Data from DAISIE (2009) database (www.europe-aliens.eu; Pergl et al. 2012) was used as a source of information on impacts of established alien species in Europe. It was also used as a basis for assignment of the pathways of their introduction to Europe for (i) vascular plants, (ii) freshwater fish, and (iii) mammals, while the EASIN catalogue (easin.jrc.ec.europa.eu; Katsanevakis et al. 2015) was used for pathway and impact classification of (iv) terrestrial invertebrates. The classification of introduction pathways follows the scheme of Hulme et al. (2008) that allows their comparison across taxonomic groups as well as between accidental and intentional introductions. Each species was assigned to one or more of the following pathway categories: (i) release (intentional introduction and release into the environment), (ii) escape (intentionally introduced as a commodity, but escaped from culture), (iii) contaminant (unintentional introduction with specified commodity), (iv) stowaway (unintentionally introduced attached to or within a transport vector), (v) corridor (unintentional spread via human transport infrastructures linking previously unconnected regions) or (vi) unaided (unintentional introduction by natural dispersal across political borders following a primary human-mediated introduction in a neighbouring region). The data do not differentiate between the pathways for initial introduction to Europe and those associated with movement among different European countries. Similarly, species are often listed as associated with more than one introduction pathway with no measure of their relative importance. In contrast to other taxonomic groups, the invertebrate data do not allow the exact area of origin to be identified for species that are native in a part of Europe and alien in another part and thus this group included only arrivals from other continents (classified as aliens to Europe in DAISIE 2009). Only species confirmed as established in at least one European country (DAISIE regions) were included in the analyses. As information on establishment status is incomplete for some regions of Europe, we also included species for which establishment could not be confirmed but that were found in five or more European regions.

As a second step, species for which introduction pathways had been identified were classified in two groups: those having an ecological impact and those for which no ecological impact had been recorded. For fish, mammals and plants, the information on ecological impacts was retrieved from DAISIE (Vilà et al. 2010). For invertebrates information in DAISIE and EASIN was updated with literature and expert opinion (M. Kenis, W. Rabitsch and A. Roques, unpublished data). Ecological impact was defined as an impact on native species or on the functioning of natural or semi-natural ecosystems in Europe or in similar climatic and environmental conditions in other continents. There was no assessment of the type of impact or its magnitude.

We tested: (i) whether different pathways (release, escape, contaminant, stowaway, corridor and unaided) are associated with higher or lower probability of causing ecological impact and (ii) whether there is a relationship between the likelihood of impacts and the number of pathways through which a species has been introduced. All analyses were based on species counts that were analysed by generalized linear models with a log-link function and Poisson distribution of errors with control for overdispersion (if needed using quasi-Poisson distributions) (Crawley 2007). If the full model including the interaction with taxon was significant, then individual models for particular taxonomic groups were used. To test in which pathways the counts were lower or higher than expected by chance, adjusted standardized residuals of G-tests were compared with critical values of a normal distribution (Řehák and Řeháková 1986). The null expectations were thus that the proportion of species with and without ecological impact within an individual pathway is the same across all pathways and that number of species with and without impact are not related to number of pathways. The test for multiple pathways was done by summing up the number of pathways per species. All analyses were performed in R 3.0.2 (R Core Team 2015).

There were 2529 vascular plant, 75 mammal, 107 fish and 1314 terrestrial invertebrate taxa (species or subspecies) with at least one pathway category assigned. The coverage of identified pathways for the taxa ranged from 98% for fish to 59% in plants (Table 1). The most frequently represented pathways differed between taxa. In plants, the most frequent pathway was escape, recorded for 58% of the total species number. Mammals had a high proportion of release and escape (49% and 41%, respectively). Among freshwater fishes, there were 43% escaped and 36% released species. In contrast, 76% of the terrestrial invertebrates were introduced as contaminants of commodities (Table 1).

