Tropical savannas

Tropical regions contain higher species diversity than any other biomes of the world. This is attributed to their extensive area that creates many habitat types, their comparatively stable and warm climate with higher primary productivity, their long growing season, and the long time since they underwent major perturbations. Tropical savannas denote communities with an expanse of grass layer and scattered woody vegetation (shrubs and trees). They are found in tropical and subtropical regions, shaped by a succession of dry and wet seasons that differentiate them from other biomes. Tropical savannas cover about 10,656,420 square miles (27.6 million square kilometers), approximately 12.5 percent of the global land mass. They comprise roughly 40 percent of Africa, and they are also represented in parts of Australia, South America, and Asia.

Thus, savanna ecology is influenced by both grasses and woody plants, generally warmer temperatures throughout the year, and two distinct dry seasons. Summers are wet, hot, and humid with most rainfall during this period 15–25 inches (381–638 millimeters), while the dry season lasts for a longer period of the year (up to eight months) with warmer temperatures. However, the amount of rainfall and humidity in the savannas are not sufficient to support forest growth. Furthermore, high evaporative demands as a result of intense illumination cause savannas to experience a net water deficit for most of the year. Tropical savannas are maintained by complex and dynamic interactions among climatic factors, topography, edaphic factors, geomorphology, herbivory (grazing and browsing), fire, and human activities.

Savannas are generally categorized based on their canopy cover, spatial layout of woody plants, and stature. One of the resultant categories is savanna grassland consisting of sparsely scattered trees or shrubs. On the other hand, savanna parkland comprises discrete patches of woody vegetation interspersed over a continuous herbaceous plant layer. Due to various abiotic influences such as soil, altitude, and topography, savannas are interrupted by bands or areas of woodlands, forests (riparian, groundwater, or montane forests) or wetlands forming what are popularly called savanna landscapes.

Tropical savannas are known to be subclimax communities that are maintained by the soil characteristics (edaphic subclimax savannas), grazing (biotic subclimax savannas), and fire (pyric subclimax savannas). Termites are especially abundant in tropical savannas of the world, and their tall termitaria are conspicuous elements of the savanna landscape. Termites are important in soil formation and their termitaria provide shelter for other animals. Termites are also important food sources for anteaters (endemics of the neotropical zoogeographic province) and aardvarks and pangolins in African savannas.

Tropical savannas are inhabited by the world's proportionately large and rapidly growing human population, and a majority of their rangelands are dominated by livestock. Large population size associated with the prevalence of poverty increases the demands for immediate environmental resources. Overharvesting of timber and nontimber products, introduction of exotic species of plants and animals, overstocking, deliberate massive killing of wildlife, and fire suppression in some areas have significantly degraded tropical savannas, changing their scenery. There is likelihood that with climate change impacts, tropical savanna might be completely replaced by a different landscape.

African Tropical Savannas

African savannas are grassland-dominated vegetation mixed with scattered and isolated trees found between latitudes 15 degrees north and 30 degrees south and longitudes of 15 degrees west and 40 degrees east. Savannas in Africa are found in several countries including Guinea Bissau, Sierra Leone, Liberia, Ghana, Somalia, Nigeria, Ethiopia, Ivory Coast, Benin, Togo, Central African Republic, Cameroon, Chad, Tanzania, Sudan, Democratic Republic of Congo, Ethiopia, Kenya, Angola, Malawi, Burundi, Uganda, Rwanda, Zimbabwe, Zambia, Mozambique, Botswana, Madagascar, and South Africa. The savannas occupy areas such as the Maasai steppe ecosystems in northern Tanzania and southern Kenya within the Somali-Maasai regional center of endemism. Savannas in Africa also include the widely spread miombo (mopane) woodlands spanning from southwestern Tanzania to Zambia, Malawi, Mozambique, Zimbabwe, parts of Botswana and Namibia, and Angola. Miombo is an extensive tropical savanna falling mostly under the Zambezian regional center of endemism, and mostly characterized by such plant species in the genera Isoberlina, Jubernadia, and Brachystegia.

