The recognition of the economic consequences of alien invasive plants in terms of water-supply costs was pivotal in the establishment of the Working for Water programme, which has spent over R3 billion in dealing with the problem while simultaneously addressing poverty relief. Given competition from other social development projects for future funding, however, there is a need to justify further alien control programmes and to maximize efficiency within the programme. This requires valuing the biodiversity benefits of alien control and improving of the evaluation methods used. The concept of ecological goods and services has been a useful political tool, but the resource-economics concept of the Total Economic Value of biodiversity forms a more useful analytical framework. Studies on the impacts of alien invasive plants in South Africa initially concentrated on water losses, but more recently have included values of direct consumptive and non-consumptive use, option and existence value, and other indirect measures. Secondary effects such as downstream changes in aquatic ecosystem functions have not been assessed. Studies have varied in their scale and scope, as well as in the 'currency' of evaluation (such as financial or economic). Several approaches have been used for valuing water losses, with initial estimates having been the most conservative. Estimates of non-water benefits have frequently involved extrapolation from site-specific investigations within the study area, or been estimated from estimates at the regional level. None of the contingent valuation studies used has been applied following internationally accepted guidelines. In water-yielding catchments, alien control programmes are easy to justify in economic terms. In other areas, this may be more difficult. Cost-benefit analyses to date have tended to include the full financial costs of clearing, whereas, in reality, the opportunity cost of labour is close to zero, and economic costs are therefore much smaller. Benefits, which accrue later, tend to be underestimated from lack of information on biodiversity values and by high discount rates. A revised approach would favour the outlook for control programmes. If this fails to secure funding, what alternatives are there? New regulations are considered suboptimal and likely to fail. Opportunities for creating incentives to clear aliens from private lands are extremely limited, and there are no incentives that can reduce future invasions. Government-funded control programmes are thus the most efficient option. Future studies will need to address the right questions using the appropriate methods, incorporate both ecological and economic dynamics, express values in the right 'currency', and use a discount rate that reflects the rights of future generations. The quality of this research will depend on relevant ecological enquiry
