Pressure from invasive species

Number of establishments

There are 3,343 non-native species reported in the Great Britain Non-native Species Information Portal (GB-NNSIP), 2,074 of which are classified as established (reproducing in the wild). However, only a relatively small number of these are considered to be invasive, due to their negative impacts on native biodiversity.

Since 1969, invasive non-native species have become more prevalent in freshwater, marine (coastal) and terrestrial environments, thereby increasing pressure on native biodiversity (Figure 1). As of 2024, there are 14 freshwater, 30 marine, and 64 terrestrial invasive species established in or along 10% of Great Britain’s land area or coastline.

Figure 1: Number of invasive non-native species established in or along 10% or more of Great Britain’s land area or coastline, 1969 to 2024

Figure 2: Changes in the extent (as listed in Table 1) of invasive non-native species in freshwater (A), marine (coastal) (B), and terrestrial (C) environments of Great Britain, 1969 to 2024

Rate of establishments

Modelled following the Solow and Costello (2004) method, the measure of rate of invasive alien species establishment shows an increasing trend in both the long term and short term (Figure 3); estimates increased from 1.59 in 1920 to 2.64 in 2022, with a total of 211 species establishing in that time. To account for lags (delays) in the detection and recording of new invasive alien species in the more recent years in the time series, the trend after 2000 is projected based on the data on establishments between 1920 and 2000.

Figure 3: Modelled rate of invasive alien species establishment, 1920 to 2022, with projection (hashed line) from 2000 onwards

Number of establishments

This measure is based on species distribution data available through the National Biodiversity Network (NBN), supplemented by expert knowledge and in house datasets of the Botanical Society of Britain and Ireland (BSBI), British Trust for Ornithology (BTO), UK Centre for Ecology & Hydrology (UKCEH), the Environment Agency (EA) and Marine Biological Association (MBA). The list in 2014 comprised of 179 species, but has been subsequently amended each year to comprise of 200 species (see technical annex for further information).

Table 1: Invasion extent of non-native species

Rate of establishments

The model shown in Figure 3 is based on real data on establishments of invasive alien species between 1920 and 2000, with a projected trend from 2000 onwards. This approach allows for a significant time lag between the establishment and observation of an invasive species (time lag model). For comparison, the same modelling approach has been applied to the same dataset including all observed data to 2022, to show how this compares to the projection from 2000 onwards. Figure 4 shows that these two models are very similar, with a potential small lag effect.

The same models were run on all alien species establishments (not just invasive alien species) in Great Britain (Figure 5) for comparison with the patterns shown for invasive alien species (Figure 4). The models for all alien species establishments suggest that there is a larger lag effect when considering this suite of species. When data since 2000 are included the trend line increases much less steeply (Figure 5), suggesting the Solow & Costello models may be sensitive to gaps in establishment data since 2000 and may not fully account for these time lags. Excluding data since 2000 is an attempt to avoid bias from incomplete data in this period.

Figure 4: Rate of invasive alien species establishment, 1920 to 2022

Figure 5: Rate of alien species establishment, 1920 to 2022

Relevance

The United Nations Convention on Biological Diversity (CBD) identifies invasive non-native species as a major threat to biodiversity because they can spread disease, hybridise with native species, modify ecosystems and drastically reduce populations of native species.

International/domestic reporting

The suite of UK Biodiversity Indicators has been revised and updated to bring it in line with the Kunming-Montreal Global Biodiversity Framework (GBF) of the Convention on Biological Diversity (CBD). Some UKBIs will be used for the forthcoming UK national reports to CBD. This indicator links to Target 6 in the Global Biodiversity Framework, which aims to reduce the introduction of invasive alien species by 50% and minimise their impact.

This indicator also feeds into the Outcome Indicator Framework (OIF), a set of indicators describing environmental change related to the ten goals within the 25 year Environment Plan. As part of the Outcome Indicator Framework, this data contributes towards the evidence base used to prepare the annual progress report for the Environmental Improvement Plan. This indicator contributes to OIF indicator H1: Abatement of the number of invasive non-native species entering and establishing against a baseline.

Web links for further information

• Metadata factsheet for GBF Headline Indicator 6.1
• Developing an indicator of the abundance, extent and impact of invasive non-native species
• DAISIE - Inventory of alien invasive species in Europe
• The Invasive Alien Species (Enforcement and Permitting) Order 2019
• NBN Atlas
• GB Non-native Species Secretariat
• The Great Britain invasive non-native species strategy

Number of establishments

Methodology

There are 200 invasive non-native species in Great Britain that are included within the indicator for the 2025 publication. These species have been assessed as extent levels 1-4 (five species were scored as extent level 0 for the 2020-2024 time period). The current indicator is the result of incremental updates to an indicator originally produced in 2014, which was developed using a two-stage process. This process began with extent estimation using a statistical method fitted to occurrence data available through the National Biodiversity Network (NBN Gateway), which were then validated and modified, where necessary, by taxonomic experts. The occurrence data used in the 2014 statistical analyses were obtained from the NBN Gateway at a minimum resolution of 10km and downloaded in October 2014. These data were divided into 10-year time slices to enable independent calculation extent for each decade. The method used to calculate area of extent was based on the approach developed the England Vascular Plant Red List (Stroh et al., 2014).

