Reducing uncertainty in impact assessments for alien species

Impact assessment is a widely used and cost-effective tool for prioritising invasive alien species. With the number of alien and invasive alien species expected to increase, reliance on impact assessment tools for the identification of species that pose the greatest threats will continue to grow. Given the importance of such assessments for management and resource allocation, it is critical to understand the uncertainty involved and what effect this may have on the outcome. Using an uncertainty typology and insects as a model taxon, we identified and classified the sources and types of uncertainty when performing impact assessments on alien species. We assessed 100 alien insect species across two rounds of assessments with each species independently assessed by two assessors. Agreement between assessors was relatively low for all three EICAT components (mechanism, severity, confidence) after the first round. For the second round, we revised guidelines and gave assessors access to each others assessments which improved agreement by between 20-30%. Of the 12 potential reasons for assessment discrepancies identified a priori, 11 occurred. The most frequent sources (and types) of uncertainty (i.e. differences between assessment outcomes for the same species) were: incomplete information searches (systematic error), unclear mechanism and/or extent of impact (subjective judgment due to a lack of knowledge), and limitations of the assessment framework (context dependence). In response to these findings, we identify actions to reduce uncertainty in the impact assessment process, particularly for assessing speciose taxa with diverse life histories such as Insecta. Evidence of environmental impact was available for most insect species, and (of the non-random original subset of species assessed) 14 or 29% of those with evidence were identified as high impact species (with either Major or Massive impact). Although uncertainty in risk assessment, including impact assessments, can never be eliminated, identifying and communicating its source and variety is a first step toward its reduction and a more reliable assessment outcome, regardless of the taxa being assessed.