Serengeti volcanic grasslands

Topics:

Conservation Biology (main) Biodiversity (main) Ecology (main)

Serengeti National Park, Tanzania Photograph by John Morrison

The world-renowned Serengeti Volcanic Grasslands ecoregion is a stage for some of the most spectacular mass game migrations in the world. Although populations fluctuate, there are an estimated 1.3 million blue wildebeest (Connochaetes taurinus), 200,000 plains zebra (Equus burchelli), and 400,000 Thomson’s gazelle (Gazella thomsoni) migrating between this ecoregion and the Southern Acacia-Commiphora bushlands and thickets ecoregion each year. The migrations continue despite a devastating rinderpest outbreak in the late 19th century and indiscriminate hunting by European settlers in the early 20th century. The area represents the last remnant of a large mammal dominated ecosystem which has existed in African at least since the Pleistocene. It is relatively well protected within National Parks and Game Reserves, but populations of black rhinoceros have been almost extirpated by illegal hunting, and there is some evidence that illegal hunting of animals for meat is increasing.

Location and General Description

The Serengeti Volcanic Grasslands lie just south of the Tanzanian/Kenyan border, close to the equator (between 2° and 4° S). Climatically, the ecoregion falls within the seasonal tropics. Mean maximum temperatures are between 24° to 27°Celsius (C), and mean minimum temperatures between 15° to 21°C. Mean annual rainfall in the Serengeti varies from 1,050 milimeters (mm) in the northwest to 550 mm in the southeast. This rainfall is strongly seasonal, with peaks between March to May, and between November and December. Rainfall is the main determinant of vegetation growth, and hence ungulate food supply.

Wildebeest crossing the Samara River in the north Serengeti. @ C.Michael hogan

The underlying soils and materials of the Serengeti plains are comprised of volcanic ash derived from a number of local volcanoes. The dormant caldera of Ngorongoro, the Kerimasi Volcano and Mount Lengai (last eruption in 1966) have all contributed volcanic ash to these soils (vertisols). These soils have characteristic plant communities, distinguishing the ecoregion from its neighbours. Topographically, the ecoregion is comprised of flat to slightly undulating grassy plains, interrupted by scattered rocky areas (Kopjes) which are parts of the Precambrian basement rocks protruding through the ash layers.

The ecoregion is classified as part of the Somali-Masai regional center of plant endemism, and covers the short grassland portion of the Greater Serengeti ecosystem. Different plant species predominate depending on depth, stability and age of the underlying ash. Among the dominant species on the dunes and short and intermediate grasslands are a variety of Sporobolus spp., Pennisetum mezianum, Eragrostis tenuifolia, Andropogon greenwayi, Panicum coloratum, Cynodon dactylon, Chloris gayana, Dactyloctenium sp., Digitaria macroblephara, along with sedges of the genus Kyllinga. In periods of severe drought, the grasslands become virtually denuded of standing vegetation.

Biodiversity Features

The Serengeti Volcanic Grasslands are vital to the cyclical movement of millions of large mammals in the region. Although populations fluctuate, there are an estimated 1.3 million Blue wildebeest (Connochaetes taurinus), 200,000 Plains zebra (Equus burchelli), and 400,000 Thomson’s gazelle (Gazella thomsoni) migrating between this ecoregion and the Acacia-Commiphora Bushland and Thicket ecoregion each year. A large number of associated mammalian predators are also involved in these movements. By the onset of the dry season (late May), the grasses on these plains have either dried out or been bitten down to stubble, exacerbated by scarcity of water. This phenomenon triggers the massive migration of wildebeest and zebra, later followed by Thomson’s gazelle and Eland (Taurotragus oryx), from the plains to the Acacia-Commiphora woodlands. At the beginning of the wet season, these animals complete the cycle, and return to the plains.