Flora and Fauna

In terms of plant species, savannas in Africa are characterized by such species as Acacia, miombo/mopane, candelabra trees, baobabs (Adansonia digitata), sausage trees (Kigelia africana), amarula plants (Sclerocarya birrea), Combretum species, star grass, elephant grass, Bermuda grass, and many more.

African savannas are known for their diversity of both invertebrates and vertebrates. Millions of animals are known to coexist and are spatially or temporally specialized to environmental resources in savannas. There are carnivores, including cats (lions, leopards, cheetahs, serval cats, caracals, and civets); dogs such as wild dogs and jackals, hyenids (aardwolves, spotted and striped hyenas); and mustelids (honey badgers). Herbivores include elephants, hippopotamuses, kudus (greater and lesser), giraffes, elands, topis, sable antelopes and roan antelopes, hartebeests, black and white rhinoceroses, wildebeests, gazelles, impalas, gerenuks, aardvarks, waterbucks, bushbucks, reedbucks, buffalo, zebras, oryx, dik diks, lemurs, and sunis. These herbivores utilize different parts and types of plants, thus reducing competition, and hence increasing coexistence between them.

There are also a large number of birds such as ostriches, vultures, chaffinch, doves, pigeons, eagles, shrikes, starlings, shoebills, storks, ducks, hammerkops, hornbills, and many others. Reptiles include snakes (venomous and nonvenomous), crocodiles, lizards, and tortoises. There are also amphibians such as frogs and toads, and many invertebrates, with insects accounting for the majority. Biota in African savannas interacts in a complex way and produces a complex food web. Some animal species such as lions, elephants, beetles, and termites are keystone species maintaining diversity of the biomes. For example, elephants play the roles of ecosystem engineers, maintaining and creating vegetation structure, which increases usable habitat for other species.

Economy and Culture

Wildlife abundance and the multitude of culture from the people residing in these areas make them attractive destinations for cultural tourism. Bushmen (San in Namibia and South Africa, Baswara in Botswana, and Hadzabe in Tanzania) and pastoralists (Maasai, Tutsis, and Ankole) persistently maintain their traditional ways of life. Other communities are composed of agropastoralists (peasants and small-scale livestock keepers). Preferred types of crops are maize, beans, peas, wheat, millet, cassava, coconuts, groundnuts, sugarcane, and cashew nuts, and the common livestock are an indigenous breed of cattle, sheep, and goats.

Threats to African Savannas

The majority of community livelihood in African savannas relies on the use of the immediate environmental resources for survival. People in these areas lack alternative livelihoods, which places direct pressure on natural biological resources, posing a greater threat to their survival. Indigenous trees are unsustainably harvested for construction, fuelwood, and charcoal production, while animals are illegally overhunted or captured for food and trade to satisfy the desires of the growing population in nearby towns and abroad for food and trophies. The tragedy of the ivory trade in recent years has increasingly jeopardized the survival of elephants.

Human-wildlife conflicts are prevalent in the African savannas. Conflicts between pastoralists on one side and African wild dogs and cheetahs on the other side have driven these wildlife species to the danger of extinction. On the other hand, contact between wildlife and domestic animals creates an outbreak of zoonotic diseases such as rabies, canine distemper, and anthrax, claiming the life of many wildlife, domestic animals, and human beings. Trampling by moving domestic animals creates loose and bare soils, which erode easily during rainy seasons but also suppress fire recurrence. The growing human population requires large areas for expansion of settlement and agriculture; this creates encroachment into protected areas and blocks wildlife migratory corridors and dispersal areas. Ultimately areas with wild animals (for example, protected areas) remain as ecological islands in the sea of human-modified landscapes.

Climate change and associated impacts invariably affect tropical African savannas resulting in changed climatic patterns in some areas. Frequent drought occurrences in the Horn of Africa (Kenya, Sudan, Ethiopia, and Somalia) dramatically reduce food production for humans and kill livestock (a major asset for most people) as a result of the loss of pasture and water.

