree and shrub colonization is inhibited and grasslands become dominant (see Figure 3(a)). Where there is greater rainfall and shorter periods of dryness, more woody species can colonize and persist. Eventually a tree cover may become so well established that it can resist burning and resembles woodland. It is only defined as ‘savanna woodland’ by the persistence of a grass and herbaceous ground cover that becomes more evident in the drier parts of the year when the taller vegetation loses its leaves, as in the miombo of southern Africa, the cerradão of Brazil, or some of the eucalypt-dominated savannas in Australia (see Figure 3(b)).
3(a) Savanna grassland landscapes at Amboseli, Kenya, with mountains of the Rift Valley in the background; (b) Woody savannas in the Brazilian cerrado, showing the nature of the herbaceous ground cover and arboreal termite nests.
As a consequence of fire, savanna plants and dependent animal and micro-organic activities are in a continuous process of alteration, and this is further accentuated by the relative unpredictability of climate and the impact of human disturbance. The result of these continual pressures is often a patchwork of different plant associations.
Geological evolution and climate change
Savannas are mostly found in the southern continents where there are the largest land masses. These land masses are remnants of the ancient supercontinent of Gondwana and reflect its geology with interconnections past and present that link the current lands together (see Figure 4). The nature and distribution of savanna plants and animals is closely allied with the evolution of the continents, and thus with the composition and structure of the underlying rocks together with the characteristically poor soils.
4. The movements of the continents following the splitting of the ancient land mass of Gondwana.
Gondwana (‘forest of the Gonds’), was named after the area in India where the geological chronology was first revealed in the 19th century. It contained a sequence of sediments dating from the Permian to Cretaceous Periods c.300–65 Mya (million years ago) that were later found to be similar, in composition and fossil remains, to sediments found in the other southern continents (see Table 1). Over this long period of time, the southern continents are believed to have contained broadly analogous floras although there were regional differences. The break-up of Gondwana started some 180 Mya in the Jurassic and continues today with fragments moving slowly northwards. It seems that the first rifting of the supercontinent happened in the west, notably South America splitting from Africa, and extended eastwards during the Cretaceous. The result of all these movements over time was the gradual shifting of varied rock formations into what are now warmer latitudes. It is the similarity in seasonal climate over recent time that strongly influences the distribution and pattern of present day vegetation. Before and after the rifting, the vegetation was dominated by conifers and ferns, but angiosperms or flowering plants were established before the final break-up and diversified in the Cretaceous Period. In this way it can be seen that latitudinal controls have had a marked effect on floras through time. While the floras are similar in appearance and function, and plant life was able to spread and colonize with worldwide distributions, animal life was less able to cross water or other physical and chemical barriers; the different fragments became isolated from one another and so evolved characteristically different forms of wildlife.
Table 1 The geological timescale
Despite the importance of geology, the distribution of savannas is usually distinguished on the basis of their climatic characteristics, with marked wet and dry seasons. However, there are several typical savanna landscapes that lie outside what might normally be considered as a characteristic savanna climate. This occurs, for example, where local factors override the influence of climate, as in the patches of savanna found within the Amazon rainforest, where exceptionally sandy soils have inhibited tree growth. Equally, there are tracts within the savanna, experiencing the same overall climatic determinants, that do not contain savanna vegetation. Examples include dry deciduous forest located within the Brazilian savanna that result from more fertile calcareous soils derived from exposures of underlying limestone.
The key features of the evolutionary story that occurred in savannas are the rise of primates from around fifty-six Mya, the appearance and expansion of grasses from early Eocene some thirty-five Mya, and the evolution of ancestral humanoids in Miocene about twenty Mya. In terms of geological time, the savannas are relatively recent arrivals, with the expansion of grass-dominated landscapes and the differentiation of vertebrate communities developing from the Tertiary Period onwards.
Biological richness
There are marked differences in biological richness throughout savanna regions. The landscapes may vary from open grasslands filled with large grazing animals and iconic predators (forming a distinctive food chain) as in many parts of Africa; to open grasslands with hardly any large animals (stretches of South America); or open savannas with fewer specialized animals (Australia). It is well known that species richness varies with area, and consequently comparisons between different continental realms or tracts with a given region can only be meaningful if the areas are of similar size. Nevertheless there are broad comparisons that are useful at this stage. The South American savannas are, for example, generally more moist, with a greater proportion of evergreen plants, a large rodent population, and few large animals; the African savannas are characteristically grasslands with a pattern of acacia trees and numerous large animals; and Australia has typically more open eucalypt woodlands and grasslands with distinctive marsupial animal populations. Overall the fauna and flora of the major continental savannas share remarkably few life forms at species level.
