Savanna Vultures: Carrion Ecology and Grassland Ecosystem Dynamics

Biome 7, the tropical and subtropical grasslands, savannas, and shrublands, is a vulture habitat in Africa and also in the Americas and Asia. Savannas cover about half the African continent and one-fifth of the land surface of the world (Scholes and Walker 1993). Savanna landscapes vary in the relative proportions of trees, shrubs and grasses, and such variability is due to both human and natural elements (Defries et al. 2010; Miller and Hackett 2011). It is important to define exactly what the savanna vegetation looks like. Savanna vegetation is highly heterogeneous (Solbrig 1993; Mistry 2000; Campbell 2005; Parr and Andersen 2006). Harris (1980: 3) points out that 'the word savanna has had an ambiguous career since it entered the English language in the 16th century'. Scholes and Walker (1993: 3) argue that 'the central concept—a tropical mixed tree-grass community—is widely accepted, but the delimitation of the boundaries has always been a problem'. Therefore they envisage the savanna as representing a 'continuum of vegetation types which have trees and grasses as their main constituents', while 'acknowledging that at the extremes, the distinction between savannas and woodlands is unavoidably arbitrary' (ibid.).

Scholes and Walker (1993: 4) define the savanna as a 'tropical vegetation type in which ecological processes, such as primary production, hydrology and nutrient cycling, are strongly influenced by both woody plants and grasses, and only weakly influenced by plants of other growth forms', a definition based on that which emerged from the International Responses of Savanna to Stresses and Disturbance (RSSD) Program (cited in Frost et al. 1986, in Scholes and Walker 1993: 4). Solbrig and Young (1993: 3) add that savanna 'designates tropical grassland with scattered trees' and is the 'most common tropical landscape unit', a point supported by Werner et al. (1990: 343) who argue that savannas are 'delineated both structurally (specifying a wood/grass composition) and climatically (seasonality of water availability)', and also by Menaut et al. (1990: 471) who note that 'the structure and dynamics of savanna vegetation is generally expressed as the balance between trees and grasses, and more easily by tree density and distribution'.

Tropical thicket, another common vulture habitat is not always classified as savanna (Scholes and Walker 1993). Tropical thicket is described as vegetation dominated by multi-stemmed woody plants (of course some multi-stemmed woody plants owe their form to cutting for firewood), and in some cases with no grasses, supporting the contention that they are 'disqualified as savannas' (ibid.). Scholes and Walker (1993: 9) however, acknowledge that due to the dynamism of compositional forms, which often sees savannas becoming thickets and vice versa, 'in the absence of better understanding of the ecology of thickets, we treat them as a class of savannas rather than as miniature dry forests'. This position is supported by Cole (1986: 16), who includes 'thicket and shrub' in the vegetation types classified as savanna. Menaut (1983: 128) noted that thickets, which may be found in 'a great diversity of environments, close to the rainforest as well as near the desert edge', are common aspects of coastal savannas.

There is a broad range of climatic types within which savanna may develop, with average annual rainfall varying from 2000 to 250 millimeters, these being intermediate between moist forest and desert (Harris 1980; Stott 1994; Bucini and Lambin 2002). In Africa, the driest savanna type is the Sahel or Sahelian savanna. The Sahel is the ecoclimatic and biogeographic zone of transition, between the Sahara desert to the north and the Sudanian savanna to the south. Having a semi-arid climate, it stretches across the southernmost extent of Northern Africa between the Atlantic Ocean and the Red Sea.

The Sahel is mostly covered in grasses, shrubs and a few trees. Grass cover is dominated by annual grass species such as Cenchrus biflorus, Schoenefeldia gracilis, and Aristida stipoides. Acacia species are the dominant trees, with Acacia tortilis the most common, along with Acacia senegal and Acacia laeta. Other tree species include Commiphora africana, Balanites aegyptiaca, Faidherbia albida, and Boscia senegalensis. In the drier, northern part of the Sahel, areas of desert shrub, including Panicum turgidum and Aristida sieberana, alternate with areas of grassland and savanna.

