Invasive alien plants and South African rivers: a proposed approach to the prioritisation of control operations

Abstract

Parallel initiatives in South Africa have been addressing the prioritisation and management of invasive alien plant species, the prioritisation of rivers for the conservation of biodiversity, and broad-scale planning for water resource management. This paper has combined aspects of these approaches to develop a composite index of prioritisation of quaternary catchments for alien plant control purposes. A simple composite index that combined estimates of; the number of invasive alien plant species present; the potential number of invasive alien plant species that would be present if they occupied the full range as determined by climatic envelope models; the degree of habitat loss in rivers; and the degree of water stress was calculated. Each of the four components contributed between one and four to the combined index, which had a range of values between four and 16. The authors used a geographic information system to map the distribution of priority catchments for invasive alien plant control. Of the 1911 quaternary catchments in South Africa and Lesotho, just over one-third (650) were in the highest priority category with an index of 13 or more. A relatively small proportion (273, or 14%) of the catchments had the maximum scores of 15 or 16. The approach identified priority areas that have not currently been identified as such, and should provide decision makers with an objective and transparent method with which to prioritise areas for the control of invasive alien plants. Authors anticipate a debate about the way in which components of the index are calculated, and the weight given to the different components, and that this will lead to the transparent evolution of the index.