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°C, and mean minimum temperatures between 15° to 21°C. Mean annual rainfall in the Serengeti varies from 1,050 mm in the northwest to 550 mm in the southeast (Sinclair et al. 2000). This rainfall is strongly seasonal, with peaks between March to May, and between November and December (Schaller 1972, Sinclair 1979). Rainfall is the main determinant of vegetation growth, and hence ungulate food supply (Sinclair 1977, McNaughton 1979, 1983).
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 Mt 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 neighbors. 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 (White 1983), 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, and sedges of the genus Kyllinga (White 1983). In periods of severe drought, the grasslands become virtually denuded of standing vegetation.
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 Southern Acacia-Commiphora Bushland and Thicket ecoregion each year (Campbell & Borner 1995, WCMC 2001). 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 eaten down to stubble, and water is scarce (Schaller 1972). This 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 (Schaller 1972).
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 (Stattersfield et al. 1998), 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 (montypic genus)), Usambaro barbet (Trachyphonus usambiro), greycrested helmet shrike (Prionops poliolophus), greybreasted spurfowl (Francolinus rufopictus), and Fischer’s lovebird (Agapornis fischeri).
The Black Rhinoceros (Diceros bicornis, CR) used to occur in the ecoregion, but has been extirpirted by poaching for its horn. A small population still survives on the floor of the Ngorongoro crater, just outside the ecoregion border. Wild 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 (Morell 1995, Dye 1996, East and Hofer 1996). Despite the loss of wild 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 (C. mesomelas) jackal, 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 plentiful, with Serengeti National Park having 34 raptor species (WCMC 2001), and six species of vulture (WCMC 2001). A variety of smaller predators including snakes, lizards, spiders and scorpions also occur here.
Contrary to popular belief, the Serengeti’s large predators account for no more than one-third of all deaths among the migratory herds (Reader 1998). 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 (Sinclair et al. 2000). Most large prey species living in big herds with precocial young (e.g. wildebeest, topi (Damaliscus lunatus) and buffalo (Syncerus caffer)) display highly synchronous birthing seasons (Estes 1966, 1976, Sinclair et al. 2000). Most large predators are territorial, and only hunt until satiated. Since 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 (Estes 1966, 1976). Calves of these species born outside of the synchronized birth period seldom survive (Kingdon 1997).
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 (Stuart et al. 1990). 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 (Stuart et al.1990).
With so many large herbivores, 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 (Bell 1971). 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 (Kingdon 1997). Studies here have shown that when ungulate grazing is removed, plant species composition and growth form changes relative to those under grazing (McNaughton et al. 1988), and that some important grass species (e.g., Andropogon greenwayi) eventually disappear in the absence of this disturbance (Belsky 1986). Other studies have found that the grasses found in grazed patches are more productive than ungrazed patches (McNaughton 1983), and that these patches are able to support larger concentrations of herbivores than stocking rates would suggest (Hiernaux and Turner 1996).
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 (Wilsey 1996). Although some research is being carried out by the Frankfurt Zoological Society (FZS) (Stuart et al. 1990), relatively little is known of the ecology of this disturbance (K. Parr, pers. comm).
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 (Reader 1998).
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 is probably large enough to ensure the survival of the habitat and its biodiversity values (WCMC 2001). 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 wild 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 be reintroduced to the Serengeti.
Types and Severity of Threats
While Masai pastoralists occupy the Ngorongoro Conservation Area, there are no people living within the Serengeti National Park. However, the western frontier of this Park has a dense resident population, growing at 4 percent a year (Packer 1996). 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 demand for land rises (WCMC 2001). 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 (Campbell and Hofer 1993, Leader-Williams 1996). At present, it is estimated that 200,000 animals within the Serengeti National Park are killed annually (WCMC 2001), in poaching operations that have graduated from subsistence to commercial levels (Stuart et al. 1990). It is hoped, however, that schemes to give local communities legal rights to manage the wildlife around their villages will reduce this (WCMC 2001). There are also plans to channel more money earned from tourist activities within the park back into the community, as the contribution from tourism to the local economy has been relatively low (Leader-Williams et al. 1996).
Wild dogs populations have been extirpated in the area. Possible explanations include stress-related diseases as a result of handling, infections acquired from local domestic dogs, competition from lions and hyaenas, demographic stochasity, food shortage or emigration (Dye 1996, East and Hofer 1996).
Infections from domestic dogs threaten other wildlife 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 (Mills 1999). Hyaenas and bat-eared foxes were also affected (Mills 1999). 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 (Roelke-Parker et al. 1996, Morell 1995). A program to vaccinate domestic dogs on the western boundaries of the park was initiated in 1996 (Bristow 1996).
Justification of Ecoregion Delineation
This ecoregion was taken directly from the vegetation unit ‘Edaphic grassland on volcanic soils’ mapped by White (1983). It reflects the discrete boundaries provided by deposits of volcanic ash, which in turn affect the composition of the grasslands and help explain the high concentrations of mammals at certain times of the year.
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Prepared by: Colleen Seymour, Mary Rowen
Reviewed by: In progress