Conservation Status

African savannas comprise major protected areas of the world, differing in their conservation status, ranging from fenced national parks such as Kruger National Park in South Africa to the free-ranging wildlife in the major natural ecosystems such as the Serengeti in Tanzania. Other notable protected areas falling under the African savannas include Selous, Rungwa, and Moyowosi Game Reserves; Ruaha and Mikumi National Parks in Tanzania; Maasai Mara, Amboseli, and Tsavo National Parks in Kenya; Luangwa Valley in Zambia; Gonarezue in Zimbabwe; and the Great Limpopo Transfrontier Park straddling the borders between Mozambique, Zimbabwe, and South Africa. These areas preserve pristine habitats for wildlife and play a crucial role as representative areas for biodiversity conservation in the region annually visited by millions of tourists. Many countries in this region are members of international conservation treaties regulating management of biological resources. These treaties include the International Union for Conservation of Nature (IUCN), Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), Convention on Biological Diversity (CBD), and the Ramsar treaty for wetland management.

South American Tropical Savannas

Tropical savanna also extends to South America in llanos and cerrado, but with relatively few species existing. Savannas occupy almost 965,255 square miles (2.5 million square kilometers) of Brazil, Colombia, and Venezuela, an area equivalent to one-quarter the size of Canada. Animals dispersing from the neighboring biomes play a key role in maintaining diversity and persistence of this region.

Llanos in South America

Flooded annually by the Orinoco River, the llanos of the Orinoco Basin of Venezuela and Columbia contain plants adapted to growing for long periods in stagnant waters. They span in a northeast direction from the foothills of the Andes Mountains in Colombia along the Orinoco River to its delta in the Atlantic Ocean. The vegetation in these areas is dominated by grass species in the genera Melinis, Panicum, Homolepis, Scleria, Paspalum, Trachipogon, Gymnopogon, and Axonopus with shrub species in the genus Casia. Fauna in the llanos includes capybara and marsh deer (mammal species adapted to a semi-aquatic life). The area also harbors the Orinoco crocodile, an endangered endemic crocodile species. Other animals include the world's extant largest boa anaconda, Orinoco turtle, giant armadillo, giant otter, black and chestnut eagle, swallow-tailed kite, broad-winged hawk, neotropical migratory shorebirds, and many species of cat fishes.

Brazilian Cerrado

The Brazilian Cerrado is an extensive open woodland savanna of short twisted trees composing 21 percent of the country. The Cerrado savanna extends from the Amazonian forest margin to outlying areas in the southern states of Sao Paulo and Parana, to Bolivia and Paraguay. The Cerrado is a moister tropical savanna with rainfall ranging from about 31 to 79 inches (800 to 2,000 millimeters), with vegetation less adapted to waterlogging compared to its northern counterparts, the llano of Venezuela and Columbia. The Cerrado has several endemic plants and animals, and is a biodiversity hot spot consisting largely of savanna ecosystem. Plants in the Cerrado are estimated at around 5,000 species, with dominant grassy level vegetation five to seven times the tree and shrub layer. Similar to other savanna, it is characterized by remarkably long dry seasons supporting drought and fire-adapted plant species. Common genera are Anisacanthus, Dicliptera, Geissomeria, Hygrophila, Jacobinia, Justicia, Lophoslachys, Mendoncia, Poikilancathus, Ruellia, Stauurogyne, Stenandrium, and Thunbergia.

The Cerrado is characterized by indigenous animal species such as edentates (tamanduas, anteaters, and armadillos), marsupials (opposums), latyrrhine monkeys (marmosets, howlers, and capuchins), rodents (agoutis, pacas, capybaras, and many mouse-sized species) and maned wolf. The Cerrado has great diversity of animal life, though not in comparison to its counterpart in Africa.

