Clearing of invasive alien plants in South Africa: a preliminary assessment of costs and progress

Abstract

This paper provides estimates of the costs of clearing important species of invasive alien plants, as well as of progress made with clearing, based on data from a recently developed GIS-based project information system. Before the deployment of the system, managers were unable to record, in any detail, information relating to the costs associated with areas cleared at different densities for a range of species. As a result of this, only superficial estimates of the impact of the Working for Water programme have been possible hitherto. While the system is not yet in full operation, data for about 60% of project expenditure from the 2002/03 financial year were available, and we used this information on which to base our analysis. These data show that the costs of clearing rise sharply with the density of invasion, reaching more that R2000/ha in some cases. These amounts do not include expenditure on herbicides, which can reach over R1000/ha for dense infestations of sprouting species. A large proportion (57%) of the costs was incurred in clearing large trees, which are perceived to have a significant influence on water resources. Large sums were also justifiably spent on species that probably are not substantial water users, including lantana, triffid weed and cacti. These plants affect biodiversity, catchment stability, and the agricultural potential of land. Some of the species on which large amounts have been spent, however, are arguably not a priority. Good progress has been made with clearing certain species. At current rates of clearing, however, many other species will not be brought under control within the next half century. This underscores both the importance of biological control as a sustainable, effective and inexpensive solution to the most intractable of the invasive alien plant problems, and the need for landowners to share the responsibility for clearing. Our estimates are preliminary, given the incomplete data on the project management system, and should be treated as such.