344 W. H. PEARSALL 



These are a typical Tertiary volcanic upland area with peaks of between 

 3,000 and 4,200 m (10,000 and 14,000 ft). The estimates of rainfall that are 

 available for all this region show averages of 760 ± 50 mm (30 ± 2 in.) per 

 annum with some regularity, but the rainfall on the plains is known to be 

 uncertain, falling below the minimum for arable husbandry {c. 500 mm) in 

 one year out of five (Glover & Robinson, 1955). The rainfall in the Crater 

 Highlands, though of the same average order, leaves a margin for charging 

 ground water springs and rivers owing to the lower evaporation rates at 

 higher altitudes. This may be illustrated by a figure showing the variation 

 in potential evaporation with altitude (Sansom, 1955). At about 2,100 m 

 (7,000 ft) the annual potential evaporation falls to 760 mm and thus at and 

 above this height there is a rainfall which would maintain permanent forest 

 vegetation and which at higher levels still, supplies a considerable margin for 

 maintaining springs and rivers throughout the year. The Crater Highlands 

 are thus a suitable dry-season grazing for the plains animals, with a short 

 migration path of about 15-25 km through the intervening Acacia woodlands 

 and scrub. Unfortunately the native Africans are the Masai, a pastoral tribe 

 that lives on blood and milk from their herds of cattle. They have similar 

 migratory habits to the plains game. They compete directly for pasture and 

 water and are rapidly replacing the plains game in this area. The Masai, 

 moreover, are steadily destroying the high level woodlands by fire. There 

 is much, though not absolutely conclusive, evidence that this forest destruc- 

 tion is having an adverse effect on the water regime, a vital matter in an arid 

 climate. 



Most of the plains game in the Serengeti region possess a very much 

 longer migration cycle. They follow the river valleys running from the 

 plateau of the Serengeti Plains down to Lake Victoria, where there are 

 alluvial pastures at about 1,350 m (4,500 ft) near the lake. These are *dambos' 

 or 'mbugas' (see Michelmore, 1939), waterlogged in the wet season so that 

 there is little or no tree growth, but good pasture in the dry. The better 

 drained sites among and surrounding these alluvial pastures are 'savannah', 

 now being heavily occupied by African cultivators. They are encroaching on 

 the pastures and, as is usual, the carbohydrate-rich and protein-poor diet 

 derived from their *shambas' (gardens) leads to an intense desire for meat, 

 so that the game animals are heavily poached for this purpose. Both the 

 normal dry- and wet-season grazing grounds of the plains game animals are 

 thus becoming subject to severe human pressure, in the former case from 

 arable cultivations, in the latter from pastoral competitors. 



The intervening migration routes are mostly 'bush' of various types, often 

 heavily infested by tsetse-fly, varying from 'savannah' through Acacia scrub 

 to thorn-scrub. The proper form of land use of this type of country is not 



