JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Summary 1. The invasion of fynbos shrublands by woody weed species can reduce the water yield from catchment areas dramatically. We modelled the consequences of uncontrolled invasion on water yield using a geographical information system (Arc/Info). 2. Five important processes were recognized: the occurrence of fire; the spread and establishment of alien plants after fire; rainfall-to-run-off ratios; growth and changes in biomass between fires; and effects of these changes on streamflow. 3. The simulations of water yield were modelled with the Arc/Info GRID module using a 200 x 200-m grid. It was assumed that the interval between fires was 15 years and that proliferation and dispersal of alien plants took place only after fires. 4. Between fires, the model simulated the growth of the vegetation and its effects on streamflow, using relationships between rainfall and run-off, and run-off and aboveground biomass. 5. Results for the Kogelberg area in the Western Cape Province showed that alien plants invaded about 40% of the grid cells within 50 years. Cover of alien plants increased from an initial estimate of 2 4% to 62 4% after 100 years. 6. Invasion of catchment areas would result in an average decrease of 347 m3 of water per hectare per year over 100 years, resulting in average losses of more than 30% of the water supply to the city of Cape Town. In individual years, where large areas would be covered by mature trees, losses would be much greater. 7. In addition, invasion of fynbos by alien plants will cause-the extinction of many plant species, increase the intensity of fires, destabilize catchment areas with resultant erosion and diminished water quality, and decrease the aesthetic appeal of mountain areas. 8. Control of alien weed species is necessary to avert the above impacts, and the costs of control operations could be justified by the savings achieved in maintaining adequate water run-off from stable catchments in the long term.

Invasive alien plants are consumptive water-users, and may have reduced river flows in South Africa by about 6.7% according to a broad-scale study. An effective programme to bring the invasions under control would cost about US$ 92 million per year for the next 20 years. This paper reports on studies of four representative catchments (the Sonderend, Keurbooms, Upper Wilge and Sabie-Sand) to assess the impacts and costs of invasions at a scale that is more relevant to managers. Several alien plant species have invaded the catchments. Non-riverine invasions are mainly Pinus and Hakea species in Sonderend and Keurbooms, eucalypts in the Upper Wilge, and pines and scramblers (e.g. Lantana camara) in the Sabie-Sand catchment. Riverine invasions are dominated by Acacia mearnsii and, to a lesser extent, A. dealbata, except in the Sabie-Sand and the lower Sonderend River where Eucalyptus species are important. About 44% of the Sonderend, 54% of the Keurbooms, 2% of the Upper Wilge and 23% of the Sabie-Sand catchments has been invaded to some degree. The corresponding reductions in the natural river flows attributed to these invasions are about 7.2, 22.1, 6.0 and 9.4%. If the invasions are not controlled they could potentially spread, and occupy 51, 77, 70%, respectively, of the first three catchments. At an annual expansion rate of 10-15% this would take about 13, 26 and 63 years, respectively. The invadable areas in the Sabie-Sand catchment are already invaded so invasions will only increase in density. It would take about 26-30 years to reach 100% canopy cover. The projected flow reductions for the four catchments would increase to 41.5, 95.5, 25.1 and 22.3%, respectively. The estimated cost of the control programmes to prevent these losses would be about US$ 13.2, 9.9, 4.1 and 6.6 million for the Sonderend, Keurbooms, Upper Wilge and Sabie-Sand catchments, respectively. Should the catchments be allowed to become fully invaded before control operations were started, then the costs would rise to US$ 86.5, 20.5, 278.0 and 11.1 million, respectively. The impacts and costs are significant and are comparable with those calculated independently for other South African catchments. Water is acknowledged to be a key constraint to economic growth in South Africa and there is considerable pressure for efficient and sustainable use of the limited water resources. The projected impacts would justify control programmes aimed at clearing alien invaders for water conservation. #

a b s t r a c tInvasive alien trees and shrubs pose significant threats to biodiversity and ecosystem services in South African fynbos ecosystems. An ambitious initiative, the Working for Water program, commenced in 1995 to reduce the extent and impact of plant invasions. Despite substantial progress, the problem remains immense, and innovative ways of improving the efficiency of control operations are urgently needed. This study sought to develop a robust conceptual framework for effective management of the most important invasive alien plant (IAP) species. Two methods were applied in exploring the complexity of problems, thereby identifying appropriate response strategies. The DPSIR (Driving forces-PressureState-Impacts-Responses) framework and the Analytic Hierarchy Process (AHP) tool were used to design a strategy for prioritizing management actions. This strategy considers explicitly the most influential factors that determine the distribution, abundance, spread and impacts of IAPs. Efficient management of IAPs is constrained by multiple interacting environmental and socio-economic factors. Factors related to the fire-prone nature of the ecosystem and the characteristics of the invasive stands emerged as pivotal features for setting spatially-explicit priorities for management. Results of the analyses provide an objective and quantifiable perspective for improving the management efficiency. We conclude that considerable progress in controlling the spread of IAPs in fynbos ecosystems could be achieved by better coordination of management practices and by improving the quality of species distribution data.

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