The Impact of Alien Species: towards an IUCN Black List

Main Points:

It has been fifty years since the International Union for Conservation of Nature (IUCN) first published its famous Red List of Threatened Species, which categorises species according to their risk of extinction. However, one of the major global consequences of human activities is the wholesale redistribution of species to areas outside their native range, whether through deliberate or accidental transfer of their populations. In Europe more than 13000 non-European species are recognized to live in the wild, while in Switzerland alone there are more than 900 such species. These “alien” species have in many cases caused substantial damage to their new environment, including extinctions of native populations and species, and the disruption of soil nutrients and water cycling.

Published in:

PLOS Biology

Study Further:

In an Essay publishing in the open access journal PLOS Biology on May 6th, an international team of leading experts from four continents, in collaboration with the IUCN, proposes a pragmatic solution to this problem. Their scheme allows ranking of alien species from different animal and plant groups into so-called “Black Lists” of harmful species according to the magnitude of their environmental impacts. It is designed to have a similar structure and logic to the widely adopted IUCN “Red List” for categorizing extinction risk. Like the IUCN Red List, the proposed Black List can be used to prioritise species for action, as required by international policies on biological invasions. The scheme can not only provide a basis for decision-making, but could also be developed into a formal indicator of progress towards the achievement of the aims of Aichi Biodiversity Target 9 of the Convention on Biological Diversity. Target 9 states that “By 2020, invasive alien species and pathways are identified and prioritized, priority species are controlled or eradicated, and measures are in place to manage pathways to prevent their introduction and establishment.”

Preventing and mitigating the impact of alien species is a major drain on limited conservation resources. There is thus a considerable incentive to understand which species are currently, or are likely to become, the most damaging (or conversely, which have minimal impacts), so that we can direct legislation and prioritize action more effectively to preventative biosecurity, early eradication, or long-term control programs. A fundamental problem here is how to compare the enormous range of impacts attributable to diverse alien taxa, acting on different levels of ecological complexity, and at different spatial and temporal scales.

The authors’ scheme defines scenarios that describe increasing levels of impact on native species by a range of different mechanisms. Scenarios are designed such that each successively higher category reflects an increase in the order of magnitude of the effects of a given impact mechanism (impacts on native individuals, populations, communities), allowing the magnitudes of impacts caused by different mechanisms to be directly compared. A species assigned to a higher impact category is considered to have had a greater deleterious impact on some aspect of an environment in which it is alien than does a species in a lower impact category.

The publication is the result of the workshops Impact of the Synthesis Centre for Biodiversity sciences (sDiv) of the German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig.


Blackburn TM, Essl F, Evans T, Hulme PE, Jeschke JM, et al. (2014) A Unified Classification of Alien Species Based on the Magnitude of their Environmental Impacts. PLoS Biol 12(5): e1001850. doi:10.1371/journal.pbio.1001850,


PD Dr. Sven Bacher, Department of Biology, Ecology & Evolution Unit, University of Fribourg, Ch. du Musée 10, CH-1700 Fribourg, Switzerland, Tel +41-26-300 88 22, Fax +41-26-300 96 98,


The sDiv workshop that led to the sImpact working group was funded by the German Research Foundation DFG (FZT 118). JP, PP, and ZM acknowledge the support from long-term research development project no. RVO 67985939 (Academy of Sciences of the Czech Republic), Centre of  xcellence PLADIAS no. 14-36079G, and grant no. P504/11/1028 (Czech Science Foundation), and institutional resources of the Ministry of Education, Youth and Sports of the Czech Republic. PP acknowledges the support by the Praemium Academiae award from the Academy of Sciences of the Czech Republic. JMJ acknowledges support from the ERA-Net BiodivERsA (project FFII), with the national funder German Research Foundation DFG (JE 288/7-1). SK acknowledges financial support  from the Swiss National Science Foundation, the DST-NRF Centre of Excellence for Invasion Biology, and the Drakenstein Trust. DMR and JRUW acknowledge support from the DST-NRF Centre of Excellence for Invasion Biology and the National Research Foundation (grants 85417 and 86894, respectively). AS acknowledges financial support of the German Academic Exchange Service (DAAD). MV acknowledges support from the Severo Ochoa Program for Centres of Excellence in R+D+I (SEV-2012-0262). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing Interests:

The authors have declared that no competing interests exist.

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