The signatory states of the Convention on Biological Diversity have committed themselves to “Prevent[ing] the introduction of, control[ling] or eradicate[ing] those alien species which threaten ecosystems, habitats or species” (CBD, Article 8 [h]). As a step towards fulfilling this obligation, the Norwegian government has decided to carry out ecological impact assessments of alien species on a regular basis. The body responsible for these assessments is the Norwegian Biodiversity Information Centre (NBIC). So far, three such rounds of assessments of alien species have been carried out in Norway, and the methodology used has been improved each time, based on the experience collected. The first assessment was purely qualitative in nature, and covered 217 selected alien species (Gederaas et al. 2007). The second assessment used a completely new set of criteria, which was semi-quantitative (Sandvik et al. 2013). It was used to assess 1383 alien species (Gederaas et al. 2013). The third impact assessment, which has been completed in 2018, covered 1532 taxa, including all alien species (within certain delimitations) known to occur in Norway (H. Sandvik et al. in prep.). The aim of this paper is to describe the method used in the third assessment, the generic ecological impact assessment of alien species (GEIAA). GEIAA is a revised version of the method that was used in the previous assessment (Sandvik et al. 2013). Due to the revision, GEIAA’s criteria are now quantitative throughout. With 1532 impact assessments in Norway and an additional 1033 impact assessments in Sweden (Strand et al. 2018), GEIAA is among the most widely applied alien species assessment schemes.

GEIAA’s set of criteria

The core of GEIAA is the set of criteria, based on which species can be assigned to five ecological impact categories from ‘no known impact’ to ‘severe impact’. Three criteria (A–C) are used to assess invasion potential, while the remaining six criteria (D–I) capture the ecological effect of species. For each species, all nine criteria are to be assessed, assigning scores between 1 and 4.

Threshold values for the criteria are summarised in Tables 1, 2 and 3. Some key terms, which are given in small capitals in the following criteria definitions, are explained in Box 1. The rationale behind each criterion and the differences between GEIAA and the previous set of criteria are outlined in Online Resource 1.

Table 1 Criteria, scores and threshold values for the classification of the invasion potential of alien species
Table 2 Criteria, scores and threshold values for the classification of the ecological effect of alien species, criteria D–G
Table 3 Criteria, scores and threshold values for the classification of the ecological effect of alien species, criteria H and I
Box 1 Definitions of key terms

Overall impact

Ecological impact is here defined as the product of invasion potential and ecological effect. For this reason, the impact of alien species on nature can best be captured using a two-dimensional figure (Fig. 1), where impact is indicated by the species’ position along two axes—the invasion axis (criteria A–C) and the effect axis (criteria D–I). On each axis separately, the relevant criteria are combined in accordance with the one-out–all-out principle. In other words, the maximum score of the six effect criteria determines the placement along the effect axis; and the maximum score of the three invasion criteria determines the placement along the invasion axis (with the reservation that criteria A and B are coupled by means of auxiliary conditions, cf. Table 1 and Table A2 in Online Resource 1).

Fig. 1
figure 1

Impact matrix. During impact assessment, a score between 1 and 4 is assigned to the invasion potential and to the ecological effect of a given species (using the numerical thresholds described in Tables 1, 2 and 3, Box 1). The ecological impact of an alien species increases with increasing invasion potential (x-axis, criteria A–C) and with increasing ecological effect (y-axis, criteria D–I), and it is classified into five impact categories (NK, LO, PH, HI, SE)

The four subcategories along each axis provide the basis for 16 possible combinations of invasion potential and ecological effects (Fig. 1). The position of a species in Fig. 1 illustrates the (risk of) impact that a species exerts on nature. The position determines, in turn, which of the five impact categories the species is placed in:

  • severe impact (SE),

  • high impact (HI),

  • potentially high impact (PH),

  • low impact (LO) or

  • no known impact (NK).

