EPPO Global Database

EPPO Reporting Service no. 02 - 2007 Num. article: 2007/038

A proposed prioritization system for the management of invasive alien plants in South Africa

Every country has invasive plants or weeds whose presence conflicts in some way with human management objectives and needs. Resources for research and control are limited, so priority should be given to species that present the biggest problems. A prioritization system designed to assess objectively invasive alien plants at a national level has been developed in South Africa. The evaluation includes a set of 17 criteria and a scoring system, and is divided into 5 modules:
  • Module A: potential invasiveness
    • Is the species known to exhibit long-distance dispersal (;5 km)?
    • Is the species invasive elsewhere, outside South Africa?
  • Module B: spatial characteristics
    • Distribution: what is the current percentage of quarter degree grid squares that countain the species in the country: 1-2%, 3-5%, 6-10%, 11-20%, 21-40%, ;40%.
    • Density: the species occurs predominantly as: individual plants, small clumps, vast monospecific stands, mixed stands with other invasive plants.
  • Module C: potential impacts
    • Reduction in biodiversity where the species occurs is: nil, minor, moderate or profound?
    • The species impact on water resources is: nil, reduction of stream flow by 10-30%, ; 30% reduction, stream no longer flows at all.
    • The negative impact of the species is: nil, ;10%, 11-30%, ;30% reduction in profit, land unusable.
    • The positive economic impact of the species is: nil, informal, small business, commercial, any two or more of the previous ones.
    • The species is poisonous to stock or humans.
  • Module D: Potential for control
    • The options for realistic chemical control of the species are: unavailable, impractical in most situations, partially successful, effective and practical
    • The options for biological control of the species are: complete control, substantial control, negligible control, no agents released yet
    • The options for mechanical control of the species are: not available, impractical in most situations, partially successful, effective and practical
    • Is there legislation to assist the control of the species?
    • Can any agency be held accountable for the introduction or proliferation of the invasive species in South Africa?
  • Module E: conflicts of interest
    • Possible conflicts of interest with the commercial sector
    • Possible conflicts of interest with the informal sector (non-commercial)
    • According to a cost-benefit analysis, the species has: substantial economic value, some economic value, limited value, no apparent value.

Total prioritization scores, calculated from criterion and module scores, were used to assess species’ priority. Prioritization scores were calculated by combining independent assessments provided by several experts, thus increasing the reliability of the rankings.
Two decision-making procedures could be used: the nominal group techniques (NGTs) and the Delphi method. Nominal group techniques involve an interactive group structure whereas the Delphi method uses the individual opinions of experts with no “face-to-face” interaction. The Delphi method has a number of advantages over NGTs, allowing a group of individuals to reach consensus without having to meet.
The total confidence score, a separate index, indicated the reliability and availability of data used to make an assessment. The final ranking was made by combining two separate indices, the total prioritization score and the total confidence score.
This prioritization system was used to study 61 plant species and assign them prioritization and confidence scores. Out of the 61 plants assessed, Lantana camara (Verbenaceae), Chromolaena odorata (Asteraceae) and Opuntia ficus-indica (Cactaceae) were those with the highest ranks. These are plants in which the greatest number of resources, human and economic, can confidently be invested for their successful control.


Robertson MP, Villet MH, Fairbanks DHK, Henderson L, Higgins SI, Hoffmann JH, Le Maitre DC, Palmer AR, Riggs I, Shackleton CM and Zimmermann HG (2003). A proposed prioritization system for the management of invasive alien plants in South Africa. South African Journal of Science 99, 37-43.