Savannas are a type of grassland, flat with scattered and widely spaced trees. They are found throughout the world in both temperate and tropical regions and can be wet or dry. Though savanna biomes are unfamiliar to many folks, they make up about 20% of Earth’s land, according to Science News Explores.
Savanna compared to forest and grassland.
A savanna or savannah is a mixed woodland-grassland biome and ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. Savannas maintain an open canopy despite a high tree density. It is often believed that savannas feature widely spaced, scattered trees. Savannas are also characterised by seasonal water availability, with the majority of rainfall confined to one season.
What is a Savanna?
Savanna, vegetation type that grows under hot, seasonally dry climatic conditions and is characterized by an open tree canopy (i.e., scattered trees) above a continuous grass understory (the vegetation layer between the forest canopy and the ground). The largest areas of savanna are found in Africa, South America, Australia, India, the Myanmar-Thailand region in Asia, and Madagascar.
Origin of Savannas
Savannas arose as rainfall progressively lessened in the edges of the tropics during the Cenozoic Era (66 million years ago to the present)-in particular, during the past 25 million years. Grasses, the dominant plants of savannas, appeared only about 50 million years ago, although it is possible that some savanna-like vegetation lacking grasses occurred earlier.
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The South American fossil record provides evidence of a well-developed vegetation, rich in grass and thought to be equivalent to modern savanna, being established by the early Miocene Epoch, about 20 million years ago. Climates across the world became steadily cooler during that period. Lower ocean surface temperatures reduced water evaporation, which slowed the whole hydrologic cycle, with less cloud formation and precipitation.
The vegetation of midlatitude regions, lying between the wet equatorial areas and the moist cool temperate zones, was affected substantially. The main regions in which savannas emerged in response to that long-term climatic change-tropical America, Africa, South Asia, and Australia-were already separated from each other by ocean barriers by that time. Plant migration across those barriers was inhibited, and the details of the emergence of savannas on each continent varied. In each region different plant and animal species evolved to occupy the new seasonally dry habitats.
In temperate regions, savannas became much more widespread, at the expense of forests, during the long, cool, dry intervals-contemporaneous with the ice ages, or glacial intervals, of the Pleistocene Epoch (2.6 million to 11,700 years ago). When human beings (Homo sapiens) first appeared, in Africa, they initially occupied the savanna. Later, as they became more adept at modifying the environment to suit their needs, they spread to Asia, Australia, and the Americas. There their impact on the nature and development of savanna vegetation was superimposed on the natural pattern, adding to the variation seen among savanna types.
The savannas of the world currently are undergoing another phase of change as modern expansion of the human population impinges on the vegetation and fauna.
Types of Savannas
- Tropical and subtropical savannas: Classified with tropical and subtropical grasslands and shrublands as the tropical and subtropical grasslands, savannas, and shrublands biome. The savannas of Africa, including the Serengeti, famous for its wildlife, are typical of this type.
- Temperate savannas: Mid-latitude savannas with wetter summers and drier winters, classified with temperate savannas and shrublands as the temperate grasslands, savannas, and shrublands biome. They cover much of the plains of southeastern Australia, northern India, Southern Africa, southeastern Argentina and Uruguay.
- Mediterranean savannas: Mid-latitude savannas in Mediterranean climate regions, with mild, rainy winters and hot, dry summers, part of the Mediterranean forests, woodlands, and scrub biome. The oak tree savannas of California fall into this category, including the Temperate Grassland of South Australia, which features eucalyptuses.
- Flooded savannas: Savannas that are flooded seasonally or year-round, classified with flooded savannas as the flooded grasslands and savannas biome, which occurs mostly in the tropics and subtropics.
- Montane savannas: Mid- to high-altitude savannas, located in a few spots around the world's high mountain regions, part of the montane grasslands and shrublands biome.
Climate and Geography
The open spaces of the world’s savannas are found mostly in the tropical areas of the globe. Savannas occur where there isn’t enough rain to support a tropical rainforest, but there is enough to keep it from becoming a desert. There is usually a dry season and a rainy season in the savanna, with strong, hot winds in the dry season and enough rain in the wet season to flood low-lying areas.
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The climate of a savanna is characterized by two distinct seasons: dry (winter) and wet (summer). Interestingly enough, the winter is drier with about 4 inches of rainfall, while the summer receives the most rainfall of around 15 to 25 inches. The time of each season varies depending on the location, but the dry season (winter) is typically longer than the wet one. The savanna temperature in summer ranges from 68°F to 86°F on average, making the weather muggy and unpleasant.
The annual rainfall ranges from 500 mm (19.69 in) to 1,270 mm (50.00 in) per year, with the precipitation being more common in six or eight months of the year, followed by a period of drought.
Flora and Fauna
Savanna vegetation is a cross between a tropical rainforest and a desert ecosystem. These biomes are covered with short and tall grasses, low-lying shrubs, and scattered large trees. For instance, some grass species come with sharp blades or a bitter taste to prevent herbivores from eating them. Long root systems also allow them to suck underground water. Trees also have their way to fit into this savanna habitat. They develop thick, rough bark to withstand wildfires and specialized trunks to store water for an extended time.