The Sahel the habitat for large herds of grazing mammals, including the scimitar-horned oryx (Oryx dammah), dama gazelle (Gazella dama), Dorcas gazelle (Gazella dorcas), red-fronted gazelle (Gazella rufifrons), the giant prehistoric buffalo (Pelorovis antiquus) and Bubal Hartebeest (Alcelaphus busephalus buselaphus). Carnivores included the lion, African wild dog and cheetah. However, populations of these larger species have been decimated by over-hunting and competition with livestock (for example Dorcas gazelle and red-fronted gazelle), endangered (Dama gazelle, African wild dog, cheetah, lion) or extinct (the Scimitar-horned oryx is probably extinct in the wild, and both the Giant Buffalo and the Bubal hartebeest are now extinct). The reduction of the large ungulates and the carnivores that fed on them is a factor for the decline in the numbers of large vultures such as the White-backed and Ruppell's griffon in this region.

The Sudanian savanna is found to the south of the Sahel. This region has more trees, and a longer rainy season. Common tree species are Combretaceae and Caesalpinioideae, with some Acacia species. The dominant grass species are usually Andropogoneae, especially the genera Andropogon and Hyparrhenia. Vast areas of the Sudanian savanna have been farmed, and culturally useful trees, such as shea, baobab, locust-bean tree and others are left standing, while farms of sorghum, maize, millet or other crops are cultivated.

The Guinea savanna is a transition zone between the Sudan savanna and the dry deciduous forest to the south. This region has a longer rainy season than the Sudan savanna and more trees, hence it is sometimes called the tree savanna. Similar to the drier savannas, large herbivores and carnivores occur, and therefore vultures are fairly common in the more open areas. South of this region, the forest-savanna mosaic is transitory to the tropical moist broadleaf forests of Equatorial Africa where most vultures are absent, except for the small Hooded vulture (Campbell 2009).

Biome 8, Temperate grasslands, savannas, and shrublands is an important home of New World vultures in southern Canada and the plains of the United States and Western Eurasia. This biome may interface with the Mediterranean forests, woodlands, and scrub or sclerophyll forests of Biome 12. Grasslands are similar to the savannas, but tend to lack trees or shrubs. These are found interspersed with savanna in Africa. There are also grasslands in South America. Examples are the Llanos grasslands of northern South America. In such regions, foraging vultures include the Yellow-headed vulture of Central and South America, and the Black and Turkey vultures.

Grasslands are also found in colder temperate regions. These include the Prairie and Pacific Grasslands of North America, the Pampas of Argentina, Brazil and Uruguay, calcareous downland, and the steppes of Europe. These are classified with temperate savannas and shrublands as the temperate grasslands, savannas, and shrublands biome. These temperate grasslands are the habitat of large herbivores, such as American and European bison, antelopes and wild horses such as the Przewalski's Horse. Carnivores such as wolves, coyotes and leopards are also found in temperate grasslands. Other animals include: deer, prairie dogs, coyotes, snakes, foxes, owls and badgers. Hence, vultures are able to live in such ecosystems.

Denizens include the Turkey and Black Vultures in the North America, and the Griffon, Cinereous and Egyptian Vultures in Eurasia. For example, the Griffon vulture has been recorded in several types of open environments (del Hoyo et al. 1994). The Cinereous vulture is recorded in scrub and arid and semi-arid alpine steppe and grasslands up to 4,500 m (Thiollay 1994). Foraging takes place over bare mountains, steppe and open grasslands and even forest edges. The Egyptian vulture forages in lowland and montane regions over open, often arid, country, and in such open country, it nests in ledges or in caves on cliffs, crags and rocky outcrops, but occasionally also in large trees, buildings, electricity pylons and even on the ground (Sara and Di Vittorio 2003; Gangoso and Palacios 2005; Naoroji 2006). Open terrain in Europe has expanded due to deforestation over the past centuries, as also has the decline in the population of larger wild ungulates that previously inhabited open temperate grasslands. Hence many such vultures rely on livestock free ranging on grassland pasture and also small mammals in open areas.