Lion (Panthera leo), with wildebeest (Connochaetes taurinus) kill, Tanzania (Photograph by Elizabeth Ruml)

Faunal endemism here is low. There are no strict or near-endemic mammals or amphibians. There is only one strictly endemic reptile, Lygodactylus grzimeki, with Mann’s dwarf gecko (Lygodactylus manni) being the next most restricted species as this occurs in only two ecoregions. The Serengeti covers part of an Endemic Bird Area, but although five following restricted range species may occur they are more linked to the adjacent portions of the Southern (and some Northern) Acacia-Commiphora bushland and thicket ecoregions. The relevant bird species are the Rufous-tailed weaver (Histurgops ruficauda (a montypic genus), Usambaro barbet (Trachyphonus usambiro), greycrested helmet shrike (Prionops poliolophus), Grey-breasted spurfowl (Francolinus rufopictus), and Fischer’s lovebird (Agapornis fischeri).

The Black Rhinoceros (Diceros bicornis, CR) used to occur in fairly sizeable numbers in the ecoregion, but has been almost extirpirted by poaching for its horn, produced by funding from Chinese sources to entice poor natives to perform the killing; the horn is prized by many Chinese people as a supposed instrument of sexual empowerment medication. A small population still survives on the floor of the Ngorongoro Crater, just outside the ecoregion border; a more geologically accurate depiction of the crater is the Ngorongoro Caldera.

Painted hunting dog (Lycaon pictus), listed as endangered by the IUCN, disappeared from Serengeti National Park in 1991. While a rabies epidemic killed three of the packs, the full cause of the disappearance remains contentious. Despite the loss of Painted hunting dog from the ecoregion, the area still has a wide array of mammalian predators including cheetah (Acinonyx jubatus), lion (Panthera leo), leopard (P. pardus), Spotted and Striped hyaena (Crocuta crocuta, Hyaena hyaena), side-striped (Canis adustus), Golden (C. aureus) and Black-backed jackal (C. mesomelas), Honey badger (Mellivora capensis), Caracal (Felis caracal), Serval (F. serval), wild cat (F. sylvestris), bat-eared fox (Otocyon megalotis), African civet (Civettictis civetta), and a number of genet and mongoose species. Avian predators are also relatively plentiful, with Serengeti National Park having thirty four raptor species, and six species of vulture. A variety of smaller predators including snakes, lizards, spiders and scorpions also occur in the Serengeti.

Thomson's gazelle (Gazella rufifrons). @ David Olson

Contrary to popular belief, the Serengeti’s large predators account for no more than one-third of all deaths among the migratory herds. Thousands of years of predator-prey coexistence have resulted in a number of anti-predator life history traits among prey species. Many live in large herds, to reduce individual chance of falling victim to predators. Some animals, such as Thomson’s and Grant’s gazelles (Gazella granti) may hide their young until they are strong enough to flee an attack. Most large prey species living in big herds with precocial young (e.g. Wildebeest, Topi (Damaliscus lunatus) and Cape buffalo (Syncerus caffer)) display highly synchronous birthing seasons. Most large predators are territorial, and only hunt until satiated. Since the young are at their most vulnerable in the first month or so of their lives, the chances of being killed are much lower for each calf if they are all born within a short period. Calves of these species born outside of the synchronized birth period seldom survive.

Several species occurring here are of international importance because of their abundance, including Eland, Waterbuck (Kobus ellipsiprymnus), wildebeest, Coke’s hartebeest (Alcelaphus buselaphus), Topi, Impala (Aepyceros melampus), Grant’s and Thompson’s gazelles, zebra and buffalo. A European population of white stork (Ciconia ciconia) has a major wintering ground here, and the Nile crocodile (Crocodylus niloticus), while persecuted out of reserve areas, is well protected within Serengeti National Park.