Among the established taxa with known introduction pathway, there were 250 vascular plants (6.2% of the total), 38 mammals (61.3%), 52 fishes (48.6%) and 80 terrestrial invertebrates (6.1%) with documented or strongly supposed ecological impact (Table 2). There was a significantly higher frequency of taxa with impact within mammals and fishes than in plants and invertebrates (two-way interaction taxon × impact χ² = 208.71; df = 3; P < 0.001). Overall, the frequency of ecological impacts differed significantly among pathways and taxa (three-way interaction taxon × pathway × impact: χ² = 29.11; df = 15; P = 0.015). Within the particular taxonomic group, the impacts were significantly different among pathways for plants (χ² = 32.54; df = 5; P < 0.001) but not so for invertebrates, mammals or fish. As discussed below, the results might be masked by lower statistical power of the test in these taxon groups due to high numbers of pathways with zeros and that mammals and fish are generally species-poor groups. For plants exerting ecological impact, the significant difference among pathways was mainly due to disproportionately more counts than expected for release, corridor and unaided pathways, and disproportionately fewer for contaminants (Table 2).

The maximum number of pathways recorded for species with ecological impact was four, represented by five plants (e.g. Elodea canadensis – Canadian waterweed, Galinsoga parviflora – gallant soldier, and Senecio vernalis – Eastern groundsel) and two fish (Oncorhynchus gorbuscha – pink salmon, and O. mykiss – rainbow trout). For mammals, the maximum number of pathways was three, recorded in six species (e.g. Cervus nippon – sika deer, Ondatra zibethicus – muskrat, Procyon lotor – raccoon). For terrestrial invertebrates with impact, the maximum number of pathways was two (Lasius neglectus – garden ant, and Linepithema humile – Argentine ant) (Fig. 1).

Figure 1.

Percentage of alien species with impact in relation to the number of introduction pathways. The height of the bar indicates the percentage of the number of taxa with impact within the taxonomic group of species that are introduced via the given number of pathways. Numbers above bars indicate the numbers of species with impact for each taxonomic group and for the given number of pathways.

The taxonomic groups did not differ in their impact related to the number of pathways (three-way interaction taxon × number of pathways × impact: χ² = 8.01; df = 9; p=0.53), but pooled across taxa, having multiple pathways increased the probability of recording impact (χ² = 170.11; df = 3; P < 0.001). Taxa associated with only one pathway were less likely to have an impact than expected by chance (G = 3.47, P < 0.001), while those associated with two and three pathways were more likely (G = 4.45, P < 0.001; G = 2.62, P < 0.01). The number of taxa without impact and introduced by four pathways was lower than expected by chance (G = 2.89, P < 0.01). Combinations of pathways per taxonomic group are shown in Appendix 1.

The relationship between impacts and pathways differed with respect to taxonomic groups, but for most taxa no major significant differences among pathways were found. For plants, pathways vary in the proportion of species with impact they deliver, while for invertebrates, fish and mammals this was not the case. For example, among escaped mammals, in a group featuring prominent examples of escaped fur animals with high ecological impacts (Neovison vison – American mink, Ondatra zibethicus – muskrat), there were no significant differences between numbers of species with and without impacts. Further, the number of species with impact arriving by a given pathway is also important. For example, the absolute number of escaped plants with impact was twice as high as that of released plants with impact, despite the difference between the two pathways not being statistically significant. Similarly, fewer than expected species of plants causing impact are introduced as contaminants, but absolute values for terrestrial invertebrates indicate a high importance of this pathway compared to release.

That pathways do not significantly affect the probability of impact of vertebrates may be related to the generally high invasion success of this group (Jeschke and Strayer 2005, Jeschke 2008), as well as to a high percentage of species with impacts compared to plants and invertebrates (Table 2). If vertebrates are introduced and establish there is a high probability of them having impact regardless of the pathway on which they arrive. Furthermore, identifying pathways causing negative environmental impacts by alien vertebrates may require more detailed analyses than for other taxonomic groups. For example, to assess the role of the pet trade, which is a subset of the escape category, it would be important to carry out analyses at a finer level than is currently the case. Considering intentional (release, escape) vs unintentional pathways (contaminant, stowaway, corridor and unaided) across all taxa, our results indicate that the latter are associated with impact less frequently than expected, and vice versa. In absolute numbers, unintentional pathways were more common for invertebrates, but not so for plants. However, the pattern is blurred by the fact that many species were introduced through several pathways, including both intentional and unintentional. For example, for plants, Pyšek et al. (2011) found that unintentionally introduced species invaded a wider range of semi-natural habitats than intentionally introduced species; hence the risk arising from unintentional introductions should not be underestimated.

