Spread of invasive species into Scotland: study

A horizon scanning study involving analysis of pathways of spread of invasive non-native species into Scotland. It considers species having the highest likelihood of arrival and establishment and the magnitude of their potential negative impact on biodiversity and ecosystems over the next 10 years.

Annex 1: Brief Summary of Horizon Scanning Approach

Briefing note circulated to all experts in advance of the horizon scanning

Prioritising Invasive Non-Native Species through Horizon Scanning for Scotland

UK Centre for Ecology & Hydrology, Crowmarsh Gifford, Oxfordshire, OX10 8BB, UK

It is useful to note that this is the first step in prioritising INNS for action. Consequently, the assessment process, while based on specific criteria, employed is by necessity crude and used for the purposes of ranking.

Consensus approach to horizon scanning

We will use an adapted version of the consensus method (Sutherland, Fleishman et al. 2011) for a horizon scanning approach previously used to derive a ranked list of potential INNS with high impact on biodiversity and ecosystems in Great Britain (Roy, Peyton et al. 2014) and Europe (Roy, Bacher et al. 2019) (Figure 1). We have extended the approach to consider human health and economic impacts.

Figure 1. Horizon scanning process, based on consensus method, to derive a ranked list of INNS which are likely to arrive, establish and have an impact over the next decade.
A diagram outlining the horizon scanning process, based on consensus method, to derive a ranked list of INNS which are likely to arrive, establish and have an impact over the next decade. A literature trawl and expert knowledge are used to derive preliminary lists of species. These are sorted and reviewed using rapid risk assessment and knowledge gathering and sharing exercises. Species are selected and then a consensus is sought from expert groups on which species should be included on the lists of plants, invertebrates, marine species and vertebrates. There is a moderation of species ranking across the groups. Expert opinion and knowledge are used to review and refine the species rankings across the groups and a consensus is reached to produce a ranked list of species which are likely to arrive, establish and impact over the next decade.

Step 1. Establishment of thematic groups

Species will be considered within five broad thematic groups:

  • Plants
  • Terrestrial invertebrates
  • Freshwater invertebrates
  • Vertebrates
  • Marine species

Step 2. Compilation of preliminary lists of potential INNS

Each thematic group will assemble preliminary lists of potential INNS that they considered to constitute the highest risk with respect to the likelihood of arrival, establishment and the magnitude of their potential negative impact on biodiversity and ecosystems or human health or economies over the next ten years.

Each thematic group will derive these lists from a combination of systematic literature searches (including academic journals, risk assessments, reports, authoritative websites and other ‘grey’ literature), checklists, floras, querying of INNS databases and their own expert knowledge. The approaches adopted by each thematic group will differ 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 (Table 1). The leaders will coordinate activities and discussion between group members throughout the process. The consultation between experts will be completed both through e-mail discussions in advance of the workshops and through the workshop breakout groups.

We will provide lists of INNS from previous exercises (Great Britain, Europe and the UK Overseas Territories). We will also provide a spreadsheet template for gathering data.

The geographic scope of the search for potential INNS will be global but with the following restrictions:

(i) Are absent in Scotland

(ii) Have documented histories of invasion and causing undesirable impacts in other regions worldwide with similar climatic conditions.

(iii) Traded within Scotland or are present in areas that have strong trade or travel connections with Scotland and where there is a recognised potential pathway for arrival.

(iv) Are present in captivity including gardens, zoological parks, aquaculture facilities and glasshouses.

For this horizon scanning exercise, we will focus on species that have not yet become established in Scotland in the wild, that is have not yet formed self-sustaining populations (Blackburn, Pysek et al. 2011). However, a few species will be included, which have formed transient local populations that have been detected and either failed to persist or been deliberately removed. In accordance with definitions outlined by the CBD[11], we will categorize species as non-native, if their arrival was likely to be mediated by human activities.

