Satellite mapping of vegetation change: human impact in an East African semi-arid savanna
A thesis submitted to the University of Oxford in application for the degree of Doctor of Philosophy; Wolfson College, April, 2001
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This thesis investigates the relationships between the plant communities of Mkomazi Game Reserve, Tanzania, and a suite of environmental and disturbance factors. Digital satellite data are combined with historical aerial photography, digital map information and ground data, to investigate vegetation change in the Reserve from 1973 to 1995.
Spatial variation in the environmental factors of altitude, soil, rainfall index, slope and aspect, and the disturbance variable of fire frequency, allowed an analysis of those most important in determining the species and structural composition of the vegetation. Spatial and temporal variation in human use (measured by human residency within the Reserve) over two time periods allowed the detection of human impact, and the ecological processes involved.
In the moist, west of the Reserve, increasing elephant populations and human bush clearance and fires during the 50s and 60s served to promote grass-dominated habitats and increase the intensity of the fire regime. Examination of aerial photographs and eye-witness accounts indicate that this was associated with a reduction and fragmentation of upland forest, and a reduction in the number and diversity of trees in the footslopes. The quality of the imagery available meant that it was impossible to determine the extent of these processes during the time period covered by this study.
In the east and central areas, largely unregulated, rapid increases in cattle numbers were associated with a reduction in grass cover and woody cover that could not be explained by other factors. The reduction in grass cover was more pronounced in drier areas that lay within a day's reach of water, and processes associated with intense, sustained cattle use were detected, although the exact extent of the area affected could not be determined due to cloud cover and limitations in the data. Post-eviction, some of these areas regained grass and bush cover, others remained with reduced cover, and the rest shifted to increased bush cover. The perturbation of a rapid increase in human and cattle numbers followed by their removal in 1988 appears to have been instrumental in shifting the east and central areas of the Reserve from the more open, grassy regime in 1973 to one that is more dominated by woody vegetation in 1995.
The findings are discussed in the context of biodiversity, conservation and the current debate over the impact of human use in semi-arid savannas.
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