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560 taxa listed under invasive species regulations, at least 11 taxa (~2%) are assumed present based on anecdotal or outdated evidence.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 presents a structured argument mapping approach to assess whether an alien species should be considered present at a site based on the credibility, specificity, sensitivity and quantity of the available evidence. This method was applied to three South African case studies. The sea urchin, Tetrapygus niger, was assessed as not present with high confidence. A population of the urchin was present at an oyster farm but has died out, notably this site was the likely site of introduction based on the putative pathway (a contaminant of oyster spat). Surveys of the surrounding area did not find any evidence of spread and there have been no reports from other oyster farms in the country. Heather, Calluna vulgaris, was classified as present with medium confidence. Heather was originally classified as not present under South African regulations, but there were verified herbarium and citizen science records. During this study an existing plant was extirpated, but it seems likely that plants are still present, albeit in low numbers, in historical ornamental collections. Leafy spurge, Euphorbia esula, was assessed as not present with medium confidence. A single unverified report was likely a misidentification of a congeneric alien found within the same region (E. terracina). Euphorbia esula has not been found in subsequent surveys of the site. In each case regulatory and research recommendations were developed as part of a formal risk analysis process in support of South Africa’s regulations. By providing a transparent and systematic methodology that incorporates various data types and multiple lines of evidence, argument maps can support decisions on the present of an alien species when evidence is conflicting, sparse, or questionable. Argument maps thus provide a useful method to support risk analyses.