Data Sources

  • GBIF Database;
  • Caribbean Invasive Alien Species Network database (CIASNET);
  • Weber (2003) Invasive Plant Species of the World;
  • Randall (2002) A Global Compendium of Weeds;
  • BSBI Distribution Database;
  • CABI Horizon Scanning Database;
  • CABI Horizon Scanning Tool;
  • CABI Invasive Species Compendium;
  • EPPO Database;
  • Global Register of Introduced and Invasive Species
  • Caribbean Invasive Alien Species Network database (CIASNET);
  • CABI Horizon Scanning Tool;
  • Global Register of Introduced and Invasive Species (GRIIS);
  • GBIF Database; Global Avian Introduction database (GAVIA);
  • CABI Invasive Species Compendium;
  • Wikipedia List of invasive species in Florida;
  • JNCC Database of non-native species occurring in UK Overseas Territories;
  • CABI horizon scanning tool;
  • Sistema Nacional de Información sobre Especies Exóticas Invasoras (Argentinian IAS database);
  • Avibase - Bird Checklists of the World
  • CABI Horizon Scanning Tool;
  • GBIF Database;
  • WORMS Database, AlgaeBase.org;
  • CABI Invasive Species Compendium;
  • NEMESIS (US Based database)

We will consider species on the GB INNS species lists present in England and Wales, but not yet present in Scotland, whereas species that are deemed likely to arrive by natural dispersal from their native range in GB will be excluded from consideration.

We will also exclude from this exercise potential invasions of microbial pathogens, bacterial, fungal, oomycetes or otherwise. While clearly of potential importance, a recent review identified significant knowledge gaps and identified 10 key areas for research (Roy, Hesketh et al. 2016). However, we do consider potential macroscopic INNS which are known vectors of disease for impact assessments.

Where invertebrates that occupy terrestrial and freshwater habitats at different stages of their life cycle have egg and larval stages bound to freshwater habitats, e.g. most mosquitoes and midges, they will be reviewed by the freshwater group. Establishment as a crucial phase in becoming invasive is likely to be determined by adaptation to the abiotic environment at the immature stages.

Species associated with brackish water habitats will be considered explicitly by the Freshwater team but will be reviewed by the Marine Team for any gaps.

The temporal scope of the horizon scanning exercise will be only species likely to arrive in the next 10 years within GB. This temporal limit informs the relevance of, for instance, long-term climate change projections.

Step 3: Scoring of species

Experts were advised that the scoring approach was not absolute but to provide an initial ranking of all potential INNS. This context was important to ensure that experts were empowered to use expert judgement alongside available evidence sources. Experts were asked to score each species within their thematic group for their separate likelihoods of: i) arrival, ii) establishment, iii) magnitude of the potential negative impact on biodiversity or ecosystems, human health or economies. A 5-point scale from 1=very low to 5=very high (Blackburn, Essl et al. 2014) was adopted. The scores from each expert within each thematic group were then compiled and discussions within the thematic groups (at the workshop) led to an overall agreed impact and confidence score for each species.

Scoring arrival

Scores for the likelihood of arrival should be 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. A score of 1 denotes that the species is considered unlikely to arrive within the chosen timeframe. A score of 5 is used to denote near-certain arrival; for example if there was a previously documented inception of the species. In the case of species already in GB (such as those held commonly in captivity or planted in gardens), the likelihood of arrival was agreed to be given a score of 5.

Scoring establishment

Having arrived, the probability of a species establishing a self-sustaining population in the wild depends on the ecological properties of both the species and the community that it is invading (Leung, Roura-Pascual et al. 2012). Scores should reflect life-history characteristics including reproductive rate and ecological features such as tolerance of a broad range of environmental conditions or availability of food supply in the introduced range.

