THE VEGETATION DIVERSITY OF PERMANENT AND SEASONAL SWAMPS OF THE OKAVANGO DELTA, BOTSWANA

摘要

The study deals with the floristic diversity of the Okavango Delta ecosystem as a means of establishing the value of this ecosystem as a storehouse of biological diversity. It achieves this by examining local and regional patterns of diversity, and by explaining these in terms of the underlying environmental gradients. The survey was laid out in February 2003 within the scope of the extensive AquaRAP Ⅱ (Aquatic Rapid Assessment Program) of Conservation International, Okavango Office. Vegetation surveys were undertaken at a total of 116 sampling points within four focal areas. The primarily aquatic assemblage in the upper reaches (Upper Panhandle, Guma Lagoon), dominated by Cyperus papyrus, Vossia cuspidata and Phragmites species, changes in the lower reaches (Moremi, south of Chief's Island) to a patchy mosaic of aquatic, semi-aquatic and terrestrial habitats and species. The species numbers for the investigation in February 2003 were significantly higher than for the comparable campaign in the dry season 2000 (AquaRAP Ⅰ). The reason for the higher species diversity for AquaRAP Ⅱ is due to the occurring rainfall in combination with a lower water level. This therefore rapidly enlarges the amount of habitats and also the species numbers, especially on the fringes of channels, lakes and small islands. In many cases the underlying gradients are simply due to variation in abiotic conditions and resources, but in others they are the product of ecosystem engineering by plants and animals. The understanding of the origin of heterogeneity is vital for the adequate conservation of biodiversity in this region.