Argument maps can support decisions to declare the presence of alien species: South Africa as a case study

Accurate alien species lists are essential for effective biodiversity policy and regulation. However, the evidence that a species is present at a site can be weak. In South Africa, of the 560 taxa listed under invasive species regulations, at least 11 taxa (~2%) are assumed present based on anecdotal or outdated evidence. This study introduces a structured argument mapping approach to evaluate whether an alien species should be considered present. Argument maps display the inferential connections between reasons and objections, and the logical connections between them, to judge the validity of the premises of an argument. We applied this approach to three South African case studies—the Chilean black urchin ( Tetrapygus niger ), heather ( Calluna vulgaris ), and leafy spurge ( Euphorbia esula )—evaluating the available evidence in terms of its credibility, specificity, sensitivity, and quantity. The method enabled a concise classification of presence ( T. niger not present with high confidence, C. vulgaris present with medium confidence, and E. esula not present with medium confidence) and informed recommendations for regulation and research. By providing a systematic method to incorporate multiple data types and make reasoning explicit, argument maps can support decisions regarding the presence of an alien species when evidence is conflicting, sparse or questionable.