Problem Statement: Fynbos vegetation is highly endemic, e.g. of some 1600 plant species that are to be found in the reserve, more than 150 are endemic. The reserve acts as a water catchment basin for a wide region (including the Cape Town Metropolitan Area). Due to the well-preserved fynbos, it also has a high potential for tourism. In addition, some parts of the reserve are used for agriculture and forestry (e.g. Pinus plantations, apple orchards, and pastures for sheep and cattle). But woody invasive alien species such as Hakea spp. and Acacia spp. are threatening the ecological balance. As a consequence, in heavily invaded areas, fire has become more intense, resulting in progressive soil erosion and diminished water flow and quality.

Methods: The data were gathered in selected test areas of the Kogelberg Biosphere Reserve, including controls which, due to periodic clearing, had remained relatively uninfested by alien species.

Outcomes and Impacts: Results suggest that if periodic clearing were discontinued, the cover of alien plants would increase from 2.4 per cent to over 63 per cent within 100 years. Invasion of catchment areas would result in an average decrease of 347m3 of water/ha/yr over 100 years, resulting in average losses of more than 30 per cent of the water supply to the city of Cape Town (already water-stressed). Continued uncontrolled invasion and gradual displacement of fynbos vegetation by alien species will irreversibly damage this unique ecosystem and affect the aesthetics and the ecotourism potential of the region. Further, given the relationship between biodiversity and water supply, subsequent loss of biodiversity will induce substantial constraints to economic development of the region. Control of alien species in moderately infested areas would cost roughly US$ 4.50 ha/yr but annual gains in water alone would greatly outweigh these costs.

Lessons: Correct identification of invasive alien species allows cost-effective control measures to be identified and implemented, with positive returns for biodiversity conservation and economic use eg. Ecotourism and improved water supplies.