‘Biodiversity’ is the collective term for the total numbers and variety of life. While the richest biomes in terms of species can be found in some rainforests and coral reefs, there are a number of savanna regions that can be termed biological hotspots. These are concentrations of organisms that have been recognized as important for their outstanding natural richness and with a pressing need for conservation. There is justifiable concern that this biodiversity is being threatened, partly from natural causes but also from increased human disturbance. For instance, the Brazilian cerrados, which were practically undeveloped before the construction of the new capital, Brasília, in the late 1950s and early 1960s, have been converted into land for cultivation and pasture at an extraordinarily rapid pace. Over 1,000 species of trees and shrubs and over 12,000 vascular plants (those with tissues for conducting water and minerals and the products of photosynthesis) have been recorded in the Brazilian cerrados. Levels such as these rival some of the moist evergreen forests. Similarly in West Africa the total plant species count has been estimated to be only a little short of that of neighbouring rainforest. Animal variety is perhaps better known and African savannas are remarkable for their profusion of wildlife. In much of Africa, humans and wildlife have co-existed for thousands of years and, until relatively recently, a reasonable balance has been struck; only over recent decades have major discordances been experienced that threaten the futures of savannas.
Savannas and human evolution
There have been numerous theories on the role of savannas and open landscapes in shaping the emergence and spread of human populations. One line of argument has been that it was the migration of primates from forest areas to savanna woodland and eventually to more open savannas that influenced the physical adaptation and social organization separating early from later humans (hominins). The early humans were hunter-gatherers who ranged over a much larger territory than the primates. The greatest variety of primates lived in Africa and this continent is generally considered to be the origin of the human species. Fossil apes living around ten to nine Mya display a measure of ground adaptation and some groups moved out into more open landscapes, for example acacia woodlands. Humanoids evolved in the late Miocene, seven to five Mya and hominins emerged in the late Tertiary to Quaternary. Homo erectus (considered the first true human) appeared around 1.8 Mya. At this stage there were possibly only some 10,000 individuals throughout Africa where there were likely to have been several points of development. The closest relations of modern humans probably migrated out of Africa five to two Mya.
A further theory argues that it was not simply a matter of physical adaptation to savanna-like conditions but the evolution of social organization and behavioural patterns that allowed humans to move more freely in non-forested environments. There was clearly an intricate web of processes influencing the physiognomy and social character of the early pioneers venturing out into more open landscapes. An interesting line of evidence suggests that the diet of early humans had a grass component, although they lived relatively close to water. The first use of fire has been postulated for a site in Kenya at around 1.6 Mya using evidence from burnt soils. Climate change may also have been a contributing factor with some evidence for a contraction in the forest cover in the late Miocene and Pliocene commensurate with an expansion of grasslands. This would have provided new challenges and opportunities, as suggested by the greater use of tools.
Developments in DNA (deoxyribonucleic acid) sequencing have added a fascinating insight into these evolutionary processes. The genetic record of life is preserved in the genomes (an organism’s complete set of DNA including all of its genes) of living species, which are passed down over generations. This permits tracking of genetic characteristics and can generate phylogenetic trees, or chronological diagrams of evolution. Our understanding of the evolution and migration of humans is likely to be greatly increased in coming years as the sampling intensity increases and more evidence becomes available. The emergence of humans and their gradual transformation of savanna landscapes may have been the outcome of a change in climate and expansion of savanna-like conditions, along with anatomical changes from four to two legs, which freed the use of hands, together with a use of fire and a change of diet, including plant and animal sources (see Chapter 5).
The sequence of this book deals first with the way in which the biome has been shaped by physical features and by the dynamic changes produced by determining environmental and biological processes. The narrative then moves on to a consideration of the differences between the world’s different savanna regions with their varied plant and wildlife and different histories of human occupation. Savannas are being consumed at an unprecedented rate, and the issues of development and conservation are more urgent than ever before. The final chapters examine the ways that people have been shaped by savannas, and have currently settled or utilized their environment, with a consideration of current issues and future directions.