An important question is whether species introduced by multiple pathways have an advantage because of a higher propagule pressure or an increased probability to reach a more diverse range of suitable sites. Unfortunately, robust data for propagule pressure that can be compared across individual pathways for the respective taxonomic groups are rarely available. If such data exist, they are limited to specific pathways such as direct release for biocontrol (Rossinelli and Bacher 2015) or landscaping (but see Lee and Chown 2009). Our knowledge thus mostly depends on proxies such as trade volume, numbers of botanic gardens, human population density or road density (Carlton and Ruiz 2005, Wilson et al. 2009, Kaluza et al. 2010, Pyšek et al. 2010; Hulme 2015b). Although some taxonomic groups such as invertebrates are highly dependent on one specific pathway, in general, the number of introduction pathways can be used as another proxy for propagule pressure. It appears that ecological impacts are more likely to occur if plants are introduced by multiple pathways. Besides profiting from increased propagule pressure, it is also possible that species introduced by multiple pathways have a greater chance of being introduced to a wider range of habitats or are also ecologically more versatile than those arriving on single pathways. In plants, the existence of multiple pathways usually includes escape from cultivation, reflecting the dominant role of horticultural introductions, which is for many species combined with introduction as contaminants. The combination of pathways that favours high impact fishes is release and escape, but these two pathways are also often accompanied with unintentional introductions. It seems that at least in these two taxonomic groups, the predisposition for opportunistic dispersal may be determined by the same traits as the ability to escape from capture or cultivation. Still, there are fishes with severe impacts introduced by a single pathway, e.g. Leuciscus leuciscus (common dace), Clarias gariepinus (African sharptooth catfish), Oreochromis niloticus (Nile tilapia), or Polyodon spathula (American paddlefish). For terrestrial invertebrates, species with impact introduced as contaminants dominate, which highlights the importance of this pathway and the fact that this pathway is responsible for high propagule pressure. However, it is likely that a large number of these species are also introduced as stowaways but this pathway is hugely underestimated because it is so difficult to assess (A. Roques, unpublished data).

Although a simple yes/no classification of ecological impact provides basic information, it is evident that impacts manifest over a wide range of magnitudes, from local population declines to global extinctions, or from minor perturbations to massively adverse ecological and economic cascades. A yes/no impact classification lumps together species with low ecological impact, e.g. Mahonia aquifolium (Oregon-grape), with high-impact species such as Fallopia spp. (knotweeds) (Kumschick et al. 2015a). Similarly, while there are over 600 alien terrestrial invertebrate species (mostly insects) classified as having an ecological impact in DAISIE, an extensive literature survey of ecological impacts attributed to invasive insects found published records for less than 10 species in Europe (Kenis et al. 2009). On the other hand, using other impact criteria, Vaes-Petignat and Nentwig (2014) described impacts for 64 of the 77 most widely spread terrestrial arthropods alien to Europe. However, despite recent attempts to classify impacts more precisely (Nentwig et al. 2010, 2016, Kumschick and Nentwig 2011, Kenis et al. 2012, Blackburn et al. 2014, Jeschke et al. 2014, Kumschick et al. 2015b, Rumlerová et al. 2016), such information is rarely available for a large number of species. Low sample size is a constraint for the statistical analysis, particularly for alien mammals and fishes, and limits the power of finding relevant patterns despite the severe impacts that these two taxonomic groups are known to have on biodiversity (Kumschick et al. 2015a). Differences in the quality of impact data (Hulme et al. 2013) among taxonomic groups are not only due to species numbers or recorded impacts, but also result from the research activity (e.g. ease of study or attractiveness). The frequent impacts of released species may be due to the fact that some of those species are introduced for a purpose that requires having an ecological impact (e.g. plant species for dune stabilization, invertebrates for biocontrol) and are better scrutinized for any potential adverse (and unintended) impacts on native species. About 110 released invertebrates (mostly biocontrol agents) have been classified as having an ecological impact in Europe (DAISIE 2009). Only three are known to have some measurable negative impact on native species, the parasitoids Cales noaki and Lysiphlebus testaceipes (Kenis et al. 2009), and the harlequin ladybird Harmonia axyridis (Roy et al. 2012, 2016).

