D.C. Le Maitre

Invasive alien trees and water resources in South Africa: case studies of the costs and benefits of management

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

Invasive plants and water resources in the Western Cape Province, South Africa: modelling the consequences of a lack of management.

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

The economic consequences of alien plant invasions: examples of impacts and approaches to sustainable management in South Africa.

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

Assessing the risk of invasive success in Pinus and Banksia in South African mountain fynbos

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. # 2002 Elsevier Science B.V. All rights reserved.

Fire behaviour in South African fynbos (Macchia) vegetation and predictions from Rothermel's fire model

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 above-ground 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 m 3 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.

The prioritisation of invasive alien plant control projects using a multi-criteria decision model informed by stakeholder input and spatial data

The invasion of natural ecosystems by alien plants is a serious environmental problem that threatens the sustainable use of benefits derived from such ecosytems. Most past studies in this field have focussed on the history, ecology and management of invasive alien species, and little work has been done on the economic aspects and consequences of invasions. This paper reviews what is known of the economic consequences of alien plant invasions in South Africa. These economic arguments have been used to successfully launch the largest environmental management programme in Africa.Ten million hectares of South Africa has been invaded by 180 alien species, but their impacts are not fully understood, although they are undoubtedly significant. The indications are that the total costs of these impacts are substantial. Selected studies show that invasions have reduced the value of fynbos ecosystems by over US

A Computer-Based System for Fire Management in the Mountains of the Cape Province, South-Africa

11.75 billion; that the total cost of invasion would be about US

Forest evaporation models: Relationships between stand growth and evaporation

3.2 billion on the Agulhas Plain alone; that the net present cost of invasion by black wattles amounts to US

System dynamic modelling to assess economic viability and risk trade-offs for ecological restoration in South Africa

1.4 billion; that invasions by red water fern have cost US

Preliminary Estimates of Nutrient Levels in Fynbos Vegetation and the Role of Fire in Nutrient Cycling

58 million; and that the cost to clear the alien plant invasions in South Africa is around US