Species that are excluded from assessments, e.g., because they are not alien species or do not fulfil the historical, geographic, ecological or taxonomic delimitations (see Box 1 and H. Sandvik et al. in prep.), are referred to as ‘not risk-assessed’ (NR). For reasons that are detailed in Online Resource 1 (§ 1.1), GEIAA does not have a category for ‘data deficiency’.

Criteria A–C: invasion potential

Invasion processes can be split into two phases, which form the basis for one criterion each: establishment and expansion. A third criterion relates to the area of ecosystems that is colonised.


Population lifetime The higher the median population lifetime of an alien species, the higher the species scores on the invasion axis (Table 1).


Expansion speed The higher the expansion speed of an alien species, the higher the species scores on the invasion axis (Table 1).


Colonisation of ecosystems The larger the area of an ecosystem colonised by an alien species, the higher the species scores on the invasion axis (Table 1).

Criteria D–I: ecological effect

Alien species are classified along the effect axis (Fig. 1) according to their negative effects upon nature. The six criteria measure ecological and genetic effects on native species as well as effects on ecosystems.


Interactions with threatened or keystone species The stronger the negative ecological interactions an alien species has with threatened or keystone species, the higher the alien species scores on the effect axis (Table 2).


Interactions with other native species The stronger the negative ecological interactions an alien species has with other native species (that are neither threatened nor keystone), the higher the alien species scores on the effect axis (Table 2).


Changes in threatened or rare ecosystems The larger the area of threatened or rare ecosystems undergoing substantial change due to an alien species, the higher the species scores on the effect axis (Table 2).


Changes in other ecosystems The larger the area of other ecosystems (that are neither threatened nor rare nor heavily modified) undergoing substantial change due to an alien species, the higher the species scores on the effect axis (Table 2).


Genetic contamination The larger the likelihood and consequence of an alien species genetically contaminating native species by introgression, the higher the alien species scores on the effect axis (Table 3).


Transmission of parasites The larger the likelihood and consequences of an alien species acting as a vector for parasites (including pathogens such as bacteria or viruses) to native hosts, the higher the alien species scores on the effect axis (Table 3).

GEIAA’s assessment procedure

In addition to the set of criteria described above, GEIAA contains guidelines on the procedure of assessment. Assessments are carried out by experts in a purpose-made web application, the Alien Species Database. This application has two interfaces: an assessment interface and a public interface. The assessments and all documentation are registered in the assessment interface (for an English test version, see, which is only accessible to the assessors, facilitates standardisation across assessors and provides a way of archiving all data. After completion of assessments and quality assurance, the results are made available in the public interface (for Norwegian assessments, see

GEIAA’s assessment procedure includes instructions on the following four aspects (for details, see Online Resource 1):

  • Time frame All assessments are to be based on historical and current effects. Assessments of criteria C–I should also consider effects that, based on documented evidence, can be expected to occur within 50 years into the future.

  • Documentation A criterion is not regarded as met unless documentation is available. In addition to the documentation regarding the nine criteria, further information is archived in the Alien Species Database, including species characteristics, distribution history and pathways of introduction and spread (Table A3 in Online Resource 1). Documentation may consist of scientific publications, but also of the assessors’ own observations or judgements and other unpublished data or analyses, provided the latter are uploaded to the Alien Species Database.

  • Uncertainty Uncertainty is reported in terms of interquartile ranges (equivalent to 50% confidence intervals).

  • Quality assurance Assessments are to be carried out by expert panels rather than single assessors. Assessors receive training from NBIC, and NBIC checks whether assessments have been following the guidelines. Before finalising assessments, the preliminary results are circulated for public comment.


