# Developing a list of invasive alien species likely to threaten biodiversity and ecosystems in the European Union

Helen E. Roy, Sven Bacher, Franz Essl, Tim Adriaens, David C. Aldridge, John D. D. Bishop, Tim M. Blackburn, Etienne Branquart, Juliet Brodie, Carles Carboneras, et al.

► To cite this version:

Helen E. Roy, Sven Bacher, Franz Essl, Tim Adriaens, David C. Aldridge, et al.. Developing a list of invasive alien species likely to threaten biodiversity and ecosystems in the European Union. Global Change Biology, 2019, 25 (3), pp.1032 - 1048. 10.1111/gcb.14527. hal-02627982

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# Developing a list of invasive alien species likely to threaten biodiversity and ecosystems in the European Union

Helen E. Roy<fnref n="1" /> ID | Sven Bacher<fnref n="2" /> ID | Franz Essl<fnref n="3" />,<fnref n="4" /> | Tim Adriaens<fnref n="5" /> | David

C. Aldridge<fnref n="6" /> | John D. D. Bishop<fnref n="7" /> | Tim M. Blackburn<fnref n="8" />,<fnref n="9" /> | Etienne Branquart<fnref n="10" /> |

Juliet Brodie<fnref n="11" /> ID | Carles Carboneras<fnref n="12" /> | Elizabeth J. Cottier-Cook<fnref n="13" /> | Gordon

H. Copp<fnref n="14" />,<fnref n="15" /> ID | Hannah J. Dean<fnref n="1" /> | Jørgen Eilenberg<fnref n="16" /> ID | Belinda Gallardo<fnref n="17" /> |

Mariana Garcia<fnref n="18" /> | Emili García-Berthou<fnref n="19" /> ID | Piero Genovesi<fnref n="20" /> | Philip E. Hulme<fnref n="21" /> |

Marc Kenis<fnref n="22" /> | Francis Kerckhof²³ | Marianne Kettunen²⁴ | Dan Minchin²⁵ |

Wolfgang Nentwig²⁶ | Ana Nieto<fnref n="18" /> | Jan Pergl²⁷ ID | Oliver L. Pescott<fnref n="1" /> | Jodey M.

Peyton<fnref n="1" /> | Cristina Preda²⁸ | Alain Roques²⁹ | Steph L. Rorke<fnref n="1" /> | Riccardo Scalera<fnref n="18" /> |

Stefan Schindler<fnref n="3" /> | Karsten Schönrogge<fnref n="1" /> | Jack Sewell<fnref n="7" /> | Wojciech Solarz³⁰ | Alan J.

A. Stewart³¹ ID | Elena Tricarico³² ID | Sonia Vanderhoeven³³ | Gerard van der

Velde³⁴,³⁵,³⁶ | Montserrat Vilà³⁷ ID | Christine A. Wood<fnref n="7" /> | Argyro Zenetos³⁸ |

Wolfgang Rabitsch<fnref n="3" />

<footnote n="1">¹Centre for Ecology & Hydrology, Wallingford, UK</footnote>

<footnote n="2">²University of Fribourg, Fribourg, Switzerland</footnote>

<footnote n="3">³Environment Agency Austria, Vienna, Austria</footnote>

<footnote n="4">⁴Division of Conservation Biology, Vegetation Ecology and Landscape Ecology, University Vienna, Vienna, Austria</footnote>

<footnote n="5">⁵Research Institute for Nature and Forest (INBO), Brussels, Belgium</footnote>

<footnote n="6">⁶Department of Zoology, University of Cambridge, Cambridge, UK</footnote>

<footnote n="7">⁷The Laboratory, The Marine Biological Association, Plymouth, UK</footnote>

<footnote n="8">⁸University College London, London, UK</footnote>

<footnote n="9">⁹Institute of Zoology, Zoological Society of London, London, UK</footnote>

<footnote n="10">¹⁰Invasive Species Unit, Service Public de Wallonie, Wallonia, Belgium</footnote>

<footnote n="11">¹¹Natural History Museum, London, UK</footnote>

<footnote n="12">¹²Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire, UK</footnote>

<footnote n="13">¹³Scottish Association for Marine Science, Scottish Marine Institute, Oban, UK</footnote>

<footnote n="14">¹⁴Centre for Environment, Fisheries and Aquaculture Science, Lowestoft, UK</footnote>

<footnote n="15">¹⁵Centre for Conservation Ecology, Bournemouth University, Poole, UK</footnote>

<footnote n="16">¹⁶Department of Plant and Environmental Sciences, University of Copenhagen, Denmark</footnote>

<footnote n="17">¹⁷Pyrenean Institute of Ecology (IPE-CSIC), Zaragoza, Spain</footnote>

<footnote n="18">¹⁸ISSG, Rome, Italy</footnote>

<footnote n="19">¹⁹GRECO, Institute of Aquatic Ecology, University of Girona, Girona, Spain</footnote>

<footnote n="20">²⁰Institute for Environmental Protection and Research ISPRA, and Chair IUCN SSC Invasive Species Specialist Group, Rome, Italy</footnote>

<footnote n="21">²¹Bio-Protection Research Centre, Lincoln University, Lincoln, New Zealand</footnote>

<footnote n="22">²²CABI, Delémont, Switzerland</footnote>

<footnote>[Correction added after first online publication on 6th February 2019 in figure captions 2, 4, 5, 6, 7].</footnote>
²³Royal Belgian Institute of Natural Sciences (RBINS), Oostende, Belgium

²⁴Institute for European Environmental Policy, London, UK

²⁵Marine Organism Investigations, Marina Village, Ballina, Killaloe, Co Clare, Ireland

²⁶University of Bern, Bern, Switzerland

²⁷Institute of Botany, The Czech Academy of Sciences, Průhonice, Czech Republic

²⁸Ovidius University of Constanta, Constanta, Romania

²⁹Institut National de la Recherche Agronomique, Zoologie Forestière, UR 0633, Ardon Orleans Cedex 2, France

³⁰Institute of Nature Conservation, Polish Academy of Sciences, Kraków, Poland

³¹University of Sussex, Brighton, UK

³²University of Florence, Firenze, Italy

³³Belgian Biodiversity Platform, Louizalaan, Brussels, Belgium

³⁴Institute for Water and Wetland Research, Radboud University, Nijmegen, The Netherlands

³⁵Naturalis Biodiversity Center, Leiden, The Netherlands

³⁶Netherlands Centre of Expertise for Exotic Species (NEC-E), Nijmegen, The Netherlands

³⁷Estación Biológica de Doñana, (EBD-CSIC), Seville, Spain

³⁸Helenic Centre for Marine Research, Anavyssos, Greece

**Correspondence**

Helen E. Roy, Centre for Ecology &
Hydrology, Wallingford, UK.

Email: hele@ceh.ac.uk

**Abstract**

The European Union (EU) has recently published its first list of invasive alien species (IAS) of EU concern to which current legislation must apply. The list comprises species known to pose great threats to biodiversity and needs to be maintained and updated. Horizon scanning is seen as critical to identify the most threatening potential IAS that do not yet occur in Europe to be subsequently risk assessed for future listing. Accordingly, we present a systematic consensus horizon scanning procedure to derive a ranked list of potential IAS likely to arrive, establish, spread and have an impact on biodiversity in the region over the next decade. The approach is unique in the continental scale examined, the breadth of taxonomic groups and environments considered, and the methods and data sources used. International experts were brought together to address five broad thematic groups of potential IAS. For each thematic group the experts first independently assembled lists of potential IAS not yet established in the EU but potentially threatening biodiversity if introduced. Experts were asked to score the species within their thematic group for their separate likelihoods of i) arrival, ii) establishment, iii) spread, and iv) magnitude of the potential negative impact on biodiversity within the EU. Experts then convened for a 2-day workshop applying consensus methods to compile a ranked list of potential IAS. From an initial working list of 329 species, a list of 66 species not yet established in the EU that were considered to be very high (8 species), high (40 species) or medium (18 species) risk species was derived. Here, we present these species highlighting the potential negative impacts and the most likely biogeographic regions to be affected by these potential IAS.

