Pressure from invasive species
Last updated: 2024
Latest data available: 2023
Introduction
Non-native species are those that have reached Great Britain by accidental human transport, deliberate human introduction, or which arrived by natural dispersal from a non-native population in Europe. Species that have arrived since 1500 are included within this indicator. Most non-native species are considered benign or positive, but some have a negative impact on native species through the spread of disease, competition for resources, or by direct consumption, parasitism or hybridisation; such species are termed invasive. Invasive non-native species have one or more of these negative impacts and a high capacity to spread to natural and semi-natural habitats. This indicator shows the change in number of invasive non-native species established across 10% or more of the land area of Great Britain, or along 10% or more of the extent of its coastline.
Data for this indicator can be found in the published datafile.
Type of indicator
Pressure indicator
Type of official statistics
Official statistic
Contents
- Assessment of change
-
Key results
- 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 2023
- 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 2023
- Further detail
- Acknowledgements
- Technical annex
- References
Assessment of change
Assessment of change in the cumulative net total number of invasive non-native species established in or along 10% or more of Great Britain’s land area or coastline
| Measure | Assessment | Time period | Result |
|---|---|---|---|
| Freshwater | Long term | 1969 to 2023 | Deteriorating |
| Marine (coastal) | Long term | 1969 to 2023 | Deteriorating |
| Terrestrial | Long term | 1969 to 2023 | Deteriorating |
Notes on the indicator assessment
Analysis of the underlying long-term trends is carried out by the data providers. See Assessing Indicators. This indicator shows a count of invasive species for each decade, unlike the majority of other indicators that report annually, therefore, it is only possible to assess long-term trends as there are insufficient data to establish trends over shorter time periods.
Key results
There are 3,343 non-native species in Great Britain, 2,074 of which are classified as established (reproducing in the wild). This indicator contains 195 non-native species that are considered to be invasive due to their negative impact on native biodiversity (47 freshwater species, 40 marine species and 108 terrestrial species).
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).
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 2023
Source: Botanical Society of Britain & Ireland; British Trust for Ornithology; UK Centre for Ecology & Hydrology; Marine Biological Association; National Biodiversity Network
Notes about Figure 1
- Each data point reported for this indicator represents the count of invasive non-native species for the preceding 10 years rather than annual data, i.e., the data point for 1969 relates to the count of invasive species present in the period 1960 to 1969.
- The 2023 data point covers a shorter time period (2020 – 2023) than the previous data points.
Invasive non-native species were categorised according to the extent of the land area or coastline of Great Britain in which they were found each decade (see Table 1). The categorisation was achieved by combining assessment of modelled distributions based (see technical annex for further information). Changes in extent levels within each habitat type are shown in Figure 2.
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 2023
Source: Botanical Society of Britain & Ireland; British Trust for Ornithology; UK Centre for Ecology & Hydrology; Marine Biological Association; National Biodiversity Network
Notes about Figures 2a-c
- Each data point reported for this indicator represents the count of invasive non-native species for the preceding 10 years, i.e., the data point for 1969 relates to the count of invasive species present in the period 1960 to 1969.
- The 2023 data point covers a shorter time period (2020 – 2023) than the previous data points.
Further detail
The indicator and background charts are 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).
The majority (189) of these species are established; six are long-term residents but not known to breed in the wild - two species of terrapin (Emys orbicularis and Trachemys scripta) and four freshwater fish (Ameiurus melas, Leuciscus idus, Salvelinus fontinalis and Oncorhynchus gorbuschas).
Table 1: Invasion extent of non-native species
| Definition | Interpretation | Extent |
|---|---|---|
| Not present in territory | Absent | 0 |
| Present in territory and either not established or with established populations that have not spread more than 10 km from their source | Not or scarcely established | 1 |
| Established populations represent less than 10% of territory, with some having arrived from further than 10 km from their source; or if more widespread then populations scattered and sparse | Established but still generally absent or at most occasional | 2 |
| Established populations present in 10% to 50% of the territory | Established and frequent in part of the territory | 3 |
| Established in more than 50% of the territory | Widespread | 4 |
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 UK Biodiversity Indicators have been reviewed in response to the latest goals and targets agreed under the Kunming-Montreal Global Biodiversity Framework (GBF) of the Convention on Biological Diversity (CBD). The indicators are being adapted, with some new indicators being developed, to better align them with the monitoring framework for the GBF. The suite of biodiversity indicators is therefore expected to change ready for forthcoming UK national reports to CBD in 2026 and 2029.
The intention is for this indicator to be modified to better align it with the requirements of the GBF monitoring framework, specifically to align with headline indicator 6.1 Rate of invasive alien species establishment, relevant to Target 6.
