The study was conducted at the regional scale in 152 recording units, known as ‘Watsonian vice-counties’, across the whole of Britain and Ireland (Fig. 1). These vice-counties (VC) were devised in 1852 with the aim of creating a series of stable geographical units for recording natural history observations and continue to be used to this day by the Botanical Society of Britain and Ireland (BSBI) and other societies (Preston et al. 2002). These vice-counties have an average size of about 2000 km², with a range of 207 to 4857 km² (Fig. 1).

Figure 1. 

Map showing vice-county boundaries in Britain and Ireland. Vice-counties participating in the study are coloured in a grey-scale representing the average impact score of the species reported.

The BSBI is the backbone of botanical recording in Britain and Ireland (Pescott et al. 2015) and appoints volunteer vice-county recorders to both collect and collate botanical records for their areas and to submit them to the BSBI centrally (https://database.bsbi.org/maps/). Vice-county recorders, therefore, have an unrivalled knowledge of the plant species, both native and non-native, that occur at a local level, including their distribution, abundance, habitats and regeneration, while also being aware of impacts on other plant species and ecosystem processes. Vice-county recorders existed long before the term ‘citizen science’ was used in Britain; however, because of their voluntary role, they would now also be considered citizen scientists (Pescott et al. 2015). Considering their expertise and their focused recording efforts (systematic recording of all plant species in contrast to unstructured or opportunistic recording) and following the classification of citizen scientists suggested by Pocock et al. (2015), these vice-county recorders would be classified as volunteer regional experts.

An online questionnaire was designed using the ‘Online surveys’ platform (www.onlinesurveys.ac.uk). The survey link was distributed to all BSBI vice-county recorders in March 2019 and the survey was closed at the end of May 2019. The questionnaire (see Suppl. material 1, Part 1) consisted of a short introduction, followed by an explanation of the criteria for scoring species’ impacts and the respondents’ confidence (Table 1). Participants were asked to name the 10 plant species that they considered were having the highest environmental impacts in their vice-counties, based on the modified EICAT scoring criteria (Table 1). Respondents were also asked to name the species with the highest impacts first, thus creating a list ranked by their perception of local impact. Each species’ impact score was assigned to a broad habitat type from a list provided, corresponding to the UK Biodiversity Action Plan habitat descriptions (http://jncc.defra.gov.uk/page-5706). In the second part of the survey, participants based in Great Britain were presented with the list of the top 10 highest ranked plant species from the 2016 GB national-level impact workshop (Booy 2019) and asked to score the impacts of these species in their vice-county using the same criteria, but only if they had not already included them in their top 10 scoring list in the previous section of the survey. Vice-county recorders from Ireland did not participate in this part of the survey, because the 2016 impact scoring was conducted for Great Britain only. In both sections, respondents could leave comments for individual species and were asked to do so, in particular, if they were aware of impacts in habitats considered as priority in the UK Biodiversity Action Plan or invasives that were known to have caused the local extinction of any native species. The questionnaire was approved through Coventry University’s ethical approval procedure (Ethic no: P87103).

We used the modified scoring categories that were applied in the 2016 national GB assessment (Booy 2019), based on the EICAT methodology (Hawkins et al. 2015). Impacts were scored, based on the five categories namely ‘massive’,’ major’, ‘moderate’, ‘minor’ and ‘minimal’ concern (Table 1). The modification of the EICAT categories in the GB assessment involved a change in the definition of the ‘major’ category, which was changed to specify that a species extinction (national or local) or a substantial, but reversible change in affected ecosystems needs to be observed, in contrast to the EICAT scheme, where both conditions need to be fulfilled (extinction and reversible change). For each species’ score, respondents were also asked to score their confidence following the definitions given in the survey (Table 1) and based on the GB assessment’s definition. In contrast to the EICAT framework and the GB assessment, which distinguished current and maximum impacts that could be reached if a species would occupy all suitable habitats, respondents in our survey were only asked to score the current impacts.

To identify the highest scoring species and to compare impact scores between species and vice-county recorders, we assigned each impact category an integer value, ranging from 1 for the ‘minimal’ category to 5 for the ‘massive’ category. We also compared the rank order in which respondents had reported the 10 species for their vice-county as they were asked to report species in the order of perceived impact magnitude from highest to lowest.