**KEYWORDS**

biological invasions, consensus approach, environmental policy, impacts, introductions, prioritization, risk assessment

1 | INTRODUCTION

There are currently more than 14,000 alien species recorded in Europe (EASIN Catalogue, https://easin.jrc.ec.europa.eu/) with more than half originating from outside EU territories, while the remainder have originated within parts of the EU and subsequently invaded others. Their numbers are rapidly increasing (Seebens et al., 2017),

and in some cases so is their rate of spread (Roques et al., 2016). A number of alien species cause serious problems for the environment and society (Vilà et al., 2010) and these are termed invasive alien species (IAS) (European Union, 2014). The European Commission has addressed the threat of IAS in their Regulation 1143/2014; at the heart of the regulation is the development of a list of IAS of EU
concern, with an explicit focus on potential future invaders, exclud-
ing some microorganisms,<fnref n="1" /> that will be targeted for action (European
Union, 2014; Genovesi, Carboneras, Vilà, & Walton, 2015). Thus, the
identification of likely future IAS is pivotal for implementing this reg-
ulation. Here, we present a horizon scanning approach to identify
likely future IAS to inform the list of IAS of EU concern.

Horizon scanning can be defined as a systematic examination of potential threats and opportunities, within a given context, and likely future developments which are at the margin of current thinking and planning (Food Standards Agency, 2018). There are a number of approaches that could be adopted for horizon scanning (Supporting information S1: Overview of approaches horizon scanning methods) with varying strengths and weaknesses depending on the context (Sutherland & Woodroof, 2009). Horizon scanning usually follows a structured process of simplification and reduction from a large set of data to a prioritized subset categorized by the most important and relevant data. A series of recent papers have provided convincing arguments that horizon scanning should play a more prominent role in environmental and conservation practice (Copp et al., 2016; Cowx, Angelopoulos, Nunn, Britton, & Copp, 2009; IPCC 2005; Ricciardi et al., 2017; Sutherland & Woodroof, 2009; Van Wilgen & Richardson, 2012) including as a tool for informing policies on IAS, particularly through preventing arrival (Copp, Templeton, & Gozlan, 2007; Shine et al., 2010).

There have been a number of horizons scanning exercises for IAS in Europe, but these have usually involved one or few taxonomic groups, such as plants (Andreu & Vilà, 2010; Thomas, 2010) or animals (Parrott et al., 2009), or distinct environments such as freshwater (Gallardo & Aldridge, 2013), specific countries (Matthews et al., 2014; Roy, Peyton et al., 2014; Matthews et al., 2017), or regions (NOBANIS 2015; Gallardo et al., 2016). Most of these approaches have relied on information from the literature coupled with impact assessment frameworks (Parrott et al., 2009; Thomas, 2010) or modelling approaches (Gallardo & Aldridge, 2013). It has been noted that wildlife diseases are lacking within horizon scanning exercises and that there is a need to address this imbalance (Roy et al., 2017).

A horizon scanning exercise for Great Britain was carried out in
2013 and illustrates the merits of using a combination of

approaches and concluding with a consensus workshop to create a
ranked list of IAS (all plant and animal taxa, excluding microorgan-
isms, across all environments) that are likely to arrive, establish and
have an impact on native biodiversity within the following 10 years
(Roy, Peyton et al., 2014). Within 2 years of publication of this list,
seven of the species ranked within the top ten had been newly
recorded within Great Britain. Most notably, the quagga mussel,
*Dreissena rostriformis bugensis*, which was given the maximum
scores for risk of arrival, establishment and impact and accordingly
ranked in the top position, was reported in October 2014 (Aldridge,
Ho, & Froufe, 2014).

There are considerable strengths to such consensus methods, par-
ticularly when information is limited, but it is important to be aware
that opinion is not knowledge (Banks, Wright, Maclean, Hann, &
Rehfisch, 2008). Indeed, it is critical that consensus methods, in which
experts are engaged, adequately address issues with respect to accu-
racy and judgement to reduce the effects of potential bias (Suther-
land & Burgman, 2015; Garnas et al., 2016). Discussions through
consensus approaches, where not just scores are communicated, but
also the insights that led to them, can reduce levels of uncertainty.
Uncertainty is inherent when dealing with data deficiency (e.g. insuffi-
cient information on species) and ambiguity in terminology, which is a
problem in invasion ecology, particularly between experts from differ-
ent taxonomic groups (Essl et al., 2016). Indicating the perceived level
of confidence of the assessments, and documenting the discussions
behind the agreed level (or score) of uncertainty, is therefore consid-
ered crucial in communicating the outcome of the exercise to a wider
scientific or public audience. During the consensus building process,
lack of evidence or contradictory information can easily be tracked
and discussed. Therefore, the method is particularly useful to inte-
grate scarce information available for many potential alien species
(Vanderhoeven et al., 2017).

Here, we present a consensus approach which was adopted for
the first EU-wide horizon scan for future IAS not native to any parts
of Europe with the potential to threaten European biodiversity. The
EU-wide horizon scan was part of a study funded by the European
Commission for prioritization of IAS (Roy, Peyton et al., 2014). This
study is unique in the continental scale examined but also the
breadth of taxonomic groups and environments considered. The pro-
posed list provides a basis for prioritizing full risk assessments of
species not yet established in the EU in order to comprehensively
evaluate the threat posed by these species to EU biodiversity. The
study may also serve as a model for future horizon scanning projects
of similar thematic or geographic scope.

2 | MATERIALS AND METHODS

We used an adapted version of the consensus method (Sutherland,
Fleishman, Mascia, Pretty, & Rudd, 2011; Roy, Peyton et al., 2014)
for a horizon scanning approach to derive a ranked list of species to
be risk assessed, hence to be further considered to derive a list of
potential IAS with high impact on biodiversity (Figure 1). It is impor-
tant to note that the process was undertaken in the framework of

<footnote n="1">¹The Regulation does not apply to:</footnote>

<footnote>a Species changing their natural range without human intervention, in response to changing ecological conditions and climate change;</footnote>

<footnote>b Genetically modified organisms as defined in point 2 of Article 2 of Directive 2001/18/EC;</footnote>

<footnote>c Pathogens that cause animal diseases; for the purpose of this Regulation, animal disease means the occurrence of infections and infestations in animals, caused by one or more pathogens transmissible to animals or to humans;</footnote>

<footnote>d Harmful organisms listed in Annex I or Annex II to Directive 2000/29/EC, and harmful organisms for which measures have been adopted in accordance with Article 16(3) of that Directive;</footnote>

<footnote>e Species listed in Annex IV to Regulation (EC) No 708/2007 when used in aquaculture;</footnote>

<footnote>f Micro-organisms manufactured or imported for use in plant protection products already authorized or for which an assessment is ongoing under Regulation (EC) No 1107/2009; or</footnote>

<footnote>g Micro-organisms manufactured or imported for use in biocidal products already authorized or for which an assessment is ongoing under Regulation (EU) No 528/2012.</footnote>
the EU Regulation 1143/2014 on IAS and accordingly the approach (and particularly scope) was in part determined by this context (Roy et al. 2014). The approach involved a sequence of critical steps:

## 2.1 | Step 1. Establishment of thematic groups

Five broad thematic groups (plants, terrestrial invertebrates, marine species, freshwater invertebrates and vertebrates) of IAS and associated experts based on taxonomy and major environments were established (Supporting information S2). The experts were selected to provide representation across Europe and ensure sufficient knowledge across taxonomic groups and environments. Group size ranged between six to nine experts and contained two co-leaders who agreed to coordinate and record activities and discussion between group members before, during and after the workshop.

