Big troubles are already here: risk assessment protocol shows high risk of many alien plants present in Portugal

Invasion by alien plants results in serious adverse impacts on biodiversity, ecosystem services, economy and social welfare, and is an ever-increasing challenge for nature conservation. Control of established invasive species is frequently very difficult and costly. Therefore, predicting which species have risk of becoming invasive is crucial both to prevent introduction of new invaders and to target high risk species already present in order to avoid their spread, particularly to areas with high conservation value. In Portugal more than 600 alien plant species are present as casuals or naturalized, and decision support tools are needed to discriminate which of these have higher probability of becoming invasive. We aimed to assess the effectiveness of the Australian Weed Risk Assessment, adapted to the Portuguese conditions (P-WRA), by evaluating 172 plant species: 49 considered as invasive; and, 123 as non-invasive species. The results showed that the P-WRA correctly identified all invasive species. As for non-invasive species, 17% were accepted, 78% rejected and 5% required further evaluation. Receiver Operating Characteristic (ROC) curve reflected high accuracy of predictions despite lower than that found in other regions. The best cutoff level for P-WRA score, maximizing the ability for classification of the protocol, was 13 resulting in more non-invasive species accepted (46%). In the end, this assessment informed that in addition to the 49 species already invasive, a high number (66) of alien plant species have invasive risk. Amongst these, a list of 20 species is proposed for targeted priority management aiming to prevent their spread. Comparison of the scores obtained with A-WRA for other regions with Mediterranean climate revealed that 17% of the species compared (78) reached different outcomes, signaling the need to be careful when extrapolating the use of previous scores. The P-WRA can be a promising screening system post-border for predicting invasive species already present in Portugal and contribute to the targeting of species for priority intervention, particularly in natural areas with high conservation value.