Predicting plant invaders in the Mediterranean through a weed risk assessment system

Risk assessment schemes have been developed to identify potential invasive species, prevent their spread and reduce their damaging effects. One of the most promising tools for detecting plant invaders is the weed risk assessment (WRA) scheme developed for Australia. Our study explores whether the Australian WRA can satisfactorily predict the invasion status of alien plants in the Mediterranean Basin by screening 100 invasive and 97 casual species in Spain. Furthermore, we analysed whether the factors taken into account in the WRA are linked to invasion likelihood (i.e., invasion status) or to impacts. The outcome was that 94% of the invasive species were rejected, 50% of the casual species were rejected and 29% of them required further evaluation. The accuracy for casuals is lower than in other studies that have tested non-invasive (i.e., casuals or non-escaped) alien species. We postulate that low accuracy for casual species could result from: (1) an incorrect “a priori” expert classification of the species status, (2) a high weight of the WRA scores given to potential impacts, and (3) casual species being prone to becoming invasive when reaching a minimum residence time threshold. Therefore, the WRA could be working as a precaution early-warning system to identify casual species with potential to become invasive.