## 2.2 | Step 2. Compilation of preliminary lists of potential IAS

Each thematic group was asked to assemble preliminary lists of potential IAS that they considered to constitute the highest risk with respect to the likelihood of arrival, establishment, spread and the magnitude of their potential negative impact on biodiversity and ecosystem services, within the EU region over the next 10 years. It was expected that each thematic group would derive these lists from a combination of systematic literature searches (including academic journals, risk assessments, reports, authoritative websites and other "grey" literature), querying of IAS databases (Supporting information S3) and their own expert knowledge. As expected, the approaches adopted by each thematic group differed slightly with respect to methods followed to derive the preliminary lists because of the diverse nature of the taxonomic groups and variation in the sources of information available (details given in Supporting information S4). However, initially all experts worked independently to provide lists of potential IAS for consideration by the entire group at a later stage.

The geographic scope of the search for potential IAS was worldwide. It was clearly stated that the lists should only include species alien to the EU, including the Macaronesian islands, but excluding other EU outermost regions, acknowledging that the EU does not encompass the entire European continent. A potential, but not exhaustive, list of search criteria included alien species that:

1. Are absent in the EU

2. Are present in countries close to or sharing a border with the EU

3. Are present in areas of the world that are climatically matched to the study region (using the Köppen-Geiger climate zones as reference)

4. Have documented histories of invasion and causing undesirable impacts in other regions worldwide

5. Are traded within the EU or are present in areas that have strong trade or travel connections with the EU and where there is a recognized potential pathway for arrival

6. Are present in captivity including zoological parks, aquaculture facilities and glass houses.

The temporal scope of the horizon scanning exercise was that only species likely to arrive in the next 10 years within the EU should be included. This temporal limit had important consequences, because it limited the relevance of, for instance, long-term climate change projections.

A simplified framework was developed following the workshop. It was decided to focus on five climatic zones based on the biogeographic regions of Europe as defined by the European Environment Agency (EEA, see http://ec.europa.eu/environment/nature/natura 2000/biogeogRegions/). A correspondence with Köppen-Geiger climate zones (Kottek, Grieser, Beck, Rudolf, & Rubel, 2006) was provided to allow extrapolation of species establishment potential based on the species distribution in other continents. For marine species (all species living within the sea), the framework was modified by adding the Baltic Sea, Mediterranean and Black Seas.

The scope of the exercise was further refined based on a number of exclusions including those already stated above:

1. Species that arrive from their native range by natural spread/dispersal without human intervention in response to changing ecological conditions or climate change

2. Parasites that cause animal diseases (including to wildlife)

3. Species or taxonomic groups that are regulated under EU legislations other than the EU Regulation 1143/2014 on IAS (e.g. EU Plant Health Legislation - Directive 2000/29/EC or EU regulation on the use of alien species in aquaculture - Regulation (EC) No 708/2007)

4. Microorganisms and fungi

5. Species having adverse impacts only in productive sectors (such as agriculture, horticulture, timber) or on human health and well-being, unless these impacts are in addition to separate impacts on native biodiversity (in which case, these additional impacts were noted, but not used as primary selection criteria).

The consultation between experts was completed both through e-mail discussions in advance of the workshop (over 6 weeks) and through the workshop breakout groups. Co-leaders of each of the thematic groups collated the lists of IAS received from the experts within their group into a single provisional list.

## 2.3 | Step 3: Scoring of species

Experts were asked to independently score each species within their thematic group for their separate likelihoods of: (a) arrival, (b) establishment, (c) spread, and (d) magnitude of the potential negative impact on biodiversity within the EU. A 5-point scale from 1 = very low to 5 = very high (Blackburn et al., 2014) was adopted to achieve an appropriate balance between accuracy and resolution. The scores from each expert within each thematic group were then compiled and discussions within the thematic groups (at the workshop) led to
<figure><img src="image_2.png" /><figcaption>**FIGURE 1** Number of species for each thematic subgroup (Freshwater invertebrates, Freshwater fish, Terrestrial invertebrates, Vertebrates, Plants and Marine species) at different stages of the horizon scanning process (preworkshop, Day 1 Subgroup Consensus, Day 2 Subgroup Consensus, Final Subgroup consensus and Final Overall Consensus). Note the Final Overall Consensus includes species that have a limited distribution within the EU and those that are considered absent from the EU; for the latter category there was a total of 66 species (with 18, 40 and 8 species considered to represent medium, high and very high threat respectively). White = unranked, dark grey = very high, light grey = high, mid grey = medium priority for risk assessment</figcaption></figure>

an overall agreed impact and confidence score for each species with
respect to likelihoods of: (a) arrival, (b) establishment, (c) spread, and
(d) impact on biodiversity. Further guidance on species scoring is
given below.

Scores for the likelihood of arrival were based on a consideration
of several relevant factors, including: previous history of invasion by
the species in other regions; the existence of a plausible introduction
pathway; qualitative consideration of volume and frequency of trade
and travel between the existing range of the species and the EU. A score of 1 denoted that the species was considered unlikely to arrive in the EU within the chosen timeframe. A score of 5 was used to denote near-certain, arrival. In the case of species already in the EU (such as those held commonly in captivity or planted in gardens), the likelihood of arrival was agreed to be the top category of 5.

Having arrived, the probability of a species establishing a self-
sustaining population in the wild will depend on the ecological prop-
erties of both the species itself and the community that it is invading
(Leung et al., 2012). Scores therefore reflected life-history character-
istics including reproductive rate and ecological features such as tol-
erance of a broad range of environmental conditions or availability
of food supply in the introduced range. Scores for likelihood of
spread were primarily determined by the dispersal ability of the spe-
cies, both natural and human-assisted, and its history and speed of
spread in other regions where invasive.

Experts were asked to score the magnitude of impact on biodi-
versity and ecosystem functions related to ecosystem services, and
the likelihood of colonization of habitats of high conservation value
(as defined by the EU Habitats Directive). Furthermore, information
was requested on the mechanisms through which each IAS could
impact biodiversity and ecosystem functions (Supporting information
S5).

The impact scoring system was modified from the ISEIA protocol (Branquart, Verreycken, Vanderhoeven, & Van Rossum, 2009; De Groot, Alkemade, Braat, Hein, & Willemen, 2010), the GB NNRA (Booy, White, & Wade, 2006) and the proposed unified framework for environmental impacts - EICAT (Blackburn et al., 2014; Hawkins et al., 2015). The descriptors of the impact scoring system are provided in Supporting information S5. Confidence levels (Supporting information S5) were attributed to each score to help focus discussions and refine the list of species but were not used formally within the consensus building (across all thematic groups). Therefore, confidence scores are not reported here but did prove useful in guiding discussion within some thematic groups.

While acknowledging that the scores were only for guidance on
ranking and not to be used as absolute, an overall risk score for each
species was calculated as the product of the individual scores for
arrival, establishment, spread and impact on biodiversity as proposed
in the Harmonia+ protocol. With a 4-criterion, 5-point scoring sys-
tem, this produces a maximum score of 625. The individual com-
pleted spreadsheets from each expert were then returned to group
leaders for collation. The objective was to reach broad consensus on
the scores within each group in advance of the workshop. This was
achieved through e-mail and Skype discussions between group mem-
bers but the workshop provided an opportunity for further refine-
ment by the experts.

**2.4 | Step 4: Expert (consensus) workshop**

The aims of the 2-day workshop were clearly outlined; then an over-
view of the IAS selected by each thematic group was presented.
These thematic group presentations were particularly important

because they informed the other participants of the range of species
and their life-histories within each group, enabling subsequent
review and moderation of the scores within the breakout sessions
for each thematic group. During the breakout session, participants
were requested to add or remove species in the light of new evi-
dence (either discovered just prior to the workshop or following
reflection from the preceding workshop presentations and discus-
sions), to justify and moderate scores through discussion and to con-
sider levels of confidence attached to scores. The thematic groups
were asked to restrict their lists to a total of 20–30 top-ranked spe-
cies. The emphasis at this stage was to use the scores as guidance
for informing the subsequent consensus-building component of the
horizon scanning approach and deriving a ranked list rather than as a
component of a full impact assessment.