We derived the list of highest scoring species for the whole region using three complementary approaches. First, we ranked species by the number of respondents who had included them in their list of 10. Second, we calculated an average impact score for each species, both for the whole sample (with an impact score of zero for vice-counties in which the species was not included in the top 10), as well as the average for only those vice-counties that included the species in their top list. Third, we added the rank scores for each species (where listed by a respondent) and ranked the overall species list by these sums. To analyse the consistency in the scoring, we followed the methodology used by Gonzáles-Moreno et al. (2019) and calculated a coefficient of variation (CV) for all species that had received at least four scores. We also used the CV to compare the variation in scoring between respondents (i.e. how they used the scale of impact categories across the 10 species).

Species’ current distribution data were extracted from the BSBI’s Distribution Database (https://database.bsbi.org/) at a 2 × 2 km gridded resolution in March 2022. As the size of the vice-counties varies, we used the percentage of total 2 × 2 km grid cells occupied per vice-county. For nine reports of species with no records in particular counties, we used an arbitrary value of one grid cell. Nomenclature and common names for vascular plants followed Stace (2019), while species’ native or non-native status was based on Preston et al. (2002). In cases where species that are native in some parts of Britain or Ireland were reported, we confirmed the status in the vice-county from where it was reported as non-native. Data on the Raunkiaer lifeform of the reported species was derived from ALIENATT, an unpublished BSBI compilation which is also the basis for this information in Henniges et al. (2022). A list of the 1,690 non-native plants confirmed in Britain and Ireland was extracted from Henniges et al. (2022). This list was used to compare the lifeform of plants (chamaephyte, geophyte, hemcryptophyte, helophyte, hydrophyte, phanerophyte, therophyte) reported in the survey using a two-sample z-test of proportions and post hoc row-wise Fisher tests with p-values adjusted for pairwise comparisons between proportions of single categories. Species reported in the survey that are native in parts of Britain and Ireland and non-native in others were excluded from this comparison (i.e. Clematis vitalba, Fagus sylvatica, Nymphoides peltata, Papaver cambricum, Pinus sylvestris, Ranunculus subgenus Batrachium, Stratiotes aloides, Symphytum tuberosum and Typha latifolia).

Where respondents had not assigned a main broad habitat type, we used the comments provided by them to assign a main habitat. Out of 75 species (9.6% of all species reported) with no habitat assignment, we allocated 61 species to habitats including the two additional habitat categories ‘urban’ and ‘brownfield’, that were not included as an option in the survey design. The remaining 14 species which could not be allocated or where the respondent indicated the species could not be assigned to a main habitat type, were categorised as ‘other’. The International Union for the Conservation of Nature’s (IUCN) Red List status of species reported to be threatened by invasive plants was checked for records in GB and Ireland separately in the respective lists as compiled in Wyse Jackson et al. (2016). We conducted all analyses in the R environment, version 4.0.3 (R Core Team 2019), including the bipartite (Dormann et al. 2008) and vegan packages (Oksanen et al. 2019).

We received responses from 86 vice-counties (a return rate of 57%), reporting a total of 123 species from 782 vice-county observations. While most recorders reported 10 species, some reported fewer and, for one vice-county, eleven species were named, giving a range of one to eleven species and an average of 9.1 (s.d. = 1.96) per vice-county. There were more than 100 observations of ‘massive’ impact species; however, the majority of reports related to evaluations of ‘major’ or ‘moderate’ impacts (Fig. 2).

Figure 2. 

Frequency of impact and confidence scores received from vice-county recorders. The number of observations relates to 123 different species reported.

The mean coefficient of variation (CV) of impact scores per species named by at least four recorders was 32% (s.d. = 9), with a range from 17% for Prunus laurocerasus from 20 reports to 51% for Gunnera tinctoria from seven reports. The CV across all recorders was 26%, based on the average score given to all species scored per vice-county of 3.3 (s.d. = 0.85). The average scores per vice-county can be seen in Fig. 1. Impatiens glandulifera, Reynoutria japonica and Rhododendron ponticum were the three species considered to have the highest impacts across all vice-counties by frequency, mean overall impact score and ranking, followed by Crassula helmsii, Heracleum mantegazzianum and Picea sitchensis in exactly the same positions by each measure (Table 2).