Economy, Culture, and Threats in the Cerrado

In the past, the Cerrado domain was sparsely populated by backwoodsmen and Indians, typically Brazilian countrymen, while llanos were shielded from human settlement and other threats. In most cases the countrymen kept livestock, cultivated crops, and utilized natural resources on a subsistence scale. In the past three decades, neoliberal changes in government policies have commercialized agriculture in the Cerrado leading to encroachment of many areas for farming. Crops like soya, maize, rice, and cassava are commonly preferred. Invasive plant species have increased in number as a result of tree plantation and livestock fodder improvement. Construction of Brazil's new capital city (Brasilia) and its associated development also created demand for the growing urban population, which increased pressure on natural resources. With livestock numbering around 48 million, it is estimated that 35 percent of the Cerrado has been destroyed.

Another threat to Brazilian savannas comes from the Brazilian steel industry, which solely depends on charcoal as source of energy. This places a remarkably high pressure on trees and is the second-largest threat after agriculture. The industry gets 71 percent of its charcoal from native trees. On the other hand, expansion of agriculture has led to dramatic changes in water use in llanos, diverting water to irrigation areas and drying up wetlands. Crops commonly cultivated are palm oil and rice, leading to fragmentation of the once continuous ecosystem. Oil exploration and extraction together with construction of dikes and water channels exacerbate habitat fragmentation. Furthermore, agrotoxins poison the ecosystem, leading to fatal consequences.

Conservation Status in the Cerrado

The value of the biodiversity-rich Cerrado and llano has always been undermined. When compared to other biomes, Cerrado is considered to be poor, and not even considered by the Brazilian constitution as a national heritage. Although only 1 percent of the area is currently conserved, local conservation organizations collaborating with the World Wildlife Fund (WWF) are advocating for conserving and rescuing the biome. In 2022, the government of Columbia, WWF, and other organizations announced a $245 million agreement to protect 30 percent of the land and 30 percent of the seas in Columbia by 2030.

Australian Tropical Savannas

Australia's tropical savannas are landscapes of dense grass and scattered trees covering approximately one-quarter of the continent, about 733,594 square miles (1.9 million square kilometers). They cross the continent from Rockhampton on the east coast, across the Gulf of Carpentaria, to the Top End of the Northern Territory, and over to The Kimberley in western Australia. Australian savannas fall within the tropical latitudinal ranges 10 degrees to 20 degrees south with medium to low mean annual rainfall (16 to 47 inches or 400 to 1,200 millimeters). Climates are characterized by two distinct seasons: the wet and the dry, with high daytime temperatures and high evapotranspiration rates. The dry season lasts five to six months, usually from May to October. The wet season lasts five to six months from December to March. Australia's tropical savannas are also referred to as the monsoonal tropics and the wet-dry tropics. The soils of Australia's tropical savannas are generally poor, with the exception of the southeast sector of the tropical savannas, which is more fertile.

Australian Fauna and Flora

Australian tropical savanna is one of the major biodiversity reservoirs which is less intensively developed compared to the temperate areas of the world, and this accounts for its relatively intact habitat compared to that of southern Australia. The savanna is home to hundreds of species of native plants, mammals, birds, reptiles, and amphibians and tens of thousands of different species of invertebrates. Many species in these groups are found nowhere else in the world. This part of Australia also has World Heritage areas like Kakadu National Park and Purnululu National Park.

Australian savannas are dominated by marsupials, mammals whose young are born undeveloped. These animals include the echidna, eastern gray kangaroo, the koala agile and whiptail wallabies, wallaroos, possums, gliders, the northern quoll, and the golden bandicoot. These animals live in or near the few trees in this area, utilizing them for shade, food, and water. Reptiles dominate in the other parts of the tropical savannas. The saltwater crocodile, which is found here, is the world's largest reptile, growing up to 26 feet (8 meters long). Associated with these are many species of birds, amphibians, and invertebrates, many of which are endemic to the region.