All the species lists from across the thematic groups were col-
lated into a single list. At this stage there were 249 species listed
(Supporting information S6). Experts were invited to justify their
scores in comparison to those of other groups, to increase the align-
ment of results among groups through a further round of review and
moderation of the lists. The lists (Supporting information S7) from
each thematic group were again combined to produce a list of 120
species. The process of sequential reduction in number of IAS priori-
tised for each thematic group is summarized in Figure 1.

All participants were then invited to review, consider and refine
the rankings of all species through plenary discussion. Leaders of
each thematic group were again asked to justify to the other work-
shop participants the scores for their top-scoring species and to
respond to queries or objections from members of other thematic
groups. It proved to be challenging, but very fruitful, to discuss rank-
ings across thematic groups. Changes to overall rankings for individ-
ual species were made only after hearing the evidence from
appropriate experts, full discussion and, if needed, majority voting.
The end result was an agreed ranked list of potential IAS derived
through discussion and broad consensus that were considered to
represent a medium, high or very high probability of arrival, estab-
lishment, spread and magnitude of impact on biodiversity and
ecosystem services (Figure 1).

2.5 | Step 5: Post workshop compilation of information on species

Following the workshop, information was gathered by the experts within the thematic groups on the likely pathways of arrival (CBD 2014), using published classifications (Supporting information S8). Additionally the biogeographic regions in the EU likely to be most threatened by each species were documented.

**2.6 | Statistical analysis**

To analyse frequencies among thematic groups in relation to threat,
pathways of arrival and membership of functional groups we used
Chi-squared tests. Count data of biogeographic regions under threat
were analysed by generalized linear models with quasi-poisson
distributions. The latter was used to account for underdispersion in the residuals (Crawley, 2012).

## 3 | RESULTS

Of the 329 species considered, a total of 66 marine, terrestrial and freshwater species were identified as having medium (18 species), high (40 species) or very high (8 species) overall threat (Table 1; Figure 2). All workshop participants agreed that the list represented the outcome of the consensus approach.

It was notable that none of the plants or terrestrial invertebrates were ranked within the very high category, but 17 plants and 9 terrestrial invertebrates were considered as posing a high probability of arrival, establishment, spread and magnitude of impact on biodiversity and ecosystem services, and thus categorized as high impact. Of the 66 species identified, plants were considered to pose a higher than average and marine species a lower than average threat ($\chi^2 = 9.32, df = 5, p < 0.05$).

### 3.1 | Native range

The highest proportions of the species identified through the horizon scanning have native ranges in Asia, North America and South America (Figure 3), which more or less mirrors the native ranges of currently established terrestrial and freshwater alien species in Europe (DAISIE, 2009). Species with native ranges in Africa are less represented in the pool of potential future invaders. The marine species are likely to originate from a range of geographic regions.

### 3.2 | Pathways of arrival

Many of the species listed were anticipated to arrive along multiple pathways (Table 1; Figure 4), but it was apparent that escape from confinement was particularly relevant to plants and vertebrates, whereas aquatic species were considered to be most likely to arrive as stowaway or via shipping, and terrestrial invertebrates as contaminants (Figure 4). While the escape pathway was also the most important one in the past for currently established aliens in Europe (60% of all known pathways ($n = 6,224$, DAISIE, 2009), the importance of the stowaway pathways was considered likely to increase for future invaders from currently 8.1% (DAISIE, 2009) to 24% (Figure 4).

Our results do not indicate that there is any one statistically significant pathway through which high risk IAS are expected to enter Europe in future ($\chi^2 = 5.3, df = 5, p = 0.38$; Figure 5).

### 3.3 | Functional groups

The species spanned a variety of functional groups (Figure 6). Primary producers dominated the species listed, while the other groups except for detritivores were almost equally represented. Furthermore, no single functional group was considered to represent a very high or high probability of threat ($\chi^2 = 7.8, df = 5, p = 0.17$).

### 3.4 | Biogeographic regions under threat

The number of EU biogeographic regions under threat from the 66 species on the final list varied between thematic groups (GLM with quasi-Poisson distribution; dispersion parameter = 0.44; analysis of deviance (type II): $\chi^2 = 21.4, df = 4, p < 0.001$), although the majority of the species were predicted to be of threat to two or more biogeographic regions (Table 1). A high number of the freshwater invertebrates and fish were anticipated to pose a threat to four or five biogeographic regions. In contrast, many of the marine species and vertebrates are likely to be restricted to two or three biogeographic regions. The terrestrial invertebrates and plant species are more evenly spread with more than two biogeographic regions predicted to be threatened in all cases. Two species were considered to pose a threat to five biogeographic regions, the Northern snakehead fish, *Channa argus*, and the black striped mussel, *Mytilopsis sallei*.

The Mediterranean, Continental, Macaronesian and Atlantic biogeographic regions are predicted to be the most threatened across all taxonomic groups (Figure 7; $\chi^2 = 108.3, df = 7, p < 0.0001$), whereas Baltic, Black Sea and Boreal biogeographic regions are predicted to be least at risk. The Alpine biogeographic region appears not to be under threat by any species. The terrestrial invertebrates, freshwater invertebrates and fish are likely to be of greatest threat to the Steppic biogeographic region.

## 4 | DISCUSSION

Biological invasions involve complex processes and the ultimate success and impact of an alien species depends on many interacting biological, environmental and societal factors. The approach to horizon scanning proposed here attempts to prioritize potential future alien species in the EU acknowledging this complexity and the lack of evidence for many species under consideration. It is important to note the inherent biases in engaging experts through consensus methods (Sutherland & Burgman, 2015). However, employing techniques such as combining independent opinions and documenting the best available evidence can improve the reliability of judgements (Sutherland & Burgman, 2015). We not only captured independent scores as a first step in compiling the species list but we embedded the consensus methods within a framework that included literature review and impact assessment ensuring an evidence-based approach which has applicability globally at various spatial and temporal scales. Ultimately our overarching aim was to systematically identify species considered to have a probability of arrival, establishment, spread and high impact on biodiversity and ecosystem services within the EU.