An initial list was derived from the GB-NNSIP (Roy et al., 2014) by selecting all non-native species within the database that are noted to have, or potentially have, a negative or strongly negative ecological effect. This list was subsequently reviewed by experts (see published datafile for full list).

The rationale for using a statistical method to produce extent estimates was to address the patchy nature of the occurrence data. Expert validation in 2014 found that the algorithmically produced extent estimates typically underestimated the true extent, largely because much of the occurrence data, particularly for earlier decades, was not readily available. As a result, most of the estimates required revision by taxonomic experts.

Since 2015, the species list and the classification of extent derived for the 2014 indicator have been reviewed and updated by taxonomic experts. Given the paucity of available occurrence data these subsequent updates of the indicator omitted the statistical estimation and the review undertaken was based on expert assessment alone.

Caveats and limitations

There are 3,343 species recorded in the GB-NNSIP; there are likely to be many more alien species in Great Britain that have not yet been recorded.

The indicator is compiled from those invasive non-native species established in or along 10% or more of Great Britain’s land area or coastline (that is, extent categories three and four). There are limitations to this approach:

• Data collection to inform the method is ongoing but due to time lags in both establishment and observation of non-native species, the list of invasive non-native species has been derived through expert assessment of observed impacts.

• The extent value is based on relatively broad categories. The extent of some species can increase multi-fold within a single category; for example, the number of invasive non-native species in 10% to 50% of the land area of Great Britain can contain a wide range of values, which can reduce the sensitivity of the indicator.

• The occurrence data obtained from the NBN may not be representative of the species distribution in each decade, especially for both the earlier and most recent time periods, because observation and detection methods have improved but there is often a time lag before occurrence data appear on the NBN. Furthermore, the availability of occurrence data reflects the intensity of survey effort applied in a time period that has subsequently been submitted to the NBN. The attribution of extent categories has, however, been supplemented by expert opinion and in some cases by more complete datasets.

Recent developments

Review of the species lists and associated extent category scores in 2025 resulted in the addition of five species. Two freshwater fish species were added for the current decade: Micropterus salmoides (extent level 1) and Rhodeus amarus (level 2). Three terrestrial invertebrate species were also added: an ant, Tapinoma pygmaeum (level 1) and a flatworm, Obama nungara, were added for 2010 and the current decade. Another flatworm species, Caenoplana variegata, was included for the past three decades: 2000 and 2010 at level 1, increasing to level 2 for the current decade.

The extent category scores of all the current species on the list were also reviewed but no changes were considered necessary this year.

Rate of establishments

Methodology

A major challenge in understanding trends in invasive species establishment is the time lag between the actual date of establishment in the wild and their discovery (Buba et al. 2024). There is frequently an additional lag between discovery and the retrospective entry of first records and establishment status into the GB-NNSIP database (see the addition of new species in Recent Developments above). These lags can be attributed to several factors:

• Some non-native species are more difficult to detect than others and this will contribute to delays in recording.
• In some cases, those wishing to publish first records of new arrivals will wait until after publication of a paper before sharing details of the new species, although this is increasingly not the case.
• Lack of awareness of the reporting route and requirements.
• Delays can also occur due to unresolved taxonomy.

This section of the indicator uses a different time series than the one used for the pressure indicator (1960-2024). The trend presented here was modelled using data from 1920 to 2000, thereby excluding the most recent 22 years of available data. This approach accounts for lags in detection and recording which can lead to underestimates in the number of establishments, particularly in the most recent years of a time series. It generates a more accurate trend by incorporating historical data that is less affected by detection delays, and by down-weighting more recent data, where establishments may be under-reported. The establishment trends presented here were modelled using the recommended Solow and Costello (2004) method, as implemented in the R package ‘alien’ (Buba et al. 2024), which assumes increasing observation effort with time.

Three invasive species are included here as long term residents, while noting that there is not yet sufficient evidence that they are fully established; these are the pond slider (Trachemys scripta), the European pond turtle (Emys orbicularis), and the pink salmon (Oncorhynchus gorbuscha).

Caveats and limitations

The measure of the rate of invasive alien species establishment is modelled based on a number of assumptions (as outlined in Methodology above) and represents the estimated rate of establishment.

Development plan

The measure on rate of invasive alien species establishment is a new aspect of this indicator. We plan to review this measure during the coming years. We would like to assess how representative it is of the UK overall and whether a future iteration could have UK coverage. In addition, the use of data describing trends in UK trade and transport networks and biological recording has potential to improve the accuracy of the indicator. Models using these additional data tended to forecast trends more accurately, but these models will require further development.

We are keen to hear from our users about these plans, the language and visualisations used in this indicator, as well as our published development plan for the biodiversity indicators: please email us.