In comparison to the GB assessment, we found that four of the top 10 species from that assessment were also in our top 10 list (Table 3) and a further three included in our list of species reported from more than 10 vice-counties (Table 2). Just one species, R. japonica, was present in all British vice-counties; however, only 68% of respondents included the species in their county’s top 10. Similar high presences in the top 10 list of vice-counties were only achieved by R. ponticum and C. helmsii (Table 2). Species where impacts are mainly recorded in coastal habitats, i.e. Carpobrotus edulis, Cotoneaster integrifolius and Spartina anglica, were only in the top 10 of a few vice-counties as these habitats are not available in inland areas. The top 10 GB list included one moss, Campylopus introflexus; however, the BSBI and their recorders do not record bryophytes and this was confirmed by 20 of the respondents who did not comment on the species’ presence in their county, with many of them indicating that moss species are outside of their remit.

Regions differed in the number of species reported (Suppl. material 1: Table S1). Looking at the frequency of individual species reported, for each region there were species in the list of species more frequently reported that were not in the overall high frequency list (Table 2). For Ireland, Allium triquetrum (5 counties) and Pinus contorta (5) were in the top 10 list of species, Claytonia sibirica (3) and Tolmiea menziesii (3) were in the top 10 list of species in the Scottish Highlands and Islands, with T. menziesii (5) also frequent in the Scottish Lowlands. C. edulis was in the top list of two of the five vice-counties participating in the region of Southwest England.

Most observations of species with impacts were related to phanerophytes, with more than half of these impacts considered to be ‘massive’ or ‘major’ (Fig. 3A). There was a significant difference (probability test of proportion: chi-squared = 51.5, d.f. = 5, p-value < 0.001) in the lifeform spectrum (Fig. 3B) of the species reported in the survey (excluding species with native occurrences in some parts of the study area) to all non-native plants included in the species list of the British and Irish flora (Henniges et al. 2022). The post hoc test of pairwise comparisons between proportions of lifeform categories found significant differences for the combined category of helophytes and hydrophytes (p < 0.001), which contribute 1.8% of the non-native flora of the British Isles, but 10.6% of the species reported to have impacts. Focusing more specifically on aquatic plants, all but three of the 14 non-native species recorded in the flora of Britain and Ireland were reported amongst the non-native species with impacts. Therophytes were the second group with significant differences in the post hoc test (p < 0.001) contributing 28% of all non-natives, but just 10% in our study (for all pairwise post-hoc comparisons, see Suppl. material 1: Table S2).

Figure 3. 

A number of observations and impact categories of 113 higher non-native plants reported in the survey and B lifeform comparison of the non-native plants with reported impacts to all 1,690 non-native plants recorded in the British and Irish flora (Henniges et al. 2022). Bars show the percentage of each sample for each lifeform, labels the number of species in each category (data for the survey in B exclude nine species that are native in some parts of the study area). Raunkiaer plant lifeforms: chamaephyte (Ch), geophyte (G), hemcryptophyte (Hc), helophyte (Hel), hydrophyte (Hy), phanerophyte (Ph), therophyte (Th).

Respondents reported the main broad habitat affected for each species when scoring the species’ impacts. The highest number of species was reported for broadleaved/mixed woodlands, followed by boundary/linear features, rivers and streams and standing open waters/canals (Suppl. material 1: Table S3). Looking more closely at which species are reported from which habitats (bipartite graph Suppl. material 3: Fig. S1), we find some species predominantly reported from single broad habitat types, for example, P. laurocerasus, Rubus spectabilis and Lamiastrum galeobdolon subsp. argentatum for broadleaved/mixed woodlands or I. glandulifera for rivers and streams. In contrast, other species are reported to have impacts in several habitats, most notably R. japonica, which was found along rivers and streams, broadleaved/mixed woodlands, urban/brownfield, inland rock, littoral habitats and boundary/linear feature habitats. In boundary/linear feature habitats, we found the greatest mixture of species that are also found in other habitat types, reflecting the diverse range of habitat conditions in this category (Suppl. material 3: Fig. S1).

Additional comments provided by respondents gave further insight into individual cases of habitat impacts. For example, one respondent reported the massive impact of Quercus ilex on calcareous grassland that had resulted in the “complete loss of chalk grassland”. A respondent from Ireland reported R. spectabilis with massive impacts in broadleaved woodlands commenting “by far the single biggest threat to what remains of our broadleaved woodlands. Have seen woodland where this is the only ground-level species”. Further priority habitats mentioned specifically in comments include oligotrophic lakes, limestone pavements, hedgerows, oak woodlands, blanket bogs and coastal sand dunes.