We identified 66 species that are currently absent from the EU and were considered to represent a medium, high or very high risk. The species identified in this horizon scanning exercise span a range of functional groups, with primary producers being numerically dominant. Escape from confinement is the pathway considered to be the most likely route of introduction for many species, particularly among plants and vertebrates. Both these patterns are consistent with already established aliens (DAISIE, 2009) and not surprising
<table><caption>TABLE 1 List of 66 potential IAS in the EU with very high (8 species; no fill), high (40; dark grey fill) or medium (18; normal text) perceived overall threat</caption><thead><tr><th rowspan="2">Species Name</th><th rowspan="2">Common Name</th><th rowspan="2">Thematic Group</th><th rowspan="2">Functional Group</th><th rowspan="2">CBD Pathway</th><th rowspan="2">Native range</th><th rowspan="2">Biogeographic regions threatened</th><th rowspan="2">Combined Risk Score</th><th rowspan="2">Com-petition</th><th rowspan="2">Pre-dation</th><th rowspan="2">Hybridization</th><th rowspan="2">Disease Transmission</th><th rowspan="2">Poisoning or toxicity</th><th rowspan="2">Bio-fouling</th><th rowspan="2">Grazing/ herbivory/ browsing</th><th rowspan="2">Interactions with invasive alien species</th><th rowspan="2">Nutrient cycling</th><th rowspan="2">Physical modification</th><th rowspan="2">Natural succession</th><th rowspan="2">Disruption to food webs</th></tr></thead><tbody><tr><td>Channa argus</td><td>Northern snakehead</td><td>Freshwater fish</td><td>Predator</td><td>Escape, Release</td><td>At</td><td>MAC, MED, ATL, CON, STE</td><td>383</td><td>X</td><td>X</td><td>X</td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td></tr><tr><td>Linnoperna fortunei</td><td>Golden mussel</td><td>Freshwater invertebrate</td><td>Filter feeder</td><td>Stowaway, Corridor, Unaided</td><td>As</td><td>MED, ATL, CON, STE</td><td>500</td><td>X</td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Orconectes ruficus</td><td>Rusty crayfish</td><td>Freshwater invertebrate</td><td>Omnivore</td><td>Escape, Release, Stowaway, Corridor, Unaided</td><td>NAm</td><td>MED, ATL, CON, STE</td><td>500</td><td>X</td><td>X</td><td>X</td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Platostus lineatus</td><td>Striped eel catfish</td><td>Marine</td><td>Predator</td><td>Unaided</td><td>WIP, TeNWP, CIP, TeAu</td><td>MED, MAC</td><td>456</td><td>X</td><td>X</td><td>X</td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td></tr><tr><td>Codium parvulum</td><td>A green alga</td><td>Marine</td><td>Primary producer</td><td>Unaided</td><td>WIP</td><td>MED, MAC</td><td>400</td><td>X</td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Crepudula onyx</td><td>Oryx slippersnail</td><td>Marine</td><td>Filter feeder</td><td>Escape, Stowaway</td><td>TiEP</td><td>ATL, MED, MAC</td><td>240</td><td>X</td><td>X</td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Mytilopsis sallei</td><td>Black striped mussel</td><td>Marine</td><td>Filter filter</td><td>Stowaway</td><td>TwWA</td><td>MED, MAC, ATL, BAL, BLK</td><td>216</td><td>X</td><td>X</td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Sclurus niger</td><td>Fox squirrel</td><td>Vertebrate</td><td>Herbivore</td><td>Escape, Release</td><td>NAm</td><td>ATL, MED, CON</td><td>405</td><td>X</td><td>X</td><td>X</td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Merone americana</td><td>White perch</td><td>Freshwater fish</td><td>Predator</td><td>Escape</td><td>NAm</td><td>MAC, MED, ATL, CON, STE</td><td>221</td><td>X</td><td>X</td><td>X</td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td></tr><tr><td>Abizia lebbeck</td><td>Indian siris</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>At</td><td>MAC, ATL, MED</td><td>300</td><td>X</td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Celastrus orbiculatus</td><td>Oriental bittersweet</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>As</td><td>ATL, BOR, CON, (MED)</td><td>500</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Chromolaena odorata</td><td>Siam weed</td><td>Plant</td><td>Primary producer</td><td>Contaminant</td><td>SAm</td><td>MAC, MED</td><td>320</td><td>X</td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td></tr><tr><td>Cinnamomum camphora</td><td>Camphor tree</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>As, At</td><td>MAC, ATL</td><td>400</td><td>X</td><td></td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td></tr><tr><td>Clematis terniflora</td><td>Leather leaf clematis</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>As, At</td><td>MAC, ATL</td><td>400</td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td></tr><tr><td>Cortaderia jubata</td><td>Purple pampas grass</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>SAm</td><td>ATL, MAC, MED</td><td>500</td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td></tr><tr><td>Crystalstegia grandiflora</td><td>Rubber vine</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>SAm</td><td>MAC, ATL, MED</td><td>320</td><td>X</td><td>X</td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Gymnocrorinis splinthoides</td><td>Senegal tea</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>As, SAm</td><td>MAC, (MED)</td><td>625</td><td>X</td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td></tr></tbody></table>

(Continues)
<table><caption>TABLE 1 (Continued)</caption><thead><tr><th rowspan="2">Species Name</th><th rowspan="2">Common Name</th><th rowspan="2">Thematic Group</th><th rowspan="2">Functional Group</th><th rowspan="2">CBD Pathway</th><th rowspan="2">Native range</th><th rowspan="2">Biogeographic regions threatened</th><th rowspan="2">Combined Risk Score</th><th colspan="5">Impact type</th><th rowspan="2">Poisoning or toxicity</th><th rowspan="2">Bio-fouling</th><th rowspan="2">Grazing/ herbivory/ browsing</th><th rowspan="2">Interactions with invasive alien species</th><th rowspan="2">Nutrient cycling</th><th rowspan="2">Physical modification</th><th rowspan="2">Natural succession</th><th rowspan="2">Disruption to food webs</th></tr><tr><th>Competition</th><th>Pre-dation</th><th>Hybridization</th><th>Disease Transmission</th></tr></thead><tbody><tr><td>Lespedeza juncea ssp. sericea (= L. cuneata)</td><td>Chinese lespedeza</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>As, Aus</td><td>ATL, CON, MAC, MED</td><td>500</td><td>X</td><td></td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Lonicera maackii</td><td>Amur honeysuckle</td><td>Plant</td><td>Primary producer</td><td>Escape, Release</td><td>As</td><td>ATL, CON, MAC, MED</td><td>500</td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td></tr><tr><td>Lonicera morrowii</td><td>Morrow's honeysuckle</td><td>Plant</td><td>Primary producer</td><td>Escape, Release</td><td>As</td><td>ATL, CON, MAC, MED</td><td>500</td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td></tr><tr><td>Lysodium japonicum</td><td>Japanese climbing fern</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>At</td><td>MAC, MED</td><td>625</td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td></td></tr><tr><td>Microstegium virgatum</td><td>Nepalese browntop</td><td>Plant</td><td>Primary producer</td><td>Contaminant</td><td>As</td><td>(ATL), CON, MAC, MED</td><td>500</td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td></td></tr><tr><td>Prosopis jujiflora</td><td>Prosopis</td><td>Plant</td><td>Primary producer</td><td>Contaminant, Escape</td><td>SAm</td><td>ATL, MAC, MED</td><td>500</td><td>X</td><td></td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td></td><td></td></tr><tr><td>Prunus comanulata</td><td>Bell flower cherry</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>As</td><td>ATL, MAC</td><td>500</td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td></td><td>X</td><td></td></tr><tr><td>Rubus rosifolius</td><td>Roseleaf raspberry</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>At, Aus</td><td>MAC</td><td>500</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Triadica sebifera (Sapium sebiferum)</td><td>Chinese tallowtree</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>As</td><td>MAC, MED</td><td>500</td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Aconitophora spicifera</td><td>A red alga</td><td>Marine</td><td>Primary producer</td><td>Stowaway</td><td>TnWA</td><td>MED, MAC</td><td>192</td><td>X</td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Gammarus fuscatus</td><td>Freshwater shrimp</td><td>Freshwater</td><td>Omnivore</td><td>Escape, Contaminant, Corridor, Stowaway</td><td>NAm</td><td>MED, ATL, CON, STE</td><td>108</td><td>X</td><td>X</td><td></td><td>X</td><td>X</td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Perna viridis</td><td>Asian green mussel</td><td>Marine</td><td>Filter feeder</td><td>Stowaway</td><td>TenNWP</td><td>MED, MAC, ATL</td><td>192</td><td>X</td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Potamocorbula amurensis</td><td>Asian basket clam</td><td>Marine</td><td>Filter feeder</td><td>Stowaway</td><td>TenNWP</td><td>MED, MAC, ATL, BLK, BAL</td><td>180</td><td>X</td><td>X</td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Symplegma reptans</td><td>Pillow-like tunicate</td><td>Marine</td><td>Filter feeder</td><td>Stowaway</td><td>TenNWP</td><td>MED, MAC, ATL, BLK</td><td>192</td><td>X</td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Aeolothes sarta</td><td>City longhorn beetle, Qetta borer</td><td>Terrestrial invertebrate</td><td>Herbivore</td><td>Contaminant</td><td>As</td><td>MED, ATL, CON, STE, BOR</td><td>99</td><td>X</td><td></td><td></td><td>X</td><td>X</td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Arrhenoths agrestis</td><td>Crazy snake worm</td><td>Terrestrial invertebrate</td><td>Detritivore</td><td>Contaminant, Stowaway</td><td>As</td><td>ATL CON</td><td>129</td><td>X</td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Pachycordyka chinensis</td><td>Asian needle ant</td><td>Terrestrial invertebrate</td><td>Omnivore</td><td>Contaminant</td><td>As</td><td>MED, ATL, CON, STE, MAC</td><td>175</td><td>X</td><td>X</td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Sirex ermok</td><td>Blue-black hornail</td><td>Terrestrial invertebrate</td><td>Herbivore</td><td>Contaminant, Escape</td><td>As</td><td>CON, STE, BOR</td><td>111</td><td>X</td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr></tbody></table>