GEIAA is a (1) generic (2) ecological (3) impact assessment of alien species based on a (4) quantitative set of criteria. These four characteristics merit elaboration:

  1. (1)

    GEIAA is generic in the sense that it is applicable to all living taxonomic groups, irrespective of phylogenetic position, habitat or status. This is corroborated by the fact that it has been used in Norway to assess 1460 species and 72 sub-specific taxa belonging to all major eukaryotic groups (‘algae’, animals, fungi, plants); occurring in marine, freshwater and terrestrial habitats; leading sessile, vagile or parasitic lives; and including both established alien species and door-knockers (H. Sandvik et al. in prep.). So far, it has not been applied to unicellular organisms or viruses, but GEIAA would presumably be applicable in those cases, too, since it worked well with pathogens such as oomycetes. Genericity is attained by avoiding taxon-specific or taxon-dependent criteria, such as population size, fecundity or dispersal distance. Instead, GEIAA uses parameters that are directly comparable (e.g., population viability, AOO, species interactions).

  2. (2)

    GEIAA assesses ecological effects in the sense that anthropocentric effects of alien species are deliberately excluded from the set of criteria. Direct or indirect, positive or negative, effects upon human health, ecosystem services, economy, aesthetics etc. are regarded as anthropocentric in this context, as is the feasibility of management measures. Information available on such effects is collected as part of the assessment procedure and made available to stakeholders and the public together with the ecological results (cf. Table A3e in Online Resource 1), but it does not affect the impact score. This is because the aim of GEIAA is a purely ecological and normatively neutral impact assessment, in analogy to the Red List, which is based on ecological criteria alone. The weighting of ecological (e.g., conservation) concerns against economic and other anthropocentric issues is a decision of a normative or political rather than a scientific nature, and it should therefore be taken by management authorities. GEIAA is meant to provide the ecological background knowledge needed by the authorities for making informed decisions on alien species management.

  3. (3)

    GEIAA defines (ecological) impact as the product of invasion potential and (ecological) effect. This definition is based on the understanding that impact is proportional to the area invaded, to the density attained, and to the per-capita effects exerted (Parker et al. 1999). As the area colonised often will be unknown and increasing, area is replaced by a species’s invasion potential. Population density and per-capita effect can be combined into a measure of per-locality ecological effect. These two factors must be multiplied, and not added together, if the ecological impact is to be quantified (cf. Branquart 2009; D’hont et al. 2015). A species will thus have a small impact whenever one of the factors is small. This is the rationale for using a two-dimensional impact matrix (Fig. 1). The concept of impact underlying GEIAA differs from some other assessment schemes (e.g., EICAT, GISS; Hawkins et al. 2015; Nentwig et al. 2016), which do not explicitly incorporate the spatial component (area invaded), so that their “impact” is equivalent to our concept of (per-locality) ecological effect. In Jeschke et al.’s (2014) framework, our definition of impact is unidirectional (by excluding positive effects), normatively neutral (by excluding human values), quantitative (see below), ecological (see above); and its spatial, temporal, taxonomic and functional scales cover impacts of the entire alien population in the assessment area within 50 years, and on all multicellular taxa at all organisational levels (gene to ecosystem).

  4. (4)

    GEIAA is a fully quantitative set of criteria in the sense that all thresholds for all criteria are numerically defined (Tables 1, 2 and 3, Box 1). Although the need for quantitative assessments is widely recognised (Lodge et al. 2006), the majority of assessment schemes is still qualitative (Verbrugge et al. 2010). Quantitative sets of criteria have several advantages over qualitative ones, including a higher degree of repeatability, testability and transparency (Tversky and Kahneman 1974; Burgman 2001; McCarthy et al. 2004). In a comparison of twelve impact assessment schemes, GEIAA obtained the highest repeatability (i.e., the lowest coefficient of variation of species scorings across assessors; González-Moreno et al. 2019), which is likely due to its quantitative nature.

GEIAA meets the 14 minimum standards that have been developed for the assessment of alien species (Roy et al. 2018; for details, see Online Resource 1, § 3). The method is currently used in Norway (where it constitutes the third generation of assessments) and in Sweden, but the principles and criteria are applicable in any country or region. In line with its generic nature, GEIAA has been used to carry out more than 2500 impact assessments of alien species in all major taxa and habitats (Strand et al. 2018; H. Sandvik et al. in prep.).