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.
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).
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:
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 http://efab.artsdatabanken.no/fab/efab/), 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 https://artsdatabanken.no/fremmedartslista2018).
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.