(Continues)
<table><caption>TABLE 1 (Continued)</caption><thead><tr><th rowspan="2">Species Name</th><th rowspan="2">Common Name</th><th rowspan="2">Thematic Group</th><th rowspan="2">Functional Group</th><th rowspan="2">CBD Pathway</th><th rowspan="2">Native range</th><th rowspan="2">Biogeographic regions threatened</th><th rowspan="2">Combined Risk Score</th><th rowspan="2">Com-petition</th><th rowspan="2">Pre-dation</th><th rowspan="2">Hybridization</th><th rowspan="2">Disease Transmission</th><th rowspan="2">Poisoning or toxicity</th><th rowspan="2">Bio-fouling</th><th rowspan="2">Grazing/ herbivory/ browsing</th><th rowspan="2">Interactions with invasive alien species</th><th rowspan="2">Nutrient cycling</th><th rowspan="2">Physical modification</th><th rowspan="2">Natural succession</th><th rowspan="2">Disruption to food webs</th></tr></thead><tbody><tr><td>Solenopsis geminata</td><td>Fire ant</td><td>Terrestrial invertebrate</td><td>Omnivore</td><td>Contaminant</td><td>NAm/ SAm</td><td>MAC, MED, ATL, CON, STE</td><td>160</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td></td><td>X</td><td></td><td>X</td><td></td><td>X</td><td></td></tr><tr><td>Solenopsis invicta</td><td>Red imported fire ant</td><td>Terrestrial invertebrate</td><td>Omnivore</td><td>Contaminant</td><td>SAm</td><td>MAC, MED</td><td>160</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Solenopsis richteri</td><td>Black imported fire ant</td><td>Terrestrial invertebrate</td><td>Omnivore</td><td>Contaminant</td><td>SAm</td><td>MAC, MED, ATL, CON, STE</td><td>128</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Tetropium gracilicorne</td><td>Fine-horned spruce beetle</td><td>Terrestrial invertebrate</td><td>Herbivore</td><td>Contaminant</td><td>As</td><td>ATL, CON, STE, BOR</td><td>128</td><td>X</td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Vespula pensylvanica</td><td>Western yellowjacker</td><td>Terrestrial invertebrate</td><td>Omnivore</td><td>Contaminant</td><td>NAm</td><td>MAC, MED, ATL, CON, STE, BOR?</td><td>99</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td></td><td>X</td><td></td><td>X</td><td></td><td>X</td><td>X</td></tr><tr><td>Bison bison</td><td>American bison</td><td>Vertebrate</td><td>Herbivore</td><td>Release</td><td>NAm</td><td>CON</td><td>338</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Boiga irregularis</td><td>Brown tree snake</td><td>Vertebrate</td><td>Predator</td><td>Escape, Release, Stowaway</td><td>Aus</td><td>MED, MAC</td><td>280</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td></td><td>X</td><td></td><td></td><td></td><td></td><td>X</td></tr><tr><td>Cynops pyrrhogaster</td><td>Japanese fire-bellied salamander</td><td>Vertebrate</td><td>Omnivore</td><td>Escape</td><td>As</td><td>CON</td><td>354</td><td>X</td><td>X</td><td>?</td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td>X</td><td>X</td></tr><tr><td>Eleutherodactylus coqui</td><td>Common coqui</td><td>Vertebrate</td><td>Predator</td><td>Escape, Contaminant, Stowaway</td><td>NAm</td><td>MED, MAC</td><td>252</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Eleutherodactylus planirostris</td><td>Greenhouse frog</td><td>Vertebrate</td><td>Predator</td><td>Escape, Stowaway</td><td>NAm</td><td>MED, MAC</td><td>288</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Hemidactylus frenatus</td><td>Common house gecko</td><td>Vertebrate</td><td>Predator</td><td>Escape, Stowaway</td><td>Aus</td><td>ATL, MED, CON</td><td>320</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Rhinella marina</td><td>Cane toad</td><td>Vertebrate</td><td>Omnivore</td><td>Contaminant, Escape, Release</td><td>SAm</td><td>MED, MAC</td><td>280</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td></td><td>X</td><td></td><td>X</td><td></td><td>X</td><td>X</td></tr><tr><td>Trichosurus vulpecula</td><td>Brushtail possum</td><td>Vertebrate</td><td>Omnivore</td><td>Escape</td><td>Aus</td><td>ATL, MED, CON, MAC</td><td>304</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Dorhinia lumholtzi</td><td></td><td>Freshwater invertebrate</td><td>Filter feeder</td><td>Contaminant, Corridor, Stowaway, Unaided</td><td>As, At, Aus</td><td>MAC, MED, CON, STE</td><td>96</td><td>X</td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td></td><td>X</td><td>X</td><td>X</td><td>X</td></tr><tr><td>Pinus potula</td><td>Mexican wesping pine</td><td>Plant</td><td>Primary producer</td><td>Escape</td><td>NAm</td><td>MAC, ATL</td><td>300</td><td>X</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ascidia sydneiensis</td><td>Green tube tunicate</td><td>Marine</td><td>Filter feeder</td><td>Contaminant, Stowaway</td><td>WIP, CIP</td><td>MED, MAC, ATL</td><td>108</td><td>X</td><td></td><td></td><td></td><td>X</td><td></td><td></td><td>X</td><td>X</td><td></td><td></td><td>X</td></tr><tr><td>Bolanus glandula</td><td>Acon barnacle</td><td>Marine</td><td>Filter feeder</td><td>Stowaway</td><td>TENEP</td><td>ATL, BAL</td><td>108</td><td>X</td><td></td><td></td><td></td><td>X</td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td></tr></tbody></table>