In the East African savannas, Acacia, Combretum, baobabs, Borassus, and Euphorbia are a common vegetation genera. Drier savannas there feature spiny shrubs and grasses, such as Andropogon, Hyparrhenia, and Themeda. Wetter savannas include Brachystegia trees and Pennisetum purpureum, and elephant grass type. West African savanna trees include Anogeissus, Combretum, and Strychnos.
Animals in the African savanna generally include the giraffe, elephant, buffalo, zebra, gnu, hippopotamus, rhinoceros, and antelope, where they rely on grass and/or tree foliage to survive.
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Savanna biomes are heaven for birdlife, especially birds of prey like eagles, buzzards, vultures, and hawks. Animals there typically have long legs for long migrations or foraging in times of scarcity. To preserve water, most of them don’t sweat but pant or release body heat through their skin and ears.
Savannas are home to both carnivores and herbivores. Herds of grazers, such as zebras, elephants, wildebeests, giraffes, gazelles, ostriches, and buffalo, are prominent in these regions. Additionally, rodents like rats and mice inhabit these vast grasslands. Savannas can’t lack predatory animals like lions, cheetahs, jackals, leopards, and hyenas.
Here is a table summarizing the common flora and fauna found in African savannas:
| Category | Examples |
|---|---|
| Trees | Acacia, Baobab, Combretum |
| Grasses | Andropogon, Hyparrhenia, Themeda |
| Herbivores | Zebras, Elephants, Wildebeests, Giraffes, Gazelles, Buffalo |
| Carnivores | Lions, Cheetahs, Jackals, Leopards, Hyenas |
| Birds of Prey | Eagles, Buzzards, Vultures, Hawks |
Great Migration/ Battle for Survival/ Wildebeest Crossing River Crocodile - Serengeti National Park
The Serengeti plain is famous for the Great Migration, the world’s largest mammal migration, with over 1.5 million mammals.
The Great Migration in the Serengeti National Park.
The Role of Wildfires
Wildfires usually occur in savanna biomes for many reasons. In an extended dry season, dry grasses and shrubs can easily catch fire. Contrary to common belief, wildfires do more good than harm for savanna biomes. Large trees, as mentioned, can survive the fires thanks to their thick, fire-resistant bark. Animals can run or fly away from the fires easily, so nothing will be left behind. The only victims are small insects, but their population is too crowded to be completely extinct.
Savannas are subject to regular wildfires and the ecosystem appears to be the result of human use of fire. Intentional controlled burns typically create fires confined to the herbaceous layer that do little long term damage to mature trees. This prevents more catastrophic wildfires that could do much more damage. However, these fires either kill or suppress tree seedlings, thus preventing the establishment of a continuous tree canopy which would prevent further grass growth.
Threats to Savannas
Around the world, savannas are threatened by human actions like logging, development, conversion to agriculture, over-grazing by livestock, and introduction of non-native plant species. Human actions are one of the biggest threats to savannas. As more people move into areas near or within savannas, they clear land to build homes, roads, and farms. Farming also often involves using machines and chemicals that damage the soil and nearby water sources. In some places, overgrazing by livestock leads to grasslands being destroyed, leaving the soil bare and dry.
Climate change is causing big shifts in the weather patterns of savannas. This makes it more difficult for grasses, trees, and animals to survive. Waterholes dry up faster, leaving animals with fewer places to drink.
Human Impact and Conservation
Prior to European settlement aboriginal land use practices, including fire, influenced vegetation and may have maintained and modified savanna flora. It has been suggested by many authors that aboriginal burning created a structurally more open savanna landscape.
The removal of grass by grazing affects the woody plant component of woodland systems in two major ways. Grasses compete with woody plants for water in the topsoil and removal by grazing reduces this competitive effect, potentially boosting tree growth. In addition to this effect, the removal of fuel reduces both the intensity and the frequency of fires which may control woody plant species.
Grazing animals can have a more direct effect on woody plants by the browsing of palatable woody species. Introduced grazing animals can also affect soil condition through physical compaction and break-up of the soil caused by the hooves of animals and through the erosion effects caused by the removal of protective plant cover. Such effects are most likely to occur on land subjected to repeated and heavy grazing.
Large areas of Australian and South American savannas have been cleared of trees, and this clearing continues today. Clearing is carried out by the grazing industry in an attempt to increase the quality and quantity of feed available for stock and to improve the management of livestock.
A number of exotic plants species have been introduced to savannas around the world. Amongst the woody plant species are serious environmental weeds such as prickly acacia (Acacia nilotica), Rubbervine (Cryptostegia grandiflora), Mesquite (Prosopis spp.), Lantana (Lantana camara and L. montevidensis) and Prickly Pear (Opuntia spp.).
Human induced climate change resulting from the greenhouse effect may result in an alteration of the structure and function of savannas. Some authors have suggested that savannas and grasslands may become even more susceptible to woody plant encroachment as a result of greenhouse induced climate change.
