Species moved by human activities beyond the limits of their native geographic ranges into areas in which they do not naturally occur (termed aliens) can cause a broad range of significant changes to recipient ecosystems; however, their impacts vary greatly across species and the ecosystems into which they are introduced. There is therefore a critical need for a standardised method to evaluate, compare, and eventually predict the magnitudes of these different impacts. Here, we propose a straightforward system for classifying alien species according to the magnitude of their environmental impacts, based on the mechanisms of impact used to code species in the International Union for Conservation of Nature (IUCN) Global Invasive Species Database, which are presented here for the first time. The classification system uses five semi-quantitative scenarios describing impacts under each mechanism to assign species to different levels of impact-ranging from Minimal to Massive-with assignment corresponding to the highest level of deleterious impact associated with any of the mechanisms. The scheme also includes categories for species that are Not Evaluated, have No Alien Population, or are Data Deficient, and a method for assigning uncertainty to all the classifications. We show how this classification system is applicable at different levels of ecological complexity and different spatial and temporal scales, and embraces existing impact metrics. In fact, the scheme is analogous to the already widely adopted and accepted Red List approach to categorising extinction risk, and so could conceivably be readily integrated with existing practices and policies in many regions.

Centre for Invasion Biology Department of Botany and Zoology Stellenbosch University Matieland South Africa

Centre for Invasion Biology Department of Botany and Zoology Stellenbosch University Matieland South Africa; South African National Biodiversity Institute Kirstenbosch National Botanical Gardens Claremont South Africa

Department of Biology Unit Ecology and Evolution University of Fribourg Fribourg Switzerland

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

Department of Ecology Faculty of Science Charles University Prague Prague Czech Republic

Environment Agency Austria Department of Biodiversity and Nature Conservation Vienna Austria

Estación Biológica de Doñana Seville Spain

German Centre for Integrative Biodiversity Research Halle Jena Leipzig Leipzig Germany

Imperial College London Ascot Berkshire United Kingdom

Institute of Botany Department of Invasion Ecology Academy of Sciences of the Czech Republic Průhonice Czech Republic

Institute of Botany Department of Invasion Ecology Academy of Sciences of the Czech Republic Průhonice Czech Republic; Department of Ecology Faculty of Science Charles University Prague Prague Czech Republic

Institute of Ecology and Evolution University of Bern Bern Switzerland

Institute of Zoology Zoological Society of London London United Kingdom; Distinguished Scientist Fellowship Program King Saud University Riyadh Saudi Arabia; Environment Institute School of Earth and Environmental Sciences University of Adelaide Adelaide South Australia Australia

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

Redpath Museum McGill University Montreal Quebec Canada

Technische Universität München Department of Ecology and Ecosystem Management Restoration Ecology Freising Weihenstephan Germany

The Bio Protection Research Centre Lincoln University Christchurch New Zealand

UFZ Helmholtz Centre for Environmental Research Department of Community Ecology Halle Germany

UFZ Helmholtz Centre for Environmental Research Department of Community Ecology Halle Germany; German Centre for Integrative Biodiversity Research Halle Jena Leipzig Leipzig Germany

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