Respondents also named 39 native plant species that were particularly negatively affected by the invasive plant they listed, including several species listed in the vascular plant Red Lists for Britain and Ireland, respectively (Table 4). Three species are classified as ‘Vulnerable’: (1) the liverwort Southbya nigrella, which reaches its northern distributional range on calcareous cliffs on the south coast of England with just 20 known sites on the Isle of Portland and one on the Isle of Wight (Blockeel et al. 2014), from where the local vice-county recorder reported Buddleja davidii having caused the “virtual loss” of the species. Similarly, the occurrence of (2) Hydrocharis morsus-ranae was the only known native site in a north-eastern English vice-county and was considered by the respondent to be “possibly shaded out” by Lemna minuta (with low confidence score). Lastly, (3) Heracleum mantegazzianum was considered responsible for the extinction of a colony of Allium oleraceum in the Scottish Lowlands. A further four species are of ‘Near Threatened’ (NT) status, including reports for Pilularia globulifera as threatened by Crassula helmsii from three vice-counties. Pilularia globulifera has also NT status in the European and global Red Lists of plants (Wyse Jackson et al. 2016). Cotoneaster integrifolius agg. was the invasive species most often associated with the direct mentioning of impacts on native species (8 cases), with reports from five vice-counties in southern England, Wales and Ireland, where it was reported to threaten the NT listed species Gentiana verna and Neotinea maculata. A further European NT species is Najas flexilis, for which Elodea canadensis was reported to be “Likely responsible for local population extinctions of Najas flexilis, Potamogeton rutilus (but hard to survey these and be sure)”.Species scoring with higher impacts had generally a higher distribution in the vice-county where they were scored than species with lower distributions (Fig. 4), (ANOVA, F(4)= 7.90, p < 0.001). Furthermore, across all vice-counties, species that were included in a county’s top 10 list were recorded from an average of 15.5% of the county’s total number of grid cells, whereas in vice-counties where the species were not included in the top list, they were recorded on the average of 10.1% grid cells (two sample t-test, t = -7.96, p < 0.001).

Figure 4. 

Distribution of reported invasive plants in vice-counties by impact category. The 2 km-scale range size of species is shown as the percentage of the total number of 2 km grid cells per vice-county.

The aim of our study was not to conduct an impact assessment following the full EICAT framework process. Our study was based on local volunteer experts reporting non-native species relying on their observations of the impacts of species in areas they are familiar with. In contrast, the EICAT approach is based on experts assessing published evidence on species’ impacts including impact mechanisms (Hawkins et al. 2015). Our respondents’ scoring of impacts was based on their observations and individual perceptions and lacks the rigour underlying published evidence. However, both approaches connect through using the EICAT assessment categories and criteria. The value of our approach lies in that it taps into a pool of knowledge that would often not make it into the published evidence assessed in the EICAT process. Species with impacts may thus be recognised earlier, because it is usually local experts that will notice changes in their environment first. Many of these records may take a long time from there to a professional investigating and publishing these impacts or may not be reported at all. This is illustrated by the number of species reported in our study that have not been considered in the GB national assessment. Studies such as ours could, therefore, be useful to feed into full EICAT assessments at national levels, not just for plants, but also for other species groups where local experts are involved in recording and monitoring. Future studies could also benefit by including a question on impact mechanisms as in the EICAT assessment, although there is always a risk that, if responding to a survey takes too much time, the response rate is likely to decrease.

Our results showed that the EICAT impact categories could be well communicated to local experts and were well-suited to be used by them. We, therefore, argue that our results are, to some extent, comparable to impact assessments using the same categories and criteria and we have confirmed this by comparing our results to the GB national level expert assessment. Ranking of species in the order they were submitted (highest impact species first) produced very similar results to the scoring using the assessment criteria. We would, nevertheless, not conclude that similar studies should just ask for a ranked list of species without providing impact definitions, as these will have guided respondents in their choice of species. However, the ranking is useful considering different interpretation of the criteria in the assessment scheme, as well as in the perceptions of impacts by individual respondents.

