The third essential variable in an invasion monitoring system is the impact that the alien species has on biodiversity and ecosystems. A standardised method for classifying alien species impacts across species and environments is needed to enable prioritisation of species for policy and management.
A method to do exactly this, called the IUCN Environmental Impact Classification for Alien Taxa (EICAT) is now available [1, 2]. It describes levels of impact on native ecosystems under 10 mechanisms of impact [1, 2, 3]. Linking impacts to scenarios in this way provides a transparent approach for assigning an aliens species to one of five sequential categories of impact: Minimal Concern, Minor, Moderate, Major, and Massive.
The different categories in the EICAT scheme, and the relationships between them are shown in figure below (reproduced from ). The process of categorisation is sequential: species are first evaluated for evidence of alien populations, then assessed for evidence of impact. If any such evidence exists, species are categorised according to the magnitude of such impacts on the basis of semi-quantitative scenarios of impact.
Relationship between alien species impact and Essential Biodiversity Variables
The ‘alien species impact’ variable is a higher level, composite variable that includes information from multiple Essential Biodiversity Variables (EBVs), such as species traits and ecosystem structure and function. It is not itself an Essential Biodiversity Variable (EBV). This variable is not observation-based but rather the outcome of a semi-quantitive, analytical and stakeholder involved process [1, 2].
Referenced material and links
 Hawkins, C.L., Bacher, S., Essl, F., Hulme, P.E., Jeschke, J.M., Kühn, I., Kumschick, S., Nentwig, W., Pergl, J., Pyšek, P., Rabitsch, W., Richardson, D.M., Vilà, M., Wilson, J.R.U., Genovesi, P., and Blackburn, T.M. (2015) Framework and guidelines for implementing the proposed IUCN Environmental Impact Classification for Alien Taxa (EICAT). Diversity and Distributions. doi: 10.1111/ddi.12379.
 McGeoch, M.A., Lythe, M. J., Henriksen, M. V., and McGrannachan, C. M (2015) Environmental impact classification for alien insects: a review of mechanisms and their biodiversity outcomes. Current Opinion in Insect Science 12, 46-53. doi:10.1016/j.cois.2015.09.004.