SummaryThis paper presents a description of the temporal and spatial distribution of rainfall within the Serengeti Ecosystem, Tanzania. Ecosystem and Regional patterns are discussed with respect to synoptic processes, vegetation zonation, phases of primary production and the distribution and migrations of large mammals.

Soils and vegetation in Ngorongoro Crater outcrop in the west (Pickering 1965). Lacustrine deposits predominate around Lake Makat and a mound field has been formed below the southwestern wall as a result of lahar (volcanic mud flow). Streams which derive debris from the northeast flank of Oldeani Mountain have built up coalescent fans on the floor near to this. Physiography The caldera lies in a volcanic block of depressions and high undulating plateaux, which average 2000 m and reach over 3350 m in elevation. The lowest point of the crater floor is at 1737 m and the highest point on the rim at 2316 m. The central plain is very flat (0-2?). Except where small hills of scoria rise above the floor, the northern and eastern sections have moderate slopes (5-13?). The walls have slopes of 35-70?, except in the northeast where some gradients are about 15?. The slopes between wall and floor vary greatly (< 10-45?). Drainage is completely internal. The Oljoro Nyuki and Munge rivers run towards the centre from the northeast ; there is also a series of springs high on the south wall and at the base of the eastern and western walls of the crater. Additional flow comes from Table 1. Mean rainfallfor Ngorongoro village over 9 years Mean rainfall (mm) 95% confidence limits (mm) January 71-7

SUMMARY Domestic and native large‐herbivore grazing within the coastal grassland of central Somalia, influenced vegetal dynamics along a grazing gradient. The gradient extended from large wells on highly overgrazed rangeland, far into rangeland that had received minimal grazing. Range condition values were used as an index of grazing intensity. Range condition along the gradient occurred as: very poor, poor, fair, and good, which represented 1–2, 4–6, 8–40, and 50–100% of the gradient respectively. Plant growth form and species cover responded differentially among the range condition classes. Species diversity increased from very poor to good condition rangeland. Floral similarity among range condition classes increased from very poor to good. Résumé Le broutage des grands herbivores domestiques et indigenes dans les savanes côtiéres de Somalie centrale influence la dynamique végétale selon un gradient. Ce gradient s'étend depuis les grandes sources dans les plaines herbeuses largement surpâturées jusqu'aux lointains pâturages qui subissent un broutage minime. On utilise des valeurs de conditions de milieu comme index intensityé du broutage. Le long du gradient, ces conditions de milieu, qualifyées de trés pauvres, pauvres, moyennes et bonnes, représented 1–2,4‐6,8–40 et 50–100% du gradient, respecti‐vement. Le mode de croissance et la couverture végétale répondent différemment selon les classes de conditions du milieu. Tant la diversité en espéces que la similarité Morale augmentent entre les plaines aux conditions ‘trés pauvres’ et celles qualifyées de ‘bonnes’.

SUMMARY Sand dune encroachment within the coastal grasslands of central Somalia influenced vegetal dynamics as a result of sand saltation and deposition. This impact was greatest near the dune bodies and decreased with distance. The soil was an Ustic Torripsamment gypsic, isohyperthermic that differed with respect to surface sand deposition. Vegetal cover and species diversity were low near the dunes and increased with distance in front of them. Cyperus chordorrihzus Chiov. and Cenchrus ciliaris L., both members of the original plant community, were the last species to succumb to the encroaching dunes. Résumé empiétement des dunes de sable sur les savanes côtiéres de Somalie centrale influence la dynamique de la végetation sous ľeffet de la salinisation et des dépôts de sable. Cet impact est le plus marqué prés du corps de la dune et décroît avec son éloignement. Le sol est un ‘Ustic Torripsamment’ gypseux et isohyperthermique qui différe en fonction des dépôts de sable en surface. Le couvert végétal et la diversité en espéces sont faibles prés des dunes et augmentent lorsqu'on s'en éloigne. Cyperus chordorrhizus chiov. et Cenchrus ciliaris L., faisant toutes deux partie de la communauté végétale originale, sont les derniéres espéces à succomber àľenvahissement des dunes.

Summary Seasonal occupancy of pastoralist campsites where livestock are bedded down at night has created islands of soil fertility within the coastal grassland of central Somalia. Soil organic matter, nitrogen, phosphorous, and potassium concentrations were greatest at the centre of the campsites and decreased towards the perimeter. The vegetation on the campsites differed from the surrounding grassland, presumably in response to soil fertility and intense grazing. The fast‐growing, grazing‐tolerant, stoloniferous grass, Cynodon dactylon, and the non‐palatable, annual forbs, Cleome tenella and Gisekia pharnaceoides, occupied the centre of the campsites. The campsite perimeters were dominated by the slow‐growing, grazing‐sensitive, perennial plants Cencrhus ciliaris and Indigofera intricata. Résumé L'occupation saisonnière des lieux de campement où le bétail est rentré pendant la nuit a créé des ilots au sol fertile dans la partie côtière de la Somalie centrale. La matière organique du sol, les concentrations en azote, phosphore et potassium, étaient plus élevées au centre du campement et diminuaient vers la périphérie. La végétation à l'intérieur des campements différait de la prairie environnante, résultat sans doute de la fertilité du sol et du pâturage intense. L'herbe à la croissance rapide, résistante au pâturage et stolonifèhe, Cynodon dactylon, et les désagréables et annuelles Cleome tenella et Gisekia pharnaceoides, occupaient le centre des campements. Le périmètre des campements était dominé par les plantes à croissance lente, sensibles au pâturage et perennes, Cenchrus ciliaris et Indigofera intricata.