We achieved a remarkably similar list to the species list in the expert assessment at GB national level. Species absences in our top list are explained by the fact that coastal habitats are not present in many areas from which we received responses (in the case of Spartina anglica) or because they were outside the taxonomic scope of vice-county recorders (e.g. Campylopus introflexus was outside the scope of most recorders). The study also identified species that were not included in the top list of species in the national impact scoring scheme, for example, Picea sitchensis, Lamiastrum galeobdolon subsp. argentatum and Prunus laurocerasus all ranked highly in our study, but did not achieve higher scores in the GB scoring, with P. laurocerasus ranked ‘moderate’ and the other two species ‘minor’. However, the GB national assessment also asked reviewers to score the potential maximum impact (an option which was not offered in our project), defined as the impact a species “would be expected to have in GB if it were established in all parts that are suitable (i.e. based on current biotic and abiotic conditions)” (Booy 2019) and all three species were scored ‘major’. This result could indicate that the evidence used in the GB scoring was out-of-date and the species’ more extensive impacts have not been documented so far or accessible to the national evaluators, who, nevertheless, realised the potential for higher impacts of these species in their potential maximum impact score.

For Ireland, the survey reported 12 species on the list of 16 high impact species identified by Kelly et al. (2013), based on risk assessments and nine of the species listed by Gioria et al (2018), based on a literature review of terrestrial plants. Species included in these two sources, but not reported in our survey for Ireland, include Reynoutria sachalinensis, Hydrocotyle ranunculoides, Carpobrotus edulis and Crassula helmsii. Species reported in our survey with ‘massive’ impacts, but missing in Kelly et al. (2013) are Picea sitchensis, Rubus spectabilis and Cotoneaster integrifolius, with the latter two being also identified by Gioria et al. (2018). Furthermore, two species included in the top ten list of species reported from Ireland in our survey, Allium triquetrum and Pinus contorta, are not included in either of these two references.

We found scoring consistency differed for individual species between vice-county recorders. This can be attributed to differences inherent to individual respondents and how they interpreted and applied the assessment criteria. Vice-county recorder may be in their roles for various lengths of time and may have different levels of commitment and experience, which could influence their scoring. Similar, their perception of the impacts of species in their vice-counties and of invasive species, in general, could influence how they scored, with underlying factors not studied in our analysis informing individual people’s perceptions (Shackleton et al. 2019). However, it is also likely that these differences can, to some extent, be explained by different introduction and spread histories of the species and/or the availability of habitats in different vice-counties. González-Moreno et al. (2019) investigated patterns of consistencies in scoring between different assessors for several invasive species and impact assessment schemes including EICAT and found, on average, a coefficient of variation (CV) of 40%. In comparison, the 32% variation for the impact score of invasive plants in our study is lower, which could be the result of higher numbers of scores for most species (i.e. between 4 to 63, compared to 5 to 8 in Gonzáles-Moreno et al. 2019), but it is also reassuring to see a comparable level of agreement at the species level, despite the fact that respondents reported from different local areas. However, we also found clear differences in scoring between different respondents, ranging from one respondent reporting 10 species all with ‘massive’ impacts for their vice-county to another one who had scored just one species (Allium paradoxum) as ‘moderate’, with the remaining nine all as either ‘minor’ (3 species) or ‘minimal’ (6). Overall, variation between recorders was 25% (CV).

While most records of non-native plants in Europe and the British Isles are from human-made habitats (i.e. industrial habitats and arable land, parks and gardens) (Lambdon et al. 2008; Stace and Crawley 2015), impacts of species in our survey were most frequently associated with woodlands. This seems reasonable, given that we asked respondents to consider environmental impacts only, which will have excluded many impacts of invasive species in human-made habitats. Linear features and boundaries were the second most mentioned habitat type; however, this habitat is more difficult to interpret as it encompasses a wide range of different linearly-arranged landscapes elements according to the UK broad habitat classification, ranging from hedgerows, tree lines and walls to dry ditches (Jackson 2000). Nevertheless, linear features are recognised to play an important role in supporting biodiversity and ecosystem services (Holland et al. 2016; Phillips et al. 2020) and the high number of impacting non-native species are a reason for concern. Equally concerning are the high number of species reported from freshwater and wetland habitats combined with the fact that 11 of the 14 aquatic species in the non-native flora of the British Isles have been reported. Urban and brownfield sites, many of which are also well known to support biodiversity (Harrison and Davies 2002; Venn et al. 2013), feature less species, but this could also be the result of these habitat types not specifically mentioned in the UK broad habitat type categorisation and, therefore, not included in our survey design.