Scoring impacts

Scores are required for each of the three impact categories (biodiversity and ecosystems (e.g. species, habitats, ecosystems and ecosystem functioning), human health or economies (Table 2). The impact scoring system has been modified from the Invasive Species Environmental Impact Assessment protocol (Branquart 2009), the GB Non-Native Risk Assessment scheme (Booy, White et al. 2006) and the proposed unified framework for environmental impacts - Environmental Impact Classification of Alien Taxa EICAT (Volery, Bacher et al. 2020) and Socio-Economic Impact Classification of Alien Taxa SEICAT (Bacher, Blackburn et al. 2017).

Confidence levels

Confidence levels (high, medium or low confidence) should be attributed to each score to help focus discussions and refine the list of species and in guiding discussion within some thematic groups (Table 3).

While acknowledging that the scores are only for guidance on ranking and not to be used as absolute, an overall risk score for each species will be calculated as the product of the individual scores for arrival, establishment and impact on biodiversity. With a 3-criterion, 5-point scoring system, this produces a maximum score of 125.

Table 2. Guidance notes for scoring impacts on biodiversity and ecosystems ( e.g. impacts on species, habitats, ecosystems and ecosystem functioning), human health or economies.
Score Impact on biodiversity and ecosystems Impact on human health Impact on economies
1 No deleterious impacts or local, short-term impact on few species or ecosystems, reversible No deleterious impacts or local, short-term reversible effects to few individuals No deleterious impacts reported
2 Local, short-term impact on communities or several ecosystems, reversible Local, short-term reversible effects to larger groups of people Negative effect on crops or livestock local, short-term and reversible; loss of revenue minor
3 Long-term impact, but little spread, no extinction Local, but irreversible effects on small groups of people or reversible effects on larger groups of people Negative effect on crops or livestock local, but irreversible
4 Long-term irreversible impact, spreading beyond the local area Local, significant irreversible effects at the regional scale or reversible effects over large areas Negative effect on crops and livestock irreversible at the regional scale (i.e. beyond local areas), or reversible over larger areas
5 Widespread, severe, long-term impact, including extinction Widespread, severe, long-term, irreversible health effects over large areas Negative effect on crops and livestock severe, irreversible over large areas

Confidence Score


  • There is direct relevant evidence to support the assessment.
  • The situation can easily be predicted.
  • There are reliable/good quality data sources on impacts of the species.
  • The interpretation of data/information is straightforward.
  • Data/information are not controversial, contradictory.


  • There is some evidence to support the assessment.
  • Some information is indirect, e.g. data from phylogenetically or functionally similar species have been used as supporting evidence.
  • The interpretation of the data is to some extent ambiguous or contradictory.


  • There is no direct evidence to support the assessment, e.g. only data from other species have been used as supporting evidence.
  • Evidence is poor and difficult to interpret, e.g. because it is strongly ambiguous.

Information on pathways

Information should be gathered throughout the workshop by the experts within the thematic groups on the likely pathways of arrival, using published classifications (Harrower, Scalera et al. 2018) (Table 2).

Step 4: Expert (consensus) workshop

Each thematic group will present an overview of the INNS to inform 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 can add or remove species, justify and moderate scores and consider levels of confidence attached to scores. All the species lists from across the thematic groups will be collated into single lists for each of the impact categories (biodiversity and ecosystems, human health or economic). Experts will be invited to justify their scores in comparison to those of other groups.

All participants will then be invited to review, consider and refine the rankings of all species through plenary discussion. Again scores will be adjusted accordingly. The end result will be an agreed ranked lists of INNS with the potential to arrive, establish and pose a threat through biodiversity and ecosystem, human health or economic impacts.

Step 5: Post workshop compilation of information on species

Following the workshop all participants will be invited to review the pathway information for the INNS identified as priorities. Additional taxonomic information and other details for the INNS will also be reviewed.

