Desert Vultures: Adaptation, Carrion Ecology, and Human Impact in Arid Ecosystems

Biome 13, desert is found in areas drier than the driest savannas and grasslands, with very little rain and little vegetation. Xeric grassland, which has little clumps of grass may be a transition between desert and dry savanna. In many cases, due to overgrazing of livestock or intensive agriculture, dry savanna may become desert. Although desertification is generally defined as land degradation where land becomes increasingly dry, and may lose water, vegetation and wildlife there are many disputed definitions, causes and contributory factors. Geist (2005) notes a widely accepted definition is that of the Princeton University Dictionary which defines it as 'the process of fertile land transforming into desert typically as a result of deforestation, drought or improper/inappropriate agriculture'. According to Geist and Lambin (2004: 817) the most authoritative definition of desertification remains that of the Convention to Combat Desertification: 'land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors, including climatic variations and human activities' (see UNEP 1994).

Desertification assumed prominence after the French colonization of West Africa (Mortimore 1989). During this period, the French Comite d'Etudes commissioned a study to investigate and explore the prehistoric expansion of the Sahara Desert. Desertification may be termed as the final stage of deforestation, after the transition passes through the stages of savanna woodland, tree savanna, grassland (also termed steppe in Europe and prairie in the United States) (Campbell 1998).

Geist and Lambin 2004 list some factors for desertification which are relevant to avian issues. These include: agricultural activities such as livestock production (nomadic/extensive grazing, intensive production), crop production (annuals, perennials); infrastructure extension such as watering/irrigation (hydrotechnical installations, dams, canals, boreholes, etc.); transport (roads); human settlements; public/private companies (oil, gas, mining, quarrying); wood extraction and related activities such as harvesting of fuelwood or pole wood (from woodlands/forests), digging for medicinal herbs and other collection of plant or animal products; and demographic factors such as migration (in- and out-migration), natural increment (fertility, mortality) population density and life-cycle features (Geist and Lambin 2004: 819).

Vultures have been observed in deserts, where there is likely to be a food source, such as cattle-herding, camel driving or seasonal migration of ungulates across patches of desert between savanna landcover patches. An example of the existence of vultures in the desert of north-central Africa is the instructive study by Wacher et al. (2013). In this study, the landscape was of Sahelian and Saharan habitats, including volcanic outcrops, plateaux and valleys of the Termit Massif, and wide, flat plains and fixed dunes of the ancient greater Lake Chad Basin and its margins. The southern part of the study area was the transition between the mainly grassland Sahelian and the light woodlands of the Sudan savannas. The northern part of the study area included Tin Toumma (Niger) and Eguey (Chad) in the southern savanna with fewer trees. The Sahelian grassland plains included trees such as Sclereocarya, Balanites aegyptiaca, Acacia raddiana and Maerua crassifolia.

Ungulates are few, the food of vultures being primarily livestock, mostly large herds of camels, cattle, donkeys, horses and smaller animals. The few wild ungulates were Dorcas gazelle (Gazella dorcas Linnaeus, 1758) and Dama gazelle (Nanger dama Pallas, 1766), Barbary sheep (Ammotragus lervia Pall., 1777) and Addax (Addax nasomaculatus de Blainville, 1816). Movements were for fresh grazing and seasonal rainfall.

In this study, six species of vultures were recorded; the Egyptian vulture, Hooded vulture, White-backed vulture, Ruppell's vulture, Lappetfaced vulture and the White-headed Vulture. The Egyptian vultures were recorded in Termit Massif and Koutous hills in Niger, and rocky outcrops of Dibella and Agadem. These locations were surrounded by treeless sand desert. These vultures could be 'residents, migrants, or opportunistic followers of nomads and their livestock temporarily based at the ancient wells and scattered clumps of Hyphaene palms that grow in the lee of the hills' (Wacher et al. 2013: 190). Hooded vultures were found closer to Sudan savanna, near human settlements and also abattoirs. White-backed vultures were also mostly associated with the Sudanian zone in the southern margin of the study area, but some moved north during the rainy season. Ruppell's vultures had colonies on prominent granite inselbergs in Sahel and Sudan savanna and used treetop nests, mostly Balanites aegyptiaca trees. They frequented livestock carcasses in the southern parts of the study area. The Lappet-faced vulture nested in either Acacia raddiana, Maerua crassifolia, or occasionally Balanites aegyptiaca in the sahel region and in the semi-desert/ sahel of the Termit Massif. The White-headed vulture was mostly found in the south in the Sudan savanna zone (Wacher et al. 2013).

Vultures are also found in the deserts of the New World. For example, the Turkey and Black vultures are common in the deserts, semi-deserts, xeric grassland, scrub, shrublands and mixed tree-grass landcover of California, Arizona, New Mexico, Nevada and Northern Mexico. A study by Dean et al. (2006) found that Turkey vultures 'are probably the most abundant avian scavengers in semi-arid shrublands' of central Mexico. Black vultures and Common ravens were also common. Dead ungulates were an important food source, and birds commonly foraged above roads. This study was echoed by the research of Hiraldo et al. (1991) in a desert border 2100 m above sea level in northern Mexico, where the diet of Turkey and Black vultures was principally small mammals and large domestic ungulates.