Influence of Grazing on the Composition of Themeda Triandra Grassland, East Africa

Abstract

As modern man settled in the grasslands, cultivated them, replaced the wild animals with domestic animals, and altered the frequency and intensity of burning, changes in the grassland vegetation and soil must have occurred. Grazing pressure on the many forage species probably changed because of differences in food preferences and in grazing intensity. Greater human populations in recent years suggest that frequency of burning may have increased but overgrazing has reduced fuel and fire intensity. Although these are recently changed conditions which are known in a general way, knowledge of their long-term effects on the vegetation and soil is more speculative. Numbers and kinds of wild animals were described by early travellers whereas the grassland composition was barely mentioned. Replacement of wild animals by domestic livestock on East African grasslands came before the advent of detailed ecological investigations; therefore original interrelationships between animals and their grassland habitat are subject to considerable conjecture. This study was conceived in the belief that East Africa contained areas where the relative influence of wild animals and domestic livestock on their grassland home could be determined. Ideally, adjacent areas were required where wild grazing animals were abundant and where domestic animals had seldom grazed, such as occur at the edges of tsetse fly infestations and in certain national game parks. Four such locations were selected for detailed study of botanical species composition and soil conditions to determine the effects of grazing by different animal species and to evaluate the influence of grazing intensity. Several difficulties were encountered during the study. First, animal numbers, hence grazing pressure, had to be evaluated by indirect means as movements of nomadic livestock and migrating game animals were poorly known. Second, sampling was possible only at one time near the end of the growing season so that early growth and early growing species were not sampled nor were the vegetational differences in years of more or less rainfall than 1958-59. Third, climatic parameters for the areas are subject to question because the available records were of short duration and some distance away. These unknowns prevent unequivocal interpretations but, on the other hand, the grasslands are composed of perennial plants which tend to ameliorate short-term variations due to climate and grazing. Vegetational changes in response to climatic variation need further study.