The distribution and abundance of the large herbivore community of Tsavo National Park,Kenya
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In Tsavo National Park, Kenya, there has existed for some years an acute problem that pivots on the interactions between elephants and trees. Elephant problems are a common feature of African National Parks, but the effects of the one in Tsavo are heightened by the marginal nature of the environment. This study describes an attempt to throw light on certain aspects of the ecology of Tsavo that are directly related to the elephant problem. All the major large herbivore species are considered, with particular reference to their numbers and distribution within the range of the Tsavo elephants and also with reference to their exploitation of areas both affected and more or \ less unaffected by elephant destruction of woodlands. Chapter 1 is a general introduction, both to the Park itself and to the study. The historical background to the formation of the Park is described, followed by a presentation of the first symptoms, in the 1950's, of the onset of an elephant problem. The background of other research that has been conducted in Tsavo over the last twelve years is discussed. Within the framework of that previous research, which had mostly been on single herbivore species, the objectives of the present work are described, being to define the geographical functioning of the large herbivore populations, to ascertain how all the species make use of the land both within the National Park and around it and to use this and other information to describe the practical implications for the conservation of large herbivores in Tsavo. The study area, being the total range occupied by the Tsavo elephant population, is defined, as is the large herbivore community. Data collection, using mainly 2 aerial and ground survey techniques appropriate to the large area (43,000 km ), is described: Chapter 2 describes the environment in which the large herbivores live.A description of the landscape includes a presentation of the major geological features of the area, their expression in terms of topographical variation, the nature and distribution of the major soil types and a description of permanent water availability. Lengthy analyses of climate are performed to discern both spatial and temporal variation in rainfall, and also spatial variation in the drying power of the atmosphere. The spatial analysis, using both contour plotting and trend surface analysis of variance, shows that there are considerable differences across the study area, the north-east being hotter, drier and less predictable than elsewhere. Temporal analysis 6f rainfall by Fourier analysis shows that there has been cyclical variation in Tsavo during the course of this century, some of the energy in the power spectra being attributable to phenomena of 36 years or more in wavelength. Studies were performed on the spatial change across the study area, of the physical structure of the vegetation. Attention was paid to mapping the areas of woodland, shrub land and grassland and also the areas of tree damage. All the information on permanent attributes of the environment, including those variables used to describe the landscape, climate and vegetation, is synthesized in a regional classification using principal components and cluster analyses; the outcome of this is that there are two distinct regions in Tsavo, one East and one West. It is stressed that these do not coincide with the two administrative halves of the National Park, Tsavo East and Tsavo West. Chapter 3 describes the animal populations. Methods of deriving population and density estimates, on the ground and in the air, are presented. Population estimates with confidence limits are derived for each species, being the mean of six separate aerial censuses. Ground census work is used for the calibration of aerial density estimates. The aerial census data on the density of each species is separated into eight components, which are extracted by division into two seasons, two regions and the land inside and outside the National Park. These eight subsets of data are tested, for each species, for significant differences. The results are presented, alongside wet and dry season distribution maps, for each species. The structure of the community as a whole is synthesized in an examination of biomass density in the eight subsets of data. Significant differences are shown between them, as is the remarkable dominance by elephants of the communities inside the Park. Chapter 4 brings together the '. data on the environment and those on tne QeDsities of each animal species, and, by a series of multiple regression analyses on the data, divided into eight subsets as before, attempts to examine the relationship between them. The results of this are combined for six of the species for which there was sufficient data, with information on reproductive strategies. Then, for each species" a synthesis is presented of the key features of its survival strategies in Tsavo, drawing on the data from Chapter "3 and from this chapter, and combining it with published information on that species, from Tsavo arid elsewhere. Chapter 5 is a general discussion. First, the suitability of the methods is discussed, both those of large-scale data collection and of multivariate analysis of that data. It is concluded that the broad geographical aims of the project justify the methods at both the collection and analysis stages. The influence on the animals of gradients of rainfall and of the other climatic factors are discussed and it is shown that the distributions of three species are directly affected by climate (two of them being absent from the arid end of the study area and one of them increasing in density there). The densities of most of the other species are influenced indirectly by climate in that rainfall has a direct effect on the primary production and thence on the carrying capacity of African savannahs. The carrying capacity is considered further in the context of the very reduced dry season ranges that are shown to apply to most of the large herbivore community. It is concluded that the effective biomass densities are very much higher than if the entire annual range is considered. In the light of recent findings on the carrying capacity of African savannahs, suggestions are made about the carrying capacity of Tsavo. The cyclical interaction between elephants and trees is still an unverified hypothesis, but data on climatic fluctuations and the patterns of tree damage are presented which sit reasonably with certain properties of two published mathematical models of the interaction. The effect that the elephant damage to woodlands may have had on herbivore density and distribution is discussed, using the multiple regression data from the previous chapter and evidence from early this century. It is concluded that elephants have invaded the Tsavo West grasslands during the last 70 years, displacing the other species in so doing, but altering the structure of the habitat rather little. In the woodlands, drastic modification of tree structural has clearly taken place, though not apparently to the detriment of the animals. The densities and occupance of the areas inside and outside the National Park are considered and it is concluded that the areas outside all support substantial wildlife populations, some of them arriving there in the wet season from inside the Park. For most species, though, the park serves a vital role and densities inside are higher than those outside. of this, particularly to elephant and rhino, is discussed. The role of the human populace around the Park boundaries as competitors for land, trees and as a hunter of elephants is both presented and discussed. Human settlement patterns around the Park are shown to be uneven, as are the strategies of exploitation of the land and the animals. The foci of elephant hunting and the rate of it are shown and, from published data on the reproductive rate of Tsavo elephants, it is demonstrated that the population must be declining at 4% per annum, probably faster. The foregoing information is borne in mind in a consideration of the role of the National Park in the conservation of the large herbivore community. Suggestions are made as to how the conservation goal might better survive the decades ahead, both by integration of the interests of the Park and neighbouring ranchers, and by exploitation of further areas for tourism. In conclusion, it is suggested that the large-scale geographical approach to the problems of herbivore populations in Tsavo has provided results that are both valid and of considerable practical importance.