Table 1. Scenarios used in EICAT and EICAT+ to assign alien taxa to impact categories.

Detailed description of the 5 semiquantitative scenarios used in EICAT [24] and EICAT+ to assess negative and positive impacts caused by alien taxa on native taxa at different levels of organization. The scenarios are used in EICAT and EICAT+ to assign alien taxa to one of 5 ascending categories of impact magnitude. Note that when there is no or inadequate information to classify an alien taxon to one of the 5 impact categories, the taxon should be classified as Data Deficient (DD).

EICAT (Environmental Impact Classification for Alien Taxa)	EICAT+ (positive Environmental Impact Classification for Alien Taxa)
Minimal Concern (MC) A taxon is considered to have impacts of Minimal Concern when it causes negligible levels of impacts, but no reduction in performance of individuals in the native biota. Note that all alien taxa have impacts on the recipient environment at some level, for example, by altering species diversity or community similarity (e.g., biotic homogenization), and for this reason, there is no category equating to “no impact.” Only taxa for which changes in the individual performance of natives have been studied but not detected are assigned an MC category. Taxa that have been evaluated under the EICAT process but for which impacts have not been assessed in any study should not be classified in this category, but rather should be classified as Data Deficient.	Minimal positive impact (ML+) A taxon is considered to have Minimal positive impacts when it interacts with at least 1 native taxon through a mechanism that can lead to positive impacts but causes only a negligible increase in the performance of individuals of the native taxon. Note that although alien taxa are often thought to cause positive impacts on native taxa—for instance, by serving as a food source for native consumers—these impacts should be considered minimal if the alien simply fulfills a functional role already held by other taxa without increasing the performance of individuals of the native taxon. Only alien taxa for which positive changes in the individual performance of native taxa have been studied but not detected are assigned an ML+ category. Alien taxa that have been evaluated under the EICAT+ process but for which impacts have not been assessed in any study should not be classified in this category, but rather should be classified as Data Deficient.
Minor impact (MN) A taxon is considered to have Minor impacts when it causes reductions in the performance of individuals in the native biota, but no declines in native population sizes, and has no impacts that would cause it to be classified in a higher impact category.	Minor positive impact (MN+) A taxon is considered to have Minor positive impacts when it causes increases (or prevents/mitigates decreases) in the performance of individuals of at least 1 native taxon, but no increases in native population sizes, and has no positive impacts that would cause it to be classified in a higher impact category.
Moderate impact (MO) A taxon is considered to have Moderate impacts when it causes declines in the population size of at least 1 native taxon, but has not been observed to lead to local extinction of a native taxon.	Moderate positive impact (MO+) A taxon is considered to have Moderate positive impacts when it causes increases (or prevents/mitigates decreases) in the population size of at least 1 native taxon but has not been observed to promote local population reestablishment, or to prevent local population extinction, of a native taxon.
Major impact (MR) A taxon is considered to have Major impacts when it causes community changes through the local or subpopulation extinction (or presumed extinction) of at least 1 native taxon, which would be reversible if the alien taxon was no longer present. Its impacts do not lead to irreversible local population, subpopulation, or global taxon extinctions.	Major positive impact (MR+) A taxon is considered to have Major positive impacts when it causes transient increases (or prevents transient decreases) in species occupancy through local or subpopulation reestablishment (or extinction prevention) of at least 1 native taxon, which would be reversed if the alien taxon was no longer present.
Massive impact (MV) A taxon is considered to have Massive impacts when it causes irreversible community changes through local, subpopulation, or global extinction (or presumed extinction) of at least 1 native taxon.	Massive positive impact (MV+) A taxon is considered to have Massive positive impacts when it causes long-lasting increases (or prevents long-lasting decreases) in species occupancy through local or subpopulation reestablishment (or extinction prevention) of at least 1 native taxon, which would remain even if the alien taxon was no longer present.