(Continues)
<table><caption>TABLE 1 (Continued)</caption><thead><tr><th rowspan="2">Species Name</th><th rowspan="2">Common Name</th><th rowspan="2">Thematic Group</th><th rowspan="2">Functional Group</th><th rowspan="2">CBD Pathway</th><th rowspan="2">Native range</th><th rowspan="2">Biogeographic regions threatened</th><th rowspan="2">Combined Risk Score</th><th colspan="7">Impact type</th><th rowspan="2">Physical modification</th><th rowspan="2">Natural succession</th><th rowspan="2">Disruption to food webs</th></tr><tr><th>Competition</th><th>Pre-dation</th><th>Hybridization</th><th>Disease Transmission</th><th>Poisoning or toxicity</th><th>Bio-fouling</th><th>Grazing/ herbivory/ browsing</th><th>Interactions with invasive alien species</th><th>Nutrient cycling</th></tr></thead><tbody><tr><td>Ciona savignyi</td><td>Pacific transparent tunicate</td><td>Marine</td><td>Filter feeder</td><td>Escape, Stowaway</td><td>TENEP, Teswa, TeAu</td><td>ATL, BLK, BAL, MED, MAC</td><td>144</td><td>X</td><td></td><td></td><td></td><td>X</td><td>X</td><td></td><td>X</td><td></td><td></td></tr><tr><td>Dictyosphaeria covermosa</td><td>Green bubble weed</td><td>Marine</td><td>Primary producer</td><td>Stowaway</td><td>WIP</td><td>MED, MAC</td><td>108</td><td>X</td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td></td><td></td></tr><tr><td>Didemnum perfluddum</td><td>A colonial tunicate</td><td>Marine</td><td>Filter feeder</td><td>Contaminant, Stowaway</td><td>Unknown</td><td>MED, MAC</td><td>128</td><td>X</td><td></td><td></td><td></td><td>X</td><td></td><td>X</td><td>X</td><td></td><td></td></tr><tr><td>Dorvillea similis</td><td>A polychaete worm</td><td>Marine</td><td>Detritivore</td><td>Stowaway, Unaided</td><td>WIP, CIP</td><td>MED, MAC</td><td>150</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Rhodosoma turcicum</td><td>A unitary tunicate</td><td>Marine</td><td>Filter feeder</td><td>Stowaway</td><td>WIP, CIP, TeNWP, TWA</td><td>MED, MAC, ATL</td><td>162</td><td>X</td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Zostera japonica</td><td>Dwarf eelgrass</td><td>Marine</td><td>Primary Producer</td><td>Contaminant</td><td>TENWP</td><td>MED, MAC, ATL, BLK, BAL</td><td>108</td><td>X</td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Agrilus auror guttatus</td><td>Gold spotted oak borer</td><td>Terrestrial invertebrate</td><td>Herbivore</td><td>Contaminant, Stowaway</td><td>Nam</td><td>MED</td><td>81</td><td>X</td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Dendrolimus superans</td><td>White-lined silk moth</td><td>Terrestrial invertebrate</td><td>Herbivore</td><td>Contaminant</td><td>As</td><td>CON, STE, BOR</td><td>128</td><td>X</td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Platypus quercivorus</td><td>Oak ambrosia beetle</td><td>Terrestrial invertebrate</td><td>Herbivore</td><td>Contaminant, Stowaway</td><td>As</td><td>MAC, MED, ATL, CON, STE</td><td>97</td><td>X</td><td></td><td></td><td></td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td></td></tr><tr><td>Boa constrictor</td><td>Boa constrictor</td><td>Vertebrate</td><td>Predator</td><td>Escape</td><td>SAm</td><td>MED</td><td>263</td><td>X</td><td>X</td><td></td><td></td><td>X</td><td></td><td></td><td></td><td>X</td><td></td></tr><tr><td>Gymnorhina tibicen</td><td>Australian magpie</td><td>Vertebrate</td><td>Omnivore</td><td>Escape</td><td>Aus</td><td>ATL, MED, CON</td><td>225</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Python molurus</td><td>Indian rock python</td><td>Vertebrate</td><td>Predator</td><td>Escape</td><td>Al</td><td>MED</td><td>263</td><td>X</td><td>X</td><td></td><td></td><td></td><td></td><td></td><td>X</td><td></td><td></td></tr><tr><td>Quelea quelta</td><td>Red billed quetzal</td><td>Vertebrate</td><td>Omnivore</td><td>Escape, Release</td><td>Afr</td><td>MED, CON</td><td>252</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Tantiscanus hudsonicus</td><td>American red squirrel</td><td>Vertebrate</td><td>Herbivore</td><td>Escape</td><td>NAm</td><td>BOR, ATL, CON</td><td>244</td><td>X</td><td>X</td><td>X</td><td></td><td>X</td><td></td><td></td><td></td><td>X</td><td></td></tr></tbody></table>

Notes. The pathway information is as recommended by the (CBD 2014). Codes for Native Range are given in the legend for Figure 3 and for Biogeographic Regions in the legend for Figure 7. Note that the combined risk score was used for guiding the discussions between experts across thematic groups and **does not** relate strictly to the final rank of the species within the list. Likely impact type is indicated by “X” in the relevant columns.
<caption>**FIGURE 2** Number of species absent from Europe (n = 66) that were considered to have a very high, high or medium probability of arrival, establishment, spread and magnitude of impact on biodiversity and ecosystem services across thematic subgroups (Freshwater invertebrates, Freshwater fish, Terrestrial invertebrates, Vertebrates, Plants and Marine species)</caption>

since many of the alien plants are anticipated to arrive as escapes from horticulture (Saul et al., 2017). Pathways within the stowaway categories are considered likely to increase in importance in terms of species introductions compared to the past (Chapman et al., 2016; Zieritz et al., 2017); this is particularly the case for marine species, but also for terrestrial invertebrates, and highlights the importance of increasing surveillance of transport vectors (Hulme, 2015; Saul et al., 2017; Pergl et al., 2017) and implementation of preventative measures. For example, the highly invasive fire ant, *Solenopsis invicta*, is likely to arrive as a stowaway in packaging (Inoue & Goka, 2009). It is important to consider the spread of IAS from countries adjacent to the region of interest but for the EU future major donor regions of IAS are also likely to be from further afield with introductions from Asia and the Americas anticipated to increase (Seebens et al., 2015; Zieritz et al., 2017). Thus, the pathways and origins of

expected future IAS are similar to the major pathways of historic invasions in Europe (DAISIE, 2009).

Apart from some general patterns, alien species introduction events have a strong stochastic component. Therefore, it is important to recognize the imperfect nature of horizon scanning lists (Nehring, Kowarik, Rabitsch, & Essl, 2013). There are undoubtedly many species that have not been considered through this horizon scanning approach that could arrive in the future. However, involving a large number of people through a semi-structured process to horizon scanning can inform the three-stage hierarchical approach proposed by the CBD for managing the impacts of IAS. Communication and cross-boundary collaborations extending beyond the EU, ensuring knowledge on IAS is shared between countries, are essential to ensure successful implementation of an IAS strategy (Hulme, Pyšek, Nentwig, & Vilà, 2009).

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>**FIGURE 3** Native range of the species absent from Europe (n = 66) considered to have at least a medium probability of arrival, establishment, spread and magnitude of impact on biodiversity and ecosystem services. The insert shows the native regions of established terrestrial and freshwater alien species in Europe (n = 6,224, from DAISIE, 2009). Note that species can occur in more than one region. Terrestrial and Freshwater (Continental TDWG categories): Afr = Africa; As = Asia Temperate; At = Asia Tropical; Aus = Australasia; NAm = North America; SAm = South America. Marine (Spalding et al., 2007): TeAu = Temperate Australasia; TeNWA = Temperate NW Atlantic; TeNWP = Temperate NW Pacific; TeSAf = Temperate Southern Africa; TrEA = Tropical Eastern Atlantic; TrEP = Tropical Eastern Pacific; TrWA = Tropical Western Atlantic; CIP = Central Indo-Pacific; WIP = Western Indo-Pacific</figcaption></figure>
<figure><img src="image_5.png" /></figure>

**FIGURE 4** Number of species absent from Europe (n = 66) considered to have at least a medium probability of arrival, establishment, spread and magnitude of impact on biodiversity and ecosystem services and their anticipated pathways of arrival across thematic subgroups (Freshwater invertebrates, Freshwater fish, Terrestrial invertebrates, Vertebrates, Plants and Marine species). The pathway classification follows (CBD, 2014). The insert shows the frequencies of known pathways of currently established aliens in Europe as taken from DAISIE (2009) (n = 6,224)

<figure><img src="image_6.png" /></figure>

**FIGURE 5** Number of species absent from Europe (n = 66) considered to have a very high, high or medium probability of arrival, establishment, spread and magnitude of impact on biodiversity and ecosystem services and their anticipated pathways of arrival. Note that species can arrive via multiple pathways. The pathway classification follows (CBD, 2014)

<figure><img src="image_7.png" /></figure>

**FIGURE 6** Number of species, absent from Europe (n = 66), within different functional groups considered to have at medium, high or very high probability of arrival, establishment, spread and impact on biodiversity and ecosystem services.

