Argument maps bring order to contested alien species presence decisions

When South Africa’s regulators list a species as invasive, the decision carries real consequences: landowners must control it, budgets are allocated against it, and international reports count it. Yet a new analysis of the country’s 560 regulated alien taxa has revealed that at least 11 of them, roughly 2 percent of the list, are assumed to be present on the strength of anecdotal, ambiguous or outdated records. A study published in NeoBiota now offers a way out of this evidentiary fog, borrowing a tool from critical thinking classrooms: the argument map.

Argument maps have been used in logic and critical thinking courses since the 1950s, but their application to biological invasions is novel. The method centres on a single claim, in this case the probabilistic statement that species X is not present in site Y. Around that claim, researchers arrange reasons that support it and objections that challenge it, each anchored to documented evidence with identified sources. Experts then estimate subjective probabilities for each line of reasoning, allowing an overall probability for the claim to be calculated. The result is a visual, traceable structure that makes every assumption and uncertainty explicit, rather than leaving them buried in expert opinion.

The stakes are higher than they might appear. Biological invasions are among the leading threats to biodiversity, and international exchange of alien species is expected to increase in the coming decades. Most introductions fail to establish, and invasive populations can even disappear on their own, which means alien species lists rapidly become outdated. Misidentifications and false presences can inflate species counts and distort policy and resource allocation. A thorough revision of alien species in the Mediterranean, for example, resulted in the exclusion of 73 species previously considered established. But premature removal carries its own danger: it can squander the last chance for eradication while a species is still within reach.

South Africa’s regulatory framework illustrates how these problems arise. The Conservation of Agricultural Resources Act of 1983 was replaced by the National Environmental Management: Biodiversity Act of 2004, whose initial lists of regulated alien species were published in 2014 and updated in 2020. The initial listings were based largely on expert opinion and perceived threat, without comprehensive verifiable evidence of presence. Concerns about several listed species prompted a structured review, which identified the 11 questionable cases. While risk analysis frameworks typically require some evaluation of whether a taxon occurs in a country, no transparent and standardised process existed for resolving cases where the evidence conflicts.

The argument map approach fills that gap with a four-part logical checkpoint. The first section asks whether the species was ever introduced, drawing on historical records, herbarium specimens, import and export databases, grey literature and citizen science observations. The second asks whether it still exists at known sites, examining survey outcomes, eradication reports, habitat suitability and the detectability of long-lived seedbanks or dormant life stages. The third asks whether it has spread to other locations, tracking putative introduction and dispersal pathways such as rivers, roads and trade routes. The fourth asks whether new or repeated introductions are occurring, evaluating whether original pathways like aquaculture or the ornamental trade remain active.

Evidence feeding into each checkpoint is scored against four criteria: credibility, reflecting peer-reviewed status and methodological rigour; specificity, measuring how directly the evidence addresses the species and location in question; sensitivity, the capacity of detection methods to minimise false negatives; and quantity, the volume of independent corroborative sources. Each criterion is rated high, medium or low. Lines of reasoning supported by direct, verifiable evidence carry more weight than those resting on absence of evidence or speculative claims, following a weight-of-evidence approach aligned with international risk evaluation practice. Expert opinion and informal observations are treated with appropriate caution, since judgements based solely on personal observation retain inherent bias.

The method was tested on three South African case studies chosen for their diverse evidentiary challenges. The Chilean black urchin, Tetrapygus niger, was recorded in 2007 at an oyster aquaculture facility in Alexander Bay, with museum specimens collected and the account published. Follow-up systematic surveys found no living individuals at the only known site, only empty shells, and no evidence of spread. The dam used for oyster aquaculture had dried up, and the kelp-dominated ecosystem showed none of the visible grazing signs typical of urchin presence. The original introduction pathway via oyster spat imports was confirmed inactive at the facility. All four reasons supporting absence were underpinned by strong, well-documented sources, while the two objections were speculative and rated low in confidence. The conclusion: not present, with high confidence. The researchers recommend changing the listing from category 1a to prohibited and adding the taxon to a watch list, with targeted surveys reinstated if trade pathways reopen.

Heather, Calluna vulgaris, told a different story. The species was listed as prohibited under the 2014 and 2016 lists, implying it was not present but posed a high risk. Yet a historical herbarium specimen from 1987, research-grade iNaturalist observations from 2008 and 2021, and ecological inferences pointed to past and potentially ongoing presence. A single population was discovered and removed, and a 2021 site visit to Table Mountain detected one flowering individual. No evidence suggested formal stocking in the horticultural industry, but the species is presumed to have been cultivated informally in private gardens. The ability of heather to form long-lived seedbanks, combined with scattered survey coverage and no post-2021 monitoring, lowered the confidence rating. The verdict: present, with medium confidence, and a recommendation that the species be regulated as a category 1a taxon pending an assessment of whether nation-wide eradication is feasible, with targeted surveys in the Knysna and Caledon districts.

Leafy spurge, Euphorbia esula, proved to be a case of probable mistaken identity. A single unverified record from 2006 in the Southern African Plant Invaders Atlas lacked any supporting herbarium specimens or photographs. Multiple extensive field surveys at and around the reported location detected no individuals, but a congener, Euphorbia terracina, which is also alien to South Africa and morphologically similar, was found in the same region. Regular mowing at the reported site along the Hennops River in Pretoria likely eliminated any above-ground presence. The coherence of the negative evidence, from repeated expert-led surveys to the absence of herbarium records, trade history or confirmed sightings, supported a conclusion of not present with medium confidence. The recommendation is to change the listing from category 1a to prohibited, with no further active surveillance warranted.

The researchers argue that argument maps complement rather than replace decision trees, which simplify complexity through categorical yes-or-no pathways, whereas argument maps retain complexity by making assumptions, uncertainties and objections transparent. The approach also exposed where confidence was lacking and where further evidence would be most useful, from inadequate long-term monitoring to poor geographic coverage. Beyond South Africa, similar uncertainties exist globally, and the team intends to incorporate the method into the Risk Analysis for Alien Taxa framework, with decisions feeding into South Africa’s list of alien taxa. By supporting evidence-based regulatory listings, the approach could contribute meaningfully to reporting progress towards Target 6 of the Global Biodiversity Framework, giving policymakers a clearer view of the trade-offs and risks involved in every listing decision.

Subject of Research: Argument maps applied to alien species presence decisions in South Africa

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

Article References: Matthys, C., Wilson, J. R. U., & Geerts, S. (2026). Argument maps can support decisions to declare the presence of alien species: South Africa as a case study. NeoBiota, 105, 113-130. https://doi.org/10.3897/neobiota.105.161591

Image Credits: AI Generated

DOI: 10.3897/neobiota.105.161591

Keywords: invasive species, argument maps, South Africa, biodiversity policy, NeoBiota, evidence evaluation, biological invasions, regulation, Argument, maps, support, decisions