Serengeti scene, Tanzania. @ Elizabeth Ruml

With so many large herbivores, present, grazing is clearly a significant disturbance in this ecoregion. Different herbivores tend to feed on different graze species and components, enabling grazing sequences by different ungulate species at the landscape scale. For example, Grant’s gazelle prefer herbs and shrub foliage, wildebeest usually feed on a wide variety of nutritious, short grasses, and topi tend to eat long grass leaves. Studies here have shown that when ungulate grazing is removed, plant species composition and growth form changes relative to those under grazing, and that some important grass species (e.g., Andropogon greenwayi) eventually disappear in the absence of this disturbance. Other studies have found that the grasses found in grazed patches are more productive than ungrazed patches, and that these patches are able to support larger concentrations of herbivores than stocking rates would suggest.

Fires, usually set by humans, are probably also an important disturbance in this ecoregion. Certain species, including Thomson’s and Grant’s gazelles, impala and wildebeest have been seen to favor grazing on the green flush that emerges after burning. Although some research is being carried out by the Frankfurt Zoological Society (FZS), relatively little is known of the ecology of this disturbance.

This area is also of great interest and importance in terms of human evolution. The ecoregion borders the Olduvai Gorge, site of the discovery of the 1.75 million-year-old remains of Australopithecus boisei and Homo habilis by Dr. Louis and Mrs. Mary Leakey.

Current Status

Much of the ecoregion’s habitat occurs within protected areas, most of which are joined into a continuous habitat block. The protected areas network includes parts of Serengeti National Park (SNP) and Ngorongoro Conservation Area, both of which have been designated as World Heritage Sites and Biosphere Reserves. The protected area network may be sufficiently large to promote the survival of most habitat many of its biodiversity values. There has been little loss of habitat within the protected areas, except for small areas used for tourist hotels. Areas outside of these protected areas have, however, experienced a rapid expansion of human settlement and agricultural development in recent years. Although the Painted hunting dog has recently disappeared from the protected areas of this ecoregion, the wide-ranging behavior of this species leaves open the possibility for recolonization. If their safety could be assured, the Black rhinoceros could also have its numbers enlarged in the Serengeti by careful introductions; however, this can only be done, once ranger staffing is sufficient to control illegal poaching. In the northern Tanzania portion of the Serengeti, a large number of Zebra and other large mammals are hunted strictly for meat consumption by indigneous tribal people; some of this killing is strictly illegal, and other takings are just ignored by rangers.

Types and Severity of Threats

While Masai pastoralists occupy the Ngorongoro Conservation Area, there are no tribal people living within the Serengeti National Park. However, the western frontier of this Park has a dense resident population, growing at about four percent a year. Not only is the human population increasing, but there is also a concomitant increase in livestock numbers, and much of the area is being converted into cropland, while the human demand for land rises. Although agriculture is the main source of income, many people have been attracted to the area by the wildlife resources and tourism opportunities the park presents. At present, it is estimated that 200,000 animals within the Serengeti National Park are killed annually, in poaching operations that have graduated from subsistence to commercial levels. It is hoped, however, that schemes to give local communities legal rights to manage the wildlife around their villages will reduce this. There are also plans to channel more money earned from tourist activities within the park back into the community, as the contribution from earlier tourism to the local economy has been relatively modest.

Painted hunting dog populations have been extirpated in the area. Possible explanations include stress-related diseases as a result of handling, interbreeding with domestic dogs, infections acquired from local domestic dogs, competition from lions and hyaenas, demographic stochasticity, food shortage or ex migration.

Infections from domestic dogs threaten other wildlife taxa in the area. An outbreak of canine distemper between January and October 1994 is estimated to have killed more than a third of all lions in Serengeti NP and neighboring Masai Mara. Hyaenas and Bat-eared foxes were also affected. The outbreak is believed to have originated amongst the roughly 30,000 domestic dogs that live in the area, most of which are not vaccinated. A program to vaccinate domestic dogs on the western boundaries of the park was initiated in 1996.

Justification of Ecoregion Delineation

This ecoregion was taken directly from the vegetation unit ‘Edaphic grassland on volcanic soils’ mapped by White. It reflects the discrete boundaries provided by deposits of volcanic ash, which in turn affect the composition of the grasslands and help to explain the high cocentrations of mammalian taxa at certain migration times of the year.

