Management of the Alien Weed, Salvinia molesta, in the wetlands of the Okavango, Botswana

Abstract

The Okavango River, supplemented by local rainfall, supplies on average 15 billion m3 of water each year to a wetland system known as the Okavango Delta or, more correctly, the Okavango alluvial fan. This wetland covers 14,000 km2 and can be divided into rivers, lakes and pools, perennial and seasonal swamps, floodplain grasslands, and rainwater and seepage pans and pools (Smith 1976, Smith 1989). The whole of the Okavango ecosystem is low in nutrients and productivity due mainly to the waters being chemically pure and the substrate of Kalahari sediments being infertile and low in elements such as nitrogen and phosphorus (Thompson 1974). Despite this, the wide spectrum of habitats throughout the Okavango provides for a high plant species:area ratio. Seasonal swamps and floodplains are heavily utilized by game and any disruption to these areas will limit the survival of Okavango wildlife (Snowy Mountains Engineering Corporation (SMEC) 1989). Other than the drying up of these systems through natural or artificial processes, a major threat to the ecosystem is invasion by alien aquatic weeds, such as Salvinia molesta(salvinia) and Eichhornia crassipes(water hyacinth)-weeds that are notorious for upsetting the ecology of wetland systems in the tropics (Denny 1985, Cook 1990). Fortunately, water hyacinth has never invaded the waterways of Botswana (Smith unpublished data) despite being a major weed in neighboring countries such as South Africa (Cilliers 1991), Zambia (Denny 1985), and Zimbabwe (Marshall 1993).