This indicator feeds into the Outcome Indicator Framework, 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
- 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
Acknowledgements
This indicator relies upon the opinions and assessments of taxonomic experts without whom it wouldn’t be possible to produce the indicator. The following lists the experts, and their affiliations, that have been involved in production of this indicator:
Kevin Walker and Tom Humphrey (Botanical Society of Britain & Ireland); Jack Sewell, Christine Wood, and John Bishop (Marine Biological Association); David Noble and John Marchant (British Trust for Ornithology); Alice Hiley (Environment Agency); John Murphy and Iwan Jones (Queen Mary University of London); Oliver Pescott and Ian Winfield (UK Centre for Ecology & Hydrology); Rob Britton (Bournemouth University)
We are also grateful to the national recording schemes and societies for regular updates on non-native species.
Technical annex
This technical annex has been adapted from the Technical Document written by: Colin A. Harrower, Stephanie L. Rorke, Helen E. Roy (UK Centre for Ecology & Hydrology) published alongside previous versions of this indicator.
Methodology
There are 195 invasive non-native species in Great Britain that are included within the indicator for the 2024 publication. These species have been assessed as extent levels 1-4 (five species were scored as extent level 0 for the 2020-2023 time period). The current indicator is the result of incremental updating of an indicator produced in 2014, which was created using a two-stage process beginning with extent estimation using a statistical process fitted to occurrence data available through the NBN Gateway which were then validated and modified, where required, by taxonomic experts. The occurrence data used in the statistical analyses conducted in 2014 were obtained from the National Biodiversity Network (NBN) Gateway with a minimum resolution of 10km, downloaded in October 2014. The extracted occurrence data were divided into 10-year time slices to enable the areas of extent to be calculated independently for each decade. The process used to calculate the area of extent was based upon the approach used to develop the England Vascular Plant Red List (Stroh et al., 2014).
An initial list was derived from the GB Non-native Species Information Portal (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 data tables for full list).
The rationale for using a statistical process to produce extent estimates was to attempt to control for the patchy nature of the occurrence data. The expert validation in 2014 determined that the extent estimates produced algorithmically typically underestimated the true extent due to much of the occurrence data not being easily available, particularly for earlier decades, and therefore most of the estimates required revision by the 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
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:
- The list of invasive non-native species has been derived through the rapid assessment of impacts based on expert opinion.
- 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, 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 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
The 2024 review saw one marine species: Mulinia lateralis added to the indicator species list, scored at extent category 1 for the current decade. Two other species from the marine environment saw increases in extent categories: Melanothamnus harveyi moving from 3 to 4 for both the current decade and 2010-2019 and Bispira polyomma from 2 to 3 for 2020. Three plant species saw increases from extent category 2 to 3 for the current decade: Acanthus mollis, Datura stramonium, and Spiraea alba x douglasii = S x billardii. One further plant score was increased from 3 to 4 for the current decade – Cotoneaster horizontalis.
Only one species score was decreased during this year’s review, this was for Lithobates catesbeianus which was reassessed as 0 from extent category one for the current decade due to the lack of confirmed sightings since 2018.
Development plan
Since our previous publication we have adapted the language and visualisations used in this indicator. We are keen to hear from our users about these changes, as well as our published development plan, please email us.
References
- Hill, M. O., Beckmann, B. C., Bishop, J. D. D., Fletcher, M. R., Lear, D. B., Marchant, J. H., Maskell, L. C., Noble, D. G., Rehfisch, M. M., Roy, H. E., Roy, S. & Sewell, J. (2009). Developing an indicator of the abundance, extent and impact of invasive non-native species. Final report. Defra.
- Roy, H. E., Preston, C. D., Harrower, C. A., Rorke, S. L., Noble, D., Sewell, J., Walker, K., Marchant, J., Seeley, B., Bishop, J., Jukes, A., Musgrove, A. & Pearman, D. (2014). GB Non-native Species Information Portal: documenting the arrival of non-native species in Britain. Biological Invasions, 16(12), 2495–2505.
- Burgman, M. A., & Fox, J. C. (2003). Bias in species range estimates from minimum convex polygons: implications for conservation and options for improved planning. Animal Conservation, 6(01), 19–28.
- Stroh, P. A., Leach, S. J., August, T. A., Walker, K. J., Pearman, D. A., Rumsey, F. J., Harrower, C. A., Fay, M. F., Martin, J. P., Pankhurst, T., Preston, C. D. & Taylor, I. (2014). A Vascular Plant Red List for England. Botanical Society of Britain and Ireland, Bristol.
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