Developing a list of invasive alien species likely to threaten biodiversity and ecosystems in the European Union

The European Union (EU) has recently published its first list of invasive alien species(IAS) of EU concern to which current legislation must apply. The list comprises speciesknown to pose great threats to biodiversity and needs to be maintained and updated.Horizon scanning is seen as critical to identify the most threatening potential IAS thatdo not yet occur in Europe to be subsequently risk assessed for future listing. Accord-ingly, we present a systematic consensus horizon scanning procedure to derive a rankedlist of potential IAS likely to arrive, establish, spread and have an impact on biodiversityin the region over the next decade. The approach is unique in the continental scaleexamined, the breadth of taxonomic groups and environments considered, and themethods and data sources used. International experts were brought together to addressfive broad thematic groups of potential IAS. For each thematic group the experts firstindependently assembled lists of potential IAS not yet established in the EU but poten-tially threatening biodiversity if introduced. Experts were asked to score the specieswithin their thematic group for their separate likelihoods of i) arrival, ii) establishment,iii) spread, and iv) magnitude of the potential negative impact on biodiversity within theEU. Experts then convened for a 2-day workshop applying consensus methods to com-pile a ranked list of potential IAS. From an initial working list of 329 species, a list of 66species not yet established in the EU that were considered to be very high (8 species),high (40 species) or medium (18 species) risk species was derived. Here, we presentthese species highlighting the potential negative impacts and the most likely biogeo-graphic regions to be affected by these potential IAS.