The management of IAS with negative impacts on the environment and on human well-being is subject to efforts at national, continental and global levels (CBD 2014, Aichi Biodiversity Target 9). The categories used for the present analysis are consistent with the standard categorization of pathways of introduction of IAS presented by the CBD and recommended for identifying and prioritizing pathways (CBD 2014). At the European scale, the new EU Regulation on IAS, entered into force on 1 January 2015 (EU 2014, Genovesi et al. 2015), calls EU Member States to identify the pathways of unintentional introduction and spread of IAS of Union concern, and to effectively manage them through specific action plans.

The CBD and EU legislation confirm that policies are focusing on the prioritization of pathways in order to prevent the introduction of IAS (Meyerson and Reaser 2003, Hulme 2009, 2011). This covers managing or preventing the introduction of new species to a particular region and mitigating their impacts by regulation of intentional and unintentional introductions (Wittenberg and Cock 2001, Caffrey et al. 2014). To make pathway management work efficiently, it needs to be built on rigorous data on impacts of alien species, and how these interact with individual pathways. Some pathways and taxonomic groups, plants and invertebrates particularly, contribute disproportionally more to the overall risk from alien species with documented impacts, and these should receive increased attention. However, to fully assess the potential of each particular pathway, not only is it necessary to consider the proportion of species with negative impacts, but also the absolute number of species introduced along each pathway.

Using proportions as a measure emphasizes the release pathway as posing greater risk, regardless of the taxonomic group, while using absolute species numbers prioritizes the escape and contaminant pathways. The other pathways associated with arrival of IAS can be assumed to be less important for management and monitoring. Legislation, early warning systems and rapid response mechanisms should be primarily targeted at intentional introductions (release and escape) and species introduced unintentionally as contaminants (for which the pathway of arrival can be identified). An accurate identification of the pathways of introduction and spread of alien species is essential for efficient management of invasions, and in this regard it is important to adopt a standard terminology and categorization, as recommended by the CBD (Hulme et al. 2016); a standardized approach will be essential in enforcing the EU Legislation, to ensure that action by EU member states is coordinated. Also, the present study highlights that the proportions of alien species with negative ecological impacts are taxon-specific, a finding that should be reflected by legislation and pathway management. However, in many cases at the present level of understanding, the best predictor of the relevance of an introduction pathway is the total number of species that are associated with it. Furthermore, we showed that the results of this study are highly dependent on the availability of data and it is necessary to better reflect the scales of impacts ranging from minimal to massive to improve understanding and management of IAS.

Therefore, we encourage further work on the approach outlined here through more detailed analyses of individual pathways, their association with IAS, consideration of spatial and temporal variation in pathway trends (Padilla and Williams 2004, Copp et al. 2010, Maceda-Veiga et al. 2013, Hulme 2015b), inclusion of more detailed descriptions of the magnitude and/or types of impact (Nentwig et al. 2010, 2016, Blackburn et al. 2014, Kumschick et al. 2015a) and consideration of other taxa that were not included in the present study such as fungi, and considering interactions and synergies between pathways (Roy et al. 2016). As for most taxa it was not possible to detect major differences in the way in which IAS arrive and so until robust and comprehensive information on impact is available, we should not focus on subtle differences between the pathways of arrival for different taxonomic groups, but instead consider the most common pathways for all taxa and pathways that are most easily managed. Thus pathways that deliver many species should become a management priority.