Neighbouring Ecoregions

Several ecoregions are tangent to the Serengeti Volcanic Grasslands:

• To the north and east lies the Northern Acacia-Commiphora Bushlands and Thickets (AT0711), which dominate much of lowland Kenya, and extend across the Tanzania border to cover a small area of northern Tanzania. This ecoregion, comprised of semi-arid mixed woodland, scrub and grassland, is somewhat protected within a well-functioning system of national parks and other reserves. However, numbers of humans and livestock are increasing outside protected areas, and nomadic pastoralism is declining in favor of settlement, causing environmental degradation through overgrazing and agricultural expansion. Water is scarce in this bushlands and thickets ecoregion, where one or both rainy seasons commonly fail. Certain species such as Grevy’s zebra have undergone severe declines because of competition with livestock for, and often exclusion from, water supplies. Poaching also threatens some of the large herbivores found here, particularly the Black rhinoceros and, until recently, the African elephant.
• To the south is the Southern Acacia-Commiphora bushlands and thickets (AT0716) cover much of northern and central Tanzania, extending into southwestern Kenya, around the eastern margins of Lake Victoria. In the heart of these wide-sweeping grasslands and associated Acacia-Commiphora woodlands, the world’s most spectacular migration of large mammals occurs each year. Wildebeests, Plains zebras, and Thomson’s gazelles traverse the Greater Serengeti Ecosystem, triggered by cyclical wet and dry seasons. Parts of the world famous Serengeti National Park and the Ngorongoro Conservation Area are located in this ecoregion; both have been designated as World Heritage Sites and Biosphere Reserves. Of the three Acacia-Commiphora Bushland and Thicket ecoregions, this southern unit receives the most rain and is closely associated with the Serengeti Volcanic Grasslands. The main threats to the ecoregion are the same as other savanna areas in the region, poaching of large mammals for body parts and meat, and expansion of pastoralism and agricultural use of the area with associated loss of tree cover.
• Further south is the Eastern Miombo woodlands (AT0706), covering an expansive swath of south Tanzania from Lake Malawi (Nyasa) in the east to close to (but not reaching) the Tanzanian Indian Ocean coast. Confined to the lower elevations of the Central African Plateau, this ecoregion is characterised by low-nutrient vegetation which grows in the dry climate and poor soil. However, the combination of smaller ecotonal habitats allows the area to support a variety of mammals, including arguably the largest populations of African elephant and Painted hunting dog on the continent. Although the presence of the Tsetse fly and the lengthy civil war in Mozambique has left the area sparsely populated, human populations and natural resource use are now increasing in the area. In addition, the Mozambican conflict has left a series of national parks and game reserves in need of rehabilitation.
• To the northwest lies East African halophytics (AT0901) encompassing two saline (soda) lakes, Lake Natron and Lakea Bahi, both situated in the eastern arm of the Great Rift Valley in Tanzania. These lakes are an inhospitable environment for most plants and animals as the water is saline, extremely hot, and subject to rapid salinity changes following rain. However, the lakes are home to large flocks of lesser and greater flamingos that breed on the mudflats that surround the lakes. The lesser flamingos filter the blue-green algae out of the saline waters, and the greater flamingos feed on copepod larvae that live in the shallow waters of the lakes. The East African Halophytics are threatened by active development and are not protected in any way. As a consequence, the long-term future of the flamingos that use the lakes as a principal breeding ground is also threatened.

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Disclaimer: This article \contains some information that was originally published by the World Wildlife Fund. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the World Wildlife Fund should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

Citation

World Wildlife Fund and C. Michael Hogan. 2014. Serengeti volcanic grasslands. Encyclopedia of Earth, NCSE, Washington DC. ed. Mark McGinley Retrieved from http://editors.eol.org/eoearth/wiki/Serengeti_volcanic_grasslands Updated and revised April 10, 2019