The structure, dynamics and changes of' the Amboseli Ecosystem

Phd Thesis

Maasai Amboseli Game Reserve, which covers some 3000 km2 of southern Kenya, was ecologically undescribed at the commencement of study. This fact, coupled with its economic importance as a natural resource and widespread concern over its apparent degradation by livestock and elephants, prompted the present study. The objectives were to provide an ecological description of the environment and large mammal communityof the Amboseli basin, the area within the Reserve of interest to conservationists. The ecological study was intended as a baseline for evaluating the management options and needs of the area. This thesis deals wi th the ecological aspects of study only. The study is divided into three sections which describe the structure, seasonal dynamics and long-term changes of the Amboseli ecosystem. The study area covers the wet and dry season ranges of the large mammal communi ty, wi th the main emphasis placed on the dry season concentration area in which the overall biomass is limited by the availability of forage. The methods of study depended primarily on sampling techniques from both ground and. aerial counts taken at regular monthly intervals. such samples formed the basis of a monitoring scheme in which a large number of parameters of the large mammal community and their environment were recorded simultaneously. This enabled an analysis to be made of the density and spatial distributions of various species in relation to pasture conditions, water availability, habitat type and other species and to determine how these relationships varied on a seasonal basis. The Amboseli basin environment was strongly influenced by late Pleistocene volcanics and an important consequence was the establishment of Lake Amboseli, a closed basin. Over the millenia tilelake became infilled and eventually dessicated. The soils remain saline and alkaline since the basin still acts as the sump of the closed Amboseli drainage system. The water table is elevated and highly mineralized, though this is appreciably less so in the soutner-nbasin where fresh water inflows from Kilimanjaro create a number of permanent swamps. Rainfall is low and erratic, generally less than 400 mm. since the area falls in the rainshadow of Kilimanjaro. The habitats are markedly influenced by the salinity and alkalinity of the basin as well as by the high water table. Most of the basin is covered in open short grass plains, and in the south by woodlands and swamps. Surrounding the basin , bushed grassland characteristic of the physiognomic type for the general region prevails. However, virtually no permanent sources of water are found here. The large mammal community is comprised of a broad .spectrum of herbivores and carnivores. Maasai and their livestock are an integral part of the community and contribute about 50% of the biomass. Most of the species using the basin do so in the dry season only. DtITing the rains the populations largely disperse into the surrounding area of Ilkisongo Maasailand. The principal areas of dispersion for wild herbivores are the Emotoroki and El Mau plains to the north and north east. Elephant disperse over a wider range towards Selengei and Namanga River. Livestock mainly use the northern slopes of Kilimanjaro,and in the north, the Lengisim area. A return to the basin follows the end of the rains. The biomass wi thin the dry season area can be correlated Witll climatic parameters. During the dry season most species of wildlife and livestock are limited to the immediate basin vicinity by their dependence on the permanent water in the swamps. \tJtihin this area most species show a changing distribution in relation to habitat throughout the dry season, although there is a large variation in habitat specificity ben/een the different species. The utilization patterns of many species are probably linked by a grazing succession. At least in the swamps the succession is probably facilitative, the early grazing by large herbivores improving the pasture for later grazing by srraller herbivores. Livestock show a similar utilization pattern to wild herbivores and it is speculated that similar feedback mechanisms operate in both cases to produce such a convergence. Both are limi ted by the availabili t-y of food wi thin the dry season range when it is exhausted during prolonged dry seasons. The mortality of livestock in a drought was found to be appreciably higher than in wildlife. The role of man in the ecosystem is paramount and factors influencing the balance of Iivestock and wildlife numbers are considered in some detail. Haasai are found to be a highly efficient exploiter of the ecosystem and the food chain efficiency 10f the pastoral system compares favourably with that of the wildlife component. In recent years water development, veterinary and medical facilities and famine relief in drought periods have made the Haasai less dependent on the ecosystem and progressively less able to manage witllin a subsistence economy. Furthermore the wildlife attractions of the area have encouraged a large tourist trade that has progressively competed with the Maasai for land rights in the basin. In the near future it can be expected that the dry season range will be gazetted as a National Park while the Maasai will be given legal tenure to land in the wet season range, areas currently poorly used in the dry season by virtue of their distance from water. In other words tllerewill be an increasing separation of human and wildlife ecology. The dual use of the wet season areas is likely to continue, but in view of the futvxe establishment of ranciling, wildlife populations may well be limited by the food available (or its accessability) in this part of their range rather than in the dry season concentration area. Large scale habitat changes have occured in Amboseli over the last two decades. The most conspicuous feature is the decline of the Acacia xanthophloea woodland, over 90% of the trees having died during this interval. The primary cause of habitat changes has been the salinization of the basin area. Increases in the long term rainfall have raised the water table some 3 to 4 m, resulting in progressive increases in the level of soluble salts in the rooting horizon of the Acacia ~- L~o.phloea and other plants. This has caused a marked shift from a hydrophytic to halophytic plant community. Such Changes also appear to have affected the compositions and possibly total biomass levels of the large mammal cor.~unity. Elephants have probably played a catalytic role .in woodland decline. There is no evidence that their populations are suffering compression from human encroachment since the migration trails used as long ago as last century are still used. There is evidence to suggest that the recent habitat changes are producing a habitat type which existed last century when the climate was slightly more moist than during the intervening period. It appears that tileearly successional stages of the basin community are subject to acceleration and reversal by changes in rainfall regime and water table fluctuations.