Open woodlands with grassy understory are the major vegetation type in Australia's tropical savannas. Eucalyptus trees of restricted height (49 to 82 feet or 15 to 25 meters) and open canopy dominate Australia's tropical savanna woodlands in areas receiving over 23 inches (600 millimeters) mean annual rainfall, with acacia or Casuarina species inhabiting the drier, and less fertile, areas. Eucalyptus species grow in warm climates and are the most important tree for timber, oil, and gum in Australia.

Australian Socioeconomic Activities

Australian savannas have a rich culture from the Aboriginal people, who have a long association with the land and much traditional knowledge of land management. The didgeridu wind instrument and the band Yothu Yindi come from the tropical savannas. Tourists and locals also value the Australian tropical savannas for their wild and spectacular natural features. The main areas of employment include the livestock, conservation, tourism, mining, and horticultural industries. Until a few decades ago, livestock was the main economic base of the tropical savannas, accompanied by lesser but more intensive uses such as mining, agriculture, or urban development.

More recently, however, mining and tourism have become the dominant economic industries. Like many other savanna ecosystems, Australian savannas are home to a pastoral industry that includes some of the most extensive operations in Australia and manages the largest area of land of any group in the northern part of the country. The livestock industry is the major user and manager of land in the tropical savannas. It contributes hundreds of millions of dollars to regional economies across northern Australia and provides employment for thousands, both directly and indirectly via service industries

Australian tropical savanna is experiencing significant population pressure and land-use changes and is vulnerable to rapid land degradation. Other threats include habitat fragmentation, deforestation, climate change and variability, as well as frequent fires.

Asian Tropical Savannas (Terai-Duar Savanna)

Asian savannas are estimated to cover an area of 13,400 square miles (34,706 square kilometers) spreading through Nepal, India, and Bhutan. These savannas are dominated by subtropical grasslands and shrublands in the Terai-Duar Savanna. The Terai or “moist land” is actually the belt of marshy grassland, savanna, and forest located at the base of the Himalayas, about 15.5 miles (25 kilometers) wide, but it is very long. The area is mainly found in the semi-arid and semi-humid climate regions of subtropical and tropical latitudes. Rainfall ranges between 12 and 60 inches (300 and 1500 millimeters) annually, with climate significantly influenced by the topography, especially the great complex of mountains that flanks the Himalayas.

The Asian savannas have originated over generations from woodland ecosystems through deforestation, abandoned cultivation, and burning and are maintained at a subclimax by repeated grazing and burning. These mixed forces have led to the formation of mosaic types of savanna communities, depending upon the age, mode of origin, and the intensity of biotic disturbance. Savanna communities at different seral stages tend to differ in their biotic composition (that is, species composition, productive potential, and nutrient cycling).

Asian Fauna and Flora

The Asian savannas are home to diverse and abundant invertebrates and vertebrates. These include diverse amphibians, birds, and reptiles such as the gharial crocodile (Gavialis gangeticus). There are also mammals, including tigers (Panthera tigris), pygmy hogs (Sus salvinius), swamp deer or barasinghas (Cervus duvauceli), and the greater one-horned rhinoceros (Rhinoceros unicornis). Asian savannas are home to three endemic birds: the spiny babbler (Turdoides nipalensis), gray-crowned prinia (Prinia cinereocapilla), and Manipur bush-quail (Perdicula manipurensis).

There are a number of plant species in the region including wild sugarcane (Saccharum spontaneum), 10-foot tall tropical reeds (Phragmitis kharka), and silky kangaroo grass (Themeda villosa). Smaller species include cranberry grass (Imperata cylindrical) and six-weeks three-awn (Aristida adscensionis). Many of the grass species found in the Terai-Duar Savanna serve as fodder for livestock and wildlife species such as elephants and rhinoceroses, as well as cover for the predatory species such as tigers.

Agriculture is the principal economic activity in the region. Other major employment sectors include livestock keeping, nature-based tourism, cultural tourism, and natural resources. The main threats to the region include poaching for wildlife species, overgrazing, clearing for cultivation, population growth, irrigation projects, and water diversion.

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