The breadth of biogeographic regions that are considered under threat by the species identified through the horizon scanning is striking, but it is notable that the Atlantic, Mediterranean, Continental and Macaronesian biogeographic regions are most at risk under current climate conditions, while the Alpine region is not. The Mediterranean biogeographic region is at risk because of the predicted

arrival of Lessepsian potential IAS from the Indo-Pacific exacerbated by the latest enlargement of the Suez Canal (Galil et al., 2015).

Climate warming is likely to play an important role in the future with respect to interactions with IAS, but not within the designated timeframe of 10 years (Walther et al., 2009; Cheng, Sakai, Matsushima, Yagi, & Hasegawa, 2010; Bellard et al., 2013). Some of the
<figure><img src="image_8.png" /><figcaption>**FIGURE 7** Number of species absent from Europe (n = 66) considered to have at least a medium probability of arrival, establishment, spread and impact on biodiversity and ecosystem services and the EU biogeographic regions they are predicted to establish within.
Abbreviations: ALP = Alpine Region, BOR = Boreal Region, ATL = Atlantic Region, CON = Continental Region, MED = Mediterranean Region, MAC = Macaronesian Region, STE = Steppic Region, BLK = Black Sea Region</figcaption></figure>

species that have been recorded but have not yet established might be able to reproduce and spread in future climates. This includes currently inhospitable regions, e.g. in the Alpine or Boreal region (Walther et al., 2009). It is essential that consideration is given to interactions between major drivers of change such as climate change, habitat destruction and pollution when predicting the likely establishment, spread and impacts of potential IAS.

The proposed lists provide a basis for prioritizing full risk assessments in order to comprehensively evaluate the threat posed by these species to the EU biodiversity. Completion of risk assessments for each species categorized as high or very high risk should be prioritized to validate the list and ensure that evidence of impacts is assessed in a rigorous and robust way. However, it would also be useful to assess a sample of those with medium risk scores as a way of checking the selection and ranking of species. Consideration of so many species requires a rapid method of assessment for arrival, establishment, spread and impact that enables effective, although approximate ranking. The crude bracketing of species as posing very high, high and medium threat was an effective way of managing the complexities of prioritizing such a long list of species spanning diverse taxonomic groups and environments. The experts were unanimously agreed that this approach increased their confidence in reaching a decision and reduced bias in the ranking, but note that the categorization is subjective. It is also important to remember that the scoring is to enable species to be prioritized for future formal risk assessment and that scores underpinned by detailed evidence should be collated during such risk assessment. Furthermore, we recommend conducting regular reviews of both the species rankings and future potential IAS that could threaten the EU, as demanded by the EU Regulation. For this purpose, dedicated species accounts should be considered and kept updated in the species data repository formally endorsed by the EU Regulation, i.e. EASIN (https://ea.sin.jrc.ec.europa.eu/).

The focus of this horizon scanning exercise was only on the negative impacts of potential IAS on biodiversity and ecosystems, with some consideration on ecosystem service impacts. Systematic consideration of ecosystem services could form an integral part of a future horizon scanning exercise (Hulme & Vilà, 2017), and

potentially evaluation of services and disservices. However, currently there is a lack of information to allow for a detailed and/or scientifically well-informed assessment of ecosystem services including socio-economic impacts, affecting the overall robustness of the scoring exercise (Roy, 2017). Therefore, biodiversity and ecosystem services impacts are recommended to be the core focus of a horizon scanning exercise with socio-economic factors included where information is available. Additionally, improving the evidence-base and developing frameworks for assessing socio-economic impacts should be a priority.

Thematic groups ranked a similar number of species as very high, high or medium priority for risk assessment, with the exception of the terrestrial invertebrate group which listed fewer species than the others. For most terrestrial invertebrates, research on impacts is focused on productive sectors, such as forestry and agriculture, or human health and well-being, rather than impacts on biodiversity. Substantial knowledge gaps for marine species (Ojaveer et al., 2015) and terrestrial invertebrates (Kenis et al., 2009; Nentwig & Vaes-Petignat, 2014) have also been recognized. Indeed, all thematic groups struggled with a lack of information to some extent. For example, over all European alien species, impacts are reported for only 10% (Vilà et al., 2010), the main shortfall being poor understanding and documentation of impacts on ecosystem services (Roy, Schonrogge et al., 2014), although it is also recognized that a high proportion of alien species might not cause notable impacts (Roy, Preston et al., 2014) and the impacts of those that do are highly-context dependent. Lack of information does not equate to absence of threat but a deliberately conservative approach was adopted whereby only those species with some supporting evidence of impacts on biodiversity were included in the list (Hulme et al., 2013). However, one of the advantages of using expert-elicitation within a consensus approach to horizon scanning is the breadth of information sources drawn upon by the group members. Furthermore, evidence is accruing and new methods are ensuring robust and repeatable approaches for assessing environmental (Blackburn et al., 2014) and socio-economic impacts (Bacher et al., 2018) including effects of IAS on ecosystem services.
Information provided by horizon scanning exercises is essential to support decision making on IAS, and to ensure an optimal use of the resources invested in prevention and early detection of possible invaders; activities that can require substantial economic investments. Therefore, regular review and refinement of the lists derived from such an approach will be critical. The horizon scanning method presented here could be extended in various ways particularly through inclusion of additional information on socio-economic impacts (Bacher et al., 2018) but also identification and prioritization of emerging and promising IAS management methods, technologies or control actions (Shine et al., 2010; Ricciardi et al., 2017). Moreover, an important future priority is the management of arrival pathways of potential IAS considered to pose a major threat to biodiversity and ecosystem services (Essl et al., 2015; Vilà & Hulme, 2017) and this horizon scanning approach could inform pathway action plans.

## ACKNOWLEDGEMENTS

The authors are grateful to the European Commission for funding the study (Invasive alien species - framework for the identification of invasive alien species of EU concern ENV.B.2/ETU/2013/0026) on which this publication is based on. The study was also supported by project COST TD1209 (Alien Challenge). JP was partly funded by long-term research development project RVO 67985939 (Academy of Sciences of the Czech Republic). MV acknowledges the Ministerio de Economía y Competitividad projects IMPLANTIN (CGL2015-65346R) and the Severo Ochoa Program for Centres of Excellence (SEV-2012-0262). GHC acknowledges the UK Department of Environment, Food & Rural Affairs for its complementary support. We thank the editor and anonymous reviewers for their insightful comments.

## AUTHORS' CONTRIBUTIONS

HER conceived the approach and led the manuscript. SB led the analyses. SB, FE, PG, SLR, TA, WR and RS contributed substantially to the writing of the manuscript. All authors contributed to the prioritization exercise including compilation of lists and rapid impact assessments and commented on the writing of the manuscript.

## ORCID

Helen E. Roy ID https://orcid.org/0000-0001-6050-679X

Sven Bacher ID https://orcid.org/0000-0001-5147-7165

Juliet Brodie ID https://orcid.org/0000-0001-7622-2564

Gordon H. Copp ID https://orcid.org/0000-0002-4112-3440

Jørgen Eilenberg ID https://orcid.org/0000-0002-9273-5252

Emili García-Berthou ID https://orcid.org/0000-0001-8412-741X

Jan Pergl ID https://orcid.org/0000-0002-0045-1974

Alan J. A. Stewart ID https://orcid.org/0000-0001-7878-8879

Elena Tricarico ID https://orcid.org/0000-0002-7392-0794

Montserrat Vilà ID https://orcid.org/0000-0003-3171-8261

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## SUPPORTING INFORMATION

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**How to cite this article:** Roy HE, Bacher S, Essl F, et al. Developing a list of invasive alien species likely to threaten biodiversity and ecosystems in the European Union. *Glob Change Biol.* 2019;25:1032–1048. https://doi.org/10.1111/gcb.14527