To the best of our knowledge, the here proposed list of 149 alien species in Europe is the most comprehensive, transparent and objective list developed to date that ranks alien species across various taxa according to their overall impacts. However, we are aware that no list will meet all expectations. Some of the species that do not appear on our list, but are included in other expert-based lists are Ailanthus altissima, Impatiens glandulifera, Diabrotica virgifera, Drosophila suzukii, Leptinotarsa decemlineata, Trachemys scripta elegans or Vespa velutina. These species do not rank highly on our list as currently their total demonstrated impacts are “only” in the range of 11–14 sum of scores and their maximal scores do not exceed a single score of 4. This indicates that we currently lack rigorous scientific proof that impacts of some of these flagship invaders are as serious as perceived by experts. Impatiens glandulifera for example, introduced over 100 years ago from India to Europe, was shown to have rather low impacts on species diversity despite its high cover (Hejda et al. 2009). Herbivorous insects such as Diabrotica virgifera or Drosophila suzukii have a high (score 4) but not devastating impact in their specialized niche but no or only low impacts in other GISS impact categories. However, the impact of a given species may change over time, thus in the future these species might cause higher impacts or additional impacts might be discovered. This also points to the fact that we need more research on the effects of many alien species, and new results might call for updating the list presented here. The same is true for future new arrivals of alien species with high impact: they may also qualify for a list of the worst alien species. Thus both aspects, improved knowledge and more alien species, are likely to generate the need for regular reanalysis, perhaps at 10 years intervals.
The comparison with other 100 worst lists reveals that our selection identifies most of the alien species that were considered as problematic by experts. Our list includes 59 of the DAISIE-100 list (DAISIE 2008). Among the excluded DAISIE-100 species, 19 are marine species, 8 herbivorous insects and 7 plants; for neither of them we found large overall impacts. Four DAISIE-100 species are of European origin, thus cannot be considered here. From the 32 species on the ISSG-100 that fit our selection criteria and occur in Europe, only 6 species (19%) did not make it on our list because their documented impacts were not high enough compared to other aliens in Europe.
The European Union published a list of “alien species of Union concern” initially containing 37 species (EU 2016). Further additions increased the list to 49 species after a complex political process (EU 2017), but more than 100 species were proposed by experts (Roy et al. 2014). Four of these 49 species do not currently occur in Europe, but although they could establish, they cannot be considered for a list of the worst aliens in Europe. Thirteen of the remaining 45 species are not on our list as they were excluded prior to screening or because they scored too low. What is more alerting, however, is that besides the overlapping 32 species found in the EU regulation and on our list, none of the remaining 117 high impact species from our list were included into the EU list of “species of Union concern” and only 16 of our first 49 species with the highest impact made it on the EU list of 49 species. Obviously, it takes more than a high impact for a species to be included on a regulated list. The EU lists a species only if it is likely that its inclusion will effectively prevent, minimize or mitigate its impact (EU 2016), and often the most widespread and/or highly impacting species are too costly to be managed effectively. Also economic interests such as with Acacia, Robinia and Eucalyptus species in forestry can prevent the inclusion on such a regulatory list.
The EU is very stringent in species selection and they require the support from their member states to be approved, therefore, such a list can only be seen as the lowest common denominator after a long compromise searching process. This could be a reason for the complete lack of marine species on the list of “EU concern”, whereas aquatic plants (10 species), crayfish (5 species) and squirrels (4 species) are well represented. In addition, the EU list does not include species which are “regulated elsewhere”, such as alien species with impact on agriculture, forestry or human health. All other mentioned 100-lists include such species which aggravates a direct comparison between political and scientific lists.
Our 149 worst species list contains 64 species that do not appear in other worst lists (DAISIE-100, ISSG-100, EU 2017). Examples include Varroa destructor (rank 8 on our list), an Asian ectoparasite of the honey bee that has been implicated in the global pollinator crisis (Potts et al. 2010); Hymenoscyphus pseudoalbidus (rank 18), the fungus responsible for ash dieback, changes in forest composition and related diversity loss (Gross et al. 2014); Carassius auratus (rank 20), the Chinese gold fish, which causes decline of native amphibians (Cats and Ferrer 2003); and the oomycete Phytophthora plurivora (rank 26), responsible for the dieback of numerous tree species, among them beech and oak (Schoebel et al. 2014). This indicates that even high-impacting alien species may escape the perception of experts. The selection process behind the list presented here, including screening of large databases of alien species and a semi-quantitative assessment with GISS which considers the published literature, is time-consuming but provides some guarantee that important species are not missed. Therefore, it is justified to recommend that many species from our list should be considered for inclusion on regulatory lists.
Many alien species on our 149 worst list do not yet have an EU-wide distribution. For a national strategy, therefore, regionalized lists would be very important. However, such subsets require detailed distribution maps and targeted collection of data on impact that are applicable to individual regions. So far, the majority of impact assessments did not follow such an approach because there is simply not enough regionally specific information.
Each of the two complementary approaches (SUM, MAX) identified slightly different sets of alien species with high impacts. The SUM approach favors species with multiple impacts in different categories while the MAX approach favors species with very high impacts in a single category. About half of the species on the final list were identified by only one of these two approaches. Depending on the stakeholders’ aim for the prioritization, one or the other might be more appropriate, but both have their merits (Nentwig et al. 2016; Blackburn et al. 2014; Bacher et al. 2017). Thus, we suggest applying either method or their combination depending on the specific needs of the stakeholders.
Our list of the worst aliens in Europe is the first compiled by using a semi-quantitative assessment across taxa and habitats. Such a transparent and reproducible procedure is crucial to ensure the authority of the resulting list. Furthermore, its broad basis of 486 analyzed species makes it less likely that important species are missed. For management purposes, it is increasingly relevant to prioritize alien species. Also politicians have to focus on key species, either for financial or for consensus reasons. In all such regards, an objective list such as the one given here, that is unbiased by expert opinion, taxonomy and environments, can be the basis for evidence based decision making. Such a list is also an ideal tool to fulfill the Aichi biodiversity target 9 that requires prioritization of invasive alien species based on scientific evidence by 2020 (CBD 2017).