5. Savannas and human evolution

Savannas have helped to shape the evolutionary pageant of human history and the dispersal of our ancestors across the continents. Although there is considerable controversy over the causes and mechanisms of this evolutionary story, ecological, environmental, and genetic evidence suggests that human-like primates first arose in savanna sites within Africa. These habitats offered more open conditions of grassland and woodland that are believed to have stimulated the development of specific physical attributes and social behaviour leading to advanced and mobile societies. ‘Savannas and human evolution’ considers the origins of savannas, the origins and movements of people, and their dispersal and migration around the globe to Australia and the Americas.

1. Savanna landscapes

What are savannas and where are they? ‘Savanna landscapes’ considers these dynamic and diverse biomes—the most widespread form of vegetation in the tropics and sub-tropics. They are areas covered by a nearly continuous grassy layer, interspersed with trees and shrubs of varying densities and heights, but their nature and distribution is affected by the local topography and is closely allied with continental evolution. Marked seasonal variation is characterized by unpredictably long wet or dry spells and there is a wide range of flora and fauna across the major continental savannas. There are numerous theories on the role of savannas in shaping the emergence and spread of human populations.

4. Wildlife and microbes

‘Wildlife and microbes’ looks at energy pathways through the savanna system and the nature of the wildlife characterizing each of the major savanna landscapes, including the woody savannas of the Brazilian cerrado, the East African grasslands, dry and wetland savannas, and the Australian savanna woodlands. This brings into consideration food chains and food webs, as well as aspects of animal ecology that determine the number and character of living organisms. The two main aspects of wildlife to consider are the individual populations of animal species and the way they integrate into communities. Life in all savannas depends upon the complexity of biological interactions between the smallest and the largest organisms.

7. Savanna futures

Constant change is the driving force in the history of savannas, and some conception of the future can be gauged from evidence of past changes and from trends experienced today. The potential futures for savanna landscapes may be depicted as two interconnected sets of processes: variations caused by natural events including climatic change; and changes resulting from human activities. ‘Savanna futures’ shows that the main concerns which could impact on the savannas can be reduced to a number of key issues: coastal savanna areas at risk of flooding and salt-water incursions; food security and food production systems; loss of biodiversity; conservation issues; urban expansion; and human health and security.

6. Changing patterns in the landscape

Relentless population growth worldwide has significantly modified savanna landscapes. ‘Changing patterns in the landscape’ considers how landscape changes differ in Latin America, Africa, and Australasia. The appearance of many savannas has been greatly affected by evolving land use. Many of the landscapes in the New World have only been occupied for relatively short lengths of time and settled only in the most favourable locations. The Old World by contrast, from Africa through India and East Asia to Australia, has experienced a long history of nomadic movement and occupation. The growth of cities in the savanna and the greatly increased pace of urbanization have placed enormous pressures on this land.

2. Shaping the savannas

Savanna landscapes are in a state of continuous change. Some of the determining factors shaping the view at any given locality are relatively short-lived. Fires, storms, seasonal variations, and some wildlife disturbances or human activities last for days, weeks, or a few months. Other factors influence the landscape over much longer stretches of time, such as the development of the underlying soil profile and more gradual processes of climatic change. ‘Shaping the savannas’ considers the drivers of this continuous change: climate; the role of vegetation; the impact of fire and burning–regrowth cycles; the influence of soils; animals and microbes; and humans, the principal determinants of the savanna landscape.

3. Savanna vegetation

Why do savannas differ? Savannas have developed in an uneasy equilibrium over time with varying pressures, leading to an amalgamation of landscape patterns. They are also the product of disturbance and conversion from other ecosystems, especially tropical forests. Each continent has developed a unique collection of plants and animals with a particular environmental evolution and history of human occupation. As a result they have followed different evolutionary pathways leading to distinct landscapes and particular challenges for their futures. ‘Savanna vegetation’ considers the primary productivity of plants, the vegetation structure, and species richness across savannas in Africa, tropical America (including Brazil, Colombia, and Venezuela), Central America and the Caribbean, and Australasia.