Movement of Commodity

Release in Nature

  • Biological control
  • Erosion control/ dune stabilization (windbreaks, hedges, …)
  • Fishery in the wild (including game fishing)
  • Hunting
  • Landscape/flora/fauna “improvement” in the wild
  • Introduction for conservation purposes or wildlife management
  • Release in nature for use (other than above, e.g., fur, transport, medical use)
  • Other intentional release

Escape from Confinement

  • Agriculture (including Biofuel feedstocks)
  • Aquaculture / mariculture
  • Botanical garden/zoo/aquaria (excluding domestic aquaria)
  • Pet/aquarium/terrarium species (including live food for such species )
  • Farmed animals (including animals left under limited control)
  • Forestry (including afforestation or reforestation)
  • Fur farms
  • Horticulture
  • Ornamental purpose other than horticulture
  • Research and ex-situ breeding (in facilities)
  • Live food and live bait
  • Other escape from confinement

Transport - Contaminant

  • Contaminant nursery material
  • Contaminated bait
  • Food contaminant (including of live food)
  • Contaminant on animals (except parasites, species transported by host/vector)
  • Parasites on animals (including species transported by host and vector)
  • Contaminant on plants (except parasites, species transported by host/vector)
  • Parasites on plants (including species transported by host and vector)
  • Seed contaminant
  • Timber trade
  • Transportation of habitat material (soil, vegetation,…)

Questions and Answers received through the preliminary consultation:

1. For the column “Already present in EU?” – should this be EU or Europe including GB or not?

Europe-wide but exclude GB.

2. Should the arrival score be for GB or specifically Scotland?

The arrival score should be for Scotland specifically – as an example a species in captivity in a zoo in the south of England may not score 5 for arrival in Scotland.

Likelihood of arrival may differ between Scotland and GB for some pathways, e.g. travel from the Continent for angling and boating.

Remember: the scoring is purposefully crude to allow initial ranking of the species so experts should not be overly concerned – that said of course it is helpful to ensure the criteria are applied as consistently as possible across groups so when the lists from all groups are merged they look sensible! The first workshop will help though in providing an opportunity for moderation of scores by comparing species across groups.

1. How is establishment defined?

Self-sustaining populations of alien species are considered established. However, this can be difficult to determine definitely so we have discussed how to deal with species that do breed in Scotland but don’t have clear evidence of a self-sustaining population yet (e.g. ring-necked parakeet).There are also examples of long-lived species that cannot breed but have sustained populations because of their longevity. We agreed to include these species on the long initial list but move them onto a separate list of species that don’t fully meet the horizon scanning criteria but we have some concerns about. Examples include Onchorhynchus gorbuscha and Trachemys scripta.

In summary it is useful to capture the species that are potentially at early establishment phase within a separate table noting that the scope of the horizon scanning includes species that are currently absent from “the wild”. However, where people have a sufficient level of uncertainty about the establishment status then I suggest taking a precautionary approach and including them for now.

Likelihood of establishment may differ also, based on climate matching and other environmental factors.

For species that are currently climatically limited it is worth thinking about the next 10 years and whether the climate might become favourable in that time-frame.

With the aphids where we’re getting alate dispersal flights in more than one year, and often at more than one site, we can say that there’s an established population *somewhere* in Scotland, even if it’s not been identified on the ground. But if all the known host plants are horticultural (and it is therefore confined to gardens, and cannot escape to the wider environment) does that still count as established?

It is fascinating to consider Fiona’s example of a species that could only establish in gardens because that is where the host plant occurs. These species could of course have a potentially high economic impact and so I would include them in the “main” HS list if doubtful about current establishment or in a table of other species of interest (i.e. evidence of early establishment) with just a short comment on why they are not in the “main” HS list.

The Notes on Impact on the scoring sheet seem to limit economic impacts to those on crops and livestock, i.e. agricultural settings – should this be extended?

Yes - be quite broad here – we should extend the economic impact to other trade such as nursery trade – indeed any economic impact – perhaps people could then note the specific sector in the comments.


Email: invasive_non-native_species@gov.scot

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