Official Statistics

Butterflies in the United Kingdom: habitat specialists and species of the wider countryside, 1976 to 2022

Updated 2 February 2024

1.Contact

Enquires on this publication to:

enviro.statistics@defra.gov.uk

Tel: 03459 335577 (Defra enquiries) Find out more about call charges at – GOV.UK (www.gov.uk)

Lead statistician: Clare Betts

Environmental Statistics and Reporting team,
Department for Environment, Food and Rural Affairs,
Mallard House,
Kings Pool,
3 Peasholme Green,
York,
YO1 7PX

Website: Biodiversity and wildlife statistics – Gov.UK

2. Summary

This release covers 2 measures of annual butterfly population abundance in the UK: the first for habitat specialist butterflies (species strongly associated with semi-natural habitats such as chalk downland) and the second for more widespread butterflies found in both semi-natural habitats and the wider countryside. Both indices contain data back to 1976 which was when the monitoring scheme started. These statistics contribute to the Biodiversity Indicators, a suite of indicators due to be updated in on 14 November 2023.

The release also includes 3 additional measures (a UK ‘all-species’ index and 2 habitat- specific measures – one on farmland and one in woodland) that do not form part of the formal assessment. The 2 habitat-specific measures only cover data from 1990 because data before that year are not sufficient at this scale to be considered representative for inclusion.

Together, these 5 composite indices provide a comprehensive view of how populations of butterflies in the UK are changing over time. It should be noted however that the overall trends identified by the measures and the performance of individual species within them can vary between the 4 countries of the UK.

These composite indices are sourced by the UK Butterfly Monitoring Scheme (UKBMS) and considered representative of butterfly trends across the UK. Recognising that monitoring coverage was biased towards good quality habitats, the UKBMS partly addressed this issue by introducing the Wider Countryside Butterfly Survey (WCBS), and these data are used alongside the UKBMS in the calculation of these indicators.

Key messages:

• Large fluctuations in numbers between years are a typical feature of butterfly populations, principally in response to weather conditions. Therefore, the statistical assessment of change is made on an analysis of the underlying smoothed, rather than unsmoothed, trends.
• Overall, 2022 was an average year for butterflies in the UK, with 47% of all species contributing to the UK indicators (24 out of 51) falling in abundance from the previous year. 2022 was the warmest year on record in the UK, although the weather fluctuated considerably each month with cooler and unsettled periods alternating with warmer, more settled spells. The rainfall was mostly below average with drought conditions in second half of the summer, especially in eastern parts of England.
• The long-term smoothed trend for habitat specialist butterflies in the UK has declined significantly since 1976, whereas the long-term smoothed trend for butterflies of the wider countryside shows no significant change.
• Assessments of the short-term smoothed trends show no significant change for either habitat specialists or species of the wider countryside in the UK since 2017.

3. Habitat specialists

Habitat specialist butterflies, which are vulnerable to semi-natural habitat loss and fragmentation, have not recovered from declines experienced in the late 1970s. These declines were mainly attributed to the knock-on effects of the drought conditions experienced in 1976.

Analysis of the smoothed trend for habitat specialist butterflies shows a significant reduction in relative abundance over this long-term period driven by decreases in 1976 to 1979. There has been no change in the abundance of habitat specialist butterflies since 2017.

Individual butterfly species fare differently within the overall trend. Habitat specialists showing the greatest long-term decline since 1976 include heath fritillary; wood white; Lulworth skipper; grayling; small pearl-bordered fritillary; pearl-bordered fritillary; high brown fritillary; white admiral; and northern brown argus. One species, silver-washed fritillary showed a significant short-term decline since 2017. Habitat specialists showing the largest long-term increases include silver-spotted skipper; black hairstreak; large heath; dark-green fritillary; and silver-washed fritillary, and of these, dark-green fritillary also showed a significant increase since 2017. Both chalk-hill blue and dingy skipper increased over the short term but showed no long-term change. Lulworth skipper increased over the short term but showed a decrease over the long term.

Figure 3.1: Trends for the abundance of habitat specialist butterflies in the UK, 1976 to 2022

Source: Butterfly Conservation, UK Centre for Ecology and Hydrology, British Trust for Ornithology, Defra, Joint Nature Conservation Committee.

