The Delta

The Okavango Delta in one of the largest undisturbed deltas remaining in the world. This dynamic riverine environment is home to a wide array of plants and animals and supplies the arid Kalahari Desert of Northern Botswana with life giving waters. This delicate swamp has been under increasing pressure from human activity. In the past the Okavango Delta was naturally protected against human invasion by the existence of the Tsetse Fly, carrier of the African Sleeping Sickness (Trypanosomiasis). With the eradication of this organism the delta was opened up to human activity and livestock grazing. Later the construction of veterinary cordon fences severed the delta, disrupting the annual migration of many herbivores, such as the wildebeest, whose population experienced a 97% decline. Moremi Game Reserve was created, which placed seven hundred square miles of the delta under protection, however the delta still faces pressure as the demands for residential space, water and grazing land continues. The development of technology has raised the standard of living resulting in inflated consumption of natural resources such as water. Botswana's 1.4 million population is steadily growing and as urbanism increases, dams and irrigation systems such as the North-South Pipe line are being proposed to meet the growing water demands. Due to sporadic and unpredictable rainfall, and the large quantities of water lost to evaporation and absorption by vegetation, the delta is already fragile. Botswana in the past has relied on diamonds and cattle for its primary sources of income, but diamonds are a dwindling nonrenewable resource and cattle are highly susceptible to contagious diseases such as the lung disease that is presently responsible for the destruction of thousands of exposed cattle in Ngamiland. Because of these threats, tourism is becoming a more viable option, and the majority of tourism is centered in the Okavango Delta area. Management of this fragile ecosystem is necessary to insure the protection of its natural balance, and before such management plans can be created, an understanding of the Okavango and its web of complex processes needs to be thoroughly studied and understood.

Hippos are one of the many animals involved in the cycles of the delta, from the development and maintenance of channels to the actual chemistry of the water. Hippos have been credited with keeping the canals open and the riverbed sediment compressed due to their preference to travel in the safety of water during their nightly passage to forage. Hippo paths often become alternate route for water during floods and back surges. These paths develop into new rivers and it is speculated that this process created both the Ngoga and Boro Rivers, and occasionally the carved area of the riverbeds along hippo paths become oxbow lakes and lagoons. It is clear that the hippos play a vital role in the ever changing physical features of the Okavango Delta.

Another study by Lauri and Oliver explores the importance of hippos in the fire cycles of the delta. Hippos prefer to graze on the short precropped grasses, and create hippo lawns. Fire is excluded from these areas due to lack of combustible material, which enables sapling trees to grow. The hippos dislike to graze in tree-populated areas and move to new feeding grounds. With hippos absent the grass is able to flourish, enabling fires to burn these areas, destroying any trees smaller than 1.8m high. After a burn the young lush grass shoots emerge and entice the hippos to return, completing the cycle.

The wildlife in the delta is heavily dependent on the rivers that nourish the otherwise semi arid desert, however, the delta waters are fairly nutrient poor, and their cycles are not well understood. It is thought that hippopotamuses play a major role in the nutrient cycling of the Okavango Delta because they graze nocturnally on land and remain submerged during the day. A single hippo can consume 80KG of grass in a night, and a large portion of these nutrients are released by the hippo into the deltas waters. It is speculated then that hippos create a nutrient highway between the land and water, and this surplus of nutrients has been linked to outbreaks of Silvia Molesta, an exotic weed.

The effects of hippo presence on the water life and other flora in the delta has not yet been documented and due to the limited knowledge on hippos, the DWNP has identified them as a priority for future research. As human encroachment on hippo habitat continues and tourism expands, hippo/human confrontations will undoubtedly increase. As hippos are forced to congregate in higher concentrations they become more stressed and hence more aggressive. Hippos are responsible for more human deaths than any other mammal in Africa, and in light of recent hippo related fatalities, steps will inevitably be taken to manage the hippos. It is crucial then that their niche in the deltas complex ecosystem be understood before the formation of any management plans.