Which taxa are alien? criteria, applications, and uncertainties

Human activities such as the transport of species to new regions and modifications of the environment are increasingly reshaping the distribution of biota. Accordingly, developing robust, repeatable, and consistent definitions of alien species that serve scientific and policy purposes has become of prime importance. We provide a set of classification criteria that are widely applicable across taxa and realms and offer guidance on their use in practice. The criteria focus on (a) acknowledging the role of assessment uncertainty, (b) incorporating time since introduction, (c) considering infraspecific taxonomic ranks, and (d) differentiating between alien species whose survival depends on explicit human assistance from those that survive without such assistance. Furthermore, we make recommendations for reducing assessment uncertainty, suggest thresholds for species assessment, and develop an assessment scheme. We illustrate the application of the assessment criteria with case studies. Finally, the implications for alien species management, policy, and research are discussed.

FE and MvK acknowledge support from the DFG-FWF project the GloNAF database (no. I2086B16, KL1866/9-1), with the national funders the Austrian Science Foundation (FWF) and the German Science Foundation (DFG). JMJ was financially supported by the DFG through grants no. JE 288/9-1 and no. JE 288/9-2. DMR and JRUW acknowledge support from the DST-NRF Centre of Excellence for Invasion Biology and the National Research Foundation of South Africa (grants no. 85417 and no. 86894). PP was supported by project no. 14-36079G of the Centre of Excellence PLADIAS of the Czech Science Foundation and the long-term research development project no. RVO 67985939 from the Czech Academy of Sciences. MV was supported by the project IMPLANTIN (no. CGL2015-65346-R).