title: A unified classification of alien species based on the magnitude of their environmental impacts.
abstract: 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.
authors.affiliation: 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 & Environmental Sciences, University of Adelaide, Adelaide, South Australia, Australia.
authors.forename: Tim M
authors.initials: TM
authors.lastname: Blackburn
authors.affiliation: Department of Conservation Biology, Vegetation and Landscape Ecology, University of Vienna, Vienna, Austria.
authors.forename: Franz
authors.initials: F
authors.lastname: Essl
authors.affiliation: Imperial College London, Ascot, Berkshire, United Kingdom.
authors.forename: Thomas
authors.initials: T
authors.lastname: Evans
authors.affiliation: The Bio-Protection Research Centre, Lincoln University, Christchurch, New Zealand.
authors.forename: Philip E
authors.initials: PE
authors.lastname: Hulme
authors.affiliation: Technische Universität München, Department of Ecology and Ecosystem Management, Restoration Ecology, Freising-Weihenstephan, Germany.
authors.forename: Jonathan M
authors.initials: JM
authors.lastname: Jeschke
authors.affiliation: UFZ - Helmholtz Centre for Environmental Research, Department of Community Ecology, Halle, Germany; German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Leipzig, Germany.
authors.forename: Ingolf
authors.initials: I
authors.lastname: Kühn
authors.affiliation: Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Matieland, South Africa.
authors.forename: Sabrina
authors.initials: S
authors.lastname: Kumschick
authors.affiliation: 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 in Prague, Prague, Czech Republic.
authors.forename: Zuzana
authors.initials: Z
authors.lastname: Marková
authors.affiliation: Department of Ecology, Faculty of Science, Charles University in Prague, Prague, Czech Republic.
authors.forename: Agata
authors.initials: A
authors.lastname: Mrugała
authors.affiliation: Institute of Ecology and Evolution, University of Bern, Bern, Switzerland.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Nentwig
authors.affiliation: Institute of Botany, Department of Invasion Ecology, Academy of Sciences of the Czech Republic, Průhonice, Czech Republic.
authors.forename: Jan
authors.initials: J
authors.lastname: Pergl
authors.affiliation: 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 in Prague, Prague, Czech Republic.
authors.forename: Petr
authors.initials: P
authors.lastname: Pyšek
authors.affiliation: Environment Agency Austria, Department of Biodiversity and Nature Conservation, Vienna, Austria.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Rabitsch
authors.affiliation: Redpath Museum, McGill University, Montreal, Quebec, Canada.
authors.forename: Anthony
authors.initials: A
authors.lastname: Ricciardi
authors.affiliation: Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Matieland, South Africa.
authors.forename: David M
authors.initials: DM
authors.lastname: Richardson
authors.affiliation: UFZ - Helmholtz Centre for Environmental Research, Department of Community Ecology, Halle, Germany.
authors.forename: Agnieszka
authors.initials: A
authors.lastname: Sendek
authors.affiliation: Estación Biológica de Doñana (EBD-CSIC), Seville, Spain.
authors.forename: Montserrat
authors.initials: M
authors.lastname: Vilà
authors.affiliation: 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.
authors.forename: John R U
authors.initials: JR
authors.lastname: Wilson
authors.affiliation: German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Leipzig, Germany.
authors.forename: Marten
authors.initials: M
authors.lastname: Winter
authors.affiliation: ISPRA, Institute for Environmental Protection and Research and Chair IUCN SSC Invasive Species Specialist Group, Rome, Italy.
authors.forename: Piero
authors.initials: P
authors.lastname: Genovesi
authors.affiliation: Department of Biology, Unit Ecology & Evolution, University of Fribourg, Fribourg, Switzerland.
authors.forename: Sven
authors.initials: S
authors.lastname: Bacher
mesh_terms: D063147:Animal Distribution / Q000502:physiology*; D000818:Animals; D044822:Biodiversity; D004777:Environment; D053476:Extinction, Biological; D020387:Food Chain; D060434:Herbivory / Q000502:physiology; D006802:Human Activities / Q000639:trends; D006801:Humans; D058865:Introduced Species / Q000706:statistics & numerical data*; D063148:Plant Dispersal / Q000502:physiology*; D010944:Plants / Q000382:microbiology / Q000469:parasitology / Q000821:virology; D011157:Population Dynamics / Q000639:trends; D011235:Predatory Behavior / Q000502:physiology; D012987:Soil / Q000737:chemistry; D013045:Species Specificity; D035501:Uncertainty
chemical_list: D012987:Soil