THE OKAVANGO DELTA
The Okavango Delta is a world-renowned destination for tourists, being a relatively pristine wetland ecosystem as well as the world’s largest RAMSAR site. The eco-tourism industry is the second most important source of income for Botswana (Mbaiwa, 2005), and much of this revenue is generated from the Okavango Delta. The value of the delta is ecological as well as economic, given the high level of biodiversity found in the region. Surprisingly little research has been carried out on this unique and fragile ecosystem.
The Okavango Delta is situated in Northern Botswana, and covers approximately 22,000 km2. The Okavango River originates in Angola, and traverses Namibia before entering Botswana in the north-western tip of the country. It passes through a valley known as the Panhandle, then spreads out into several channels and waterways that make their way towards the distal regions of the delta (Wolski et al., 2006). Please follow this link for a map of the Okavango Delta, including camps and concessions.
Despite its semi-arid climate, the Okavango Delta is host to a wide variety of wildlife, including 1,300 plant species, 444 bird species, 71 species of fish, 64 reptilian species, 33 amphibian species and 122 species of mammal (Ramberg et al., 2006). This high level of biodiversity is largely due to the unusual hydrological features of the delta.
The delta goes through a cycle of three seasons: the rainy season (December to March inclusive), the flooding season (April to August inclusive) and the dry season (September to November inclusive) (Bonyongo, 2004). A mean of 490 mm of rain per year falls during the rainy season, (McCarthy, 2006). In addition, rains falling in Angola swell the Okavango River and are distributed across the delta in an annual flood which increases the size of the inundated area from 5, 000 km2 to 6, 000 – 12, 000 km2, depending on the extent, duration and direction of the flood (Wolski et al., 2006). The ecology of the delta is partially governed by the flooding regime that determines the degree of inundation across the area. This, combined with factors such as soil quality, is responsible for the proportional availability of different habitat types, including the vegetation that they support and, ultimately, the population levels of animals living in the region.
Habitats found in the delta include primary floodplain, secondary floodplain, tertiary floodplain, grassland, open acacia, riparian woodland, open mopane and dense mopane. Primary floodplain is flooded throughout the year. Secondary floodplain floods on a regular basis and tertiary floodplain is inundated only occasionally. Floodplains and grasslands have few shrubs and even fewer trees. Each habitat is characterised by different vegetation depending on water levels, soil characteristics and sunlight availability.
The annual flood does not always follow the same routes across the delta. Factors including vegetation growth, channel modification by hippopotami (Hippopotamus amphibius) and tectonic activity can cause some channels to be blocked while others are opened (Kgathi et al., 2006). This can result in some areas drying up whilst others are exposed to more water than in previous years, which inevitably leads to a change in the vegetation.
Changes in flow patterns have historically been caused by those natural factors, but there are also human factors that could provoke a large scale change in the quantity of water that reaches the delta. There are several proposals to divert water from stretches of the Okavango River in Namibia and Angola to provide water for towns, as well as for agricultural irrigation (Murray-Hudson et al., 2006). There are plans to construct dams on the river, and also to clear land around the watercourse to build dwellings and farms. All of these activities would draw water away from the Okavango Delta. However, Murray-Hudson et al. (2006) suggest that potential permutations resulting from climate change could have the greatest impact on the hydro-ecology of the delta.
Human activities, combined with climatic variations, could lead to a significant reduction in the amount of water flowing into the delta. This reduction would create a cascade of effects, including smaller areas of floodplain, larger areas of dry land and changes in proportional availability of many grass species, as well as the decline of several animal species. Among others, the numbers of some mammals dependent on the rich grasses of the floodplains could be expected to decline, including the red lechwe (Kobus leche) and the African buffalo (Syncerus caffer) (Murray-Hudson et al., 2006).