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 species known 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 that do not yet occur in Europe to be subsequently risk assessed for future listing. Accordingly, we present a systematic consensus horizon scanning procedure to derive a ranked list of potential IAS likely to arrive, establish, spread and have an impact on biodiversity in the region over the next decade. The approach is unique in the continental scale examined, the breadth of taxonomic groups and environments considered, and the methods and data sources used. International experts were brought together to address five broad thematic groups of potential IAS. For each thematic group the experts first independently assembled lists of potential IAS not yet established in the EU but potentially threatening biodiversity if introduced. Experts were asked to score the species within 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 the EU. Experts then convened for a 2-day workshop applying consensus methods to compile a ranked list of potential IAS. From an initial working list of 329 species, a list of 66 species 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 present these species highlighting the potential negative impacts and the most likely biogeographic regions to be affected by these potential IAS.

Belgian Biodiversity Platform Louizalaan Brussels Belgium

Bio Protection Research Centre Lincoln University Lincoln New Zealand

CABI Delémont Switzerland

Centre for Conservation Ecology Bournemouth University Poole UK

Centre for Ecology and Hydrology Wallingford UK

Centre for Environment Fisheries and Aquaculture Science Lowestoft UK

Department of Plant and Environmental Sciences University of Copenhagen Denmark

Department of Zoology University of Cambridge Cambridge UK

Division of Conservation Biology Vegetation Ecology and Landscape Ecology University Vienna Vienna Austria

Environment Agency Austria Vienna Austria

Estación Biológica de Doñana Seville Spain

GRECO Institute of Aquatic Ecology University of Girona Girona Spain

Helenic Centre for Marine Research Anavyssos Greece

Institut National de la Recherche Agronomique Zoologie Forestière UR 0633 Ardon Orleans Cedex 2 France

Institute for Environmental Protection and Research ISPRA and Chair IUCN SSC Invasive Species Specialist Group Rome Italy

Institute for European Environmental Policy London UK

Institute for Water and Wetland Research Radboud University Nijmegen The Netherlands

Institute of Botany The Czech Academy of Sciences Průhonice Czech Republic

Institute of Nature Conservation Polish Academy of Sciences Kraków Poland

Institute of Zoology Zoological Society of London London UK

Invasive Species Unit Service Public de Wallonie Wallonia Belgium

ISSG Rome Italy

Marine Organism Investigations Marina Village Ballina Killaloe Co Clare Ireland

Natural History Museum London UK

Naturalis Biodiversity Center Leiden The Netherlands

Netherlands Centre of Expertise for Exotic Species Nijmegen The Netherlands

Ovidius University of Constanta Constanta Romania

Pyrenean Institute of Ecology Zaragoza Spain

Research Institute for Nature and Forest Brussels Belgium

Royal Belgian Institute of Natural Sciences Oostende Belgium

Royal Society for the Protection of Birds The Lodge Sandy Bedfordshire UK

Scottish Association for Marine Science Scottish Marine Institute Oban UK

The Laboratory The Marine Biological Association Plymouth UK

University College London London UK

University of Bern Bern Switzerland

University of Florence Firenze Italy

University of Fribourg Fribourg Switzerland

University of Sussex Brighton UK

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