View the data for Figure 3.1

Download the data for Figure 3.1 in csv format

Figure 3.2: Long-term and short-term changes in individual species trends for habitat specialist butterflies in the UK, 1976 to 2022

Source: Butterfly Conservation, UK Centre for Ecology and Hydrology, British Trust for Ornithology, Defra, Joint Nature Conservation Committee.

View the data for Figure 3.2

Download the data for Figure 3.2 in csv format

Notes for Figures 3.1 and 3.2:

• This indicator includes individual measures for 22 species of butterflies, the England farmland butterfly index, however, only includes 21 trends. This is because an aggregate trend is used for small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris); these 2 species have been combined due to historical difficulties with distinguishing between them in the field.
• Figure 3.2 shows the percentage of species within the indicator that have shown a statistically significant increase, a statistically significant decrease or no statistically significant change (little or no change) over the long-term and short-term assessment periods.

4. Species of the wider countryside

The butterflies of the wider countryside index for the UK has not changed in either the long-term (1976 to 2022) or the short-term (2017 to 2022) (Figure 4.1).

Individual butterfly species again fare differently within the overall trend. Species of the wider countryside showing the greatest long-term declines include wall; small tortoiseshell; and white-letter hairstreak. No wider countryside butterfly species have declined in the short term. The ringlet; comma; speckled wood; and marbled white show the largest increases over the long term, but no species of the wider countryside butterflies have increased in abundance in the short term.

Figure 4.1: Trends for the abundance of butterflies of the wider countryside in the UK, 1976 to 2022

Source: Butterfly Conservation, UK Centre for Ecology and Hydrology, British Trust for Ornithology, Defra, Joint Nature Conservation Committee.

View the data for Figure 4.1

Download the data for Figure 4.1 in csv format

Figure 4.2: Long-term and short-term changes in individual species trends for butterflies of the wider countryside in the UK, 1976 to 2022

Source: Butterfly Conservation, UK Centre for Ecology and Hydrology, British Trust for Ornithology, Defra, Joint Nature Conservation Committee.

View the data for Figure 4.2

Download the data for Figure 4.2 in csv format

Notes for Figure 4.1 and 4.2:

• This indicator includes individual measures for 24 species of butterflies, the England woodland butterfly index, however, only includes 23 trends. This is because an aggregate trend is used for small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris); these 2 species have been combined due to historical difficulties with distinguishing between them in the field.
• Figure 4.2 shows the percentage of species within the indicator that have shown a statistically significant increase, a statistically significant decrease or no statistically significant change (little or no change) over the long-term and short-term assessment periods.

5. Additional wider countryside indices

Butterflies of the wider countryside on farmland

Analysis of the smoothed trend for butterflies of the wider countryside on farmland over the long-term period (1990 – 2022) shows a slight decline. There has been no change in the abundance of butterflies of the wider countryside on farmland since 2017 (Figure 5.1).

Individual butterfly species fare differently within the overall stable long-term trend. Species showing the largest significant long-term decline on farmland include small tortoiseshell; wall; Scotch Argus; and gatekeeper. One species, the large skipper showed a significant decrease on farmland over the short term (since 2017). Six species, white-letter hairstreak; the ringlet; brimstone; speckled wood; marbled white; and orange-tip increased on farmland over the long term. Only one species, the large skipper, has decreased over the short term since 2017, while the remaining species have not changed.

Figure 5.1: Trends for the abundance of butterflies of the wider countryside on UK farmland, 1990 to 2022

Source: Butterfly Conservation, UK Centre for Ecology and Hydrology, British Trust for Ornithology, Defra, Joint Nature Conservation Committee.

View the data for Figure 5.1

Download the data for Figure 5.1 in csv format

Figure 5.2: Long-term and short-term changes in individual species trends for all species of butterflies resident in England, 1976 to 2022

Source: Butterfly Conservation, UK Centre for Ecology and Hydrology, British Trust for Ornithology, Defra, Joint Nature Conservation Committee.

View the data for Figure 5.2

Download the data for Figure 5.2 in csv format

Notes for Figures 5.1 and 5.2:

• This indicator includes individual measures for 50 species of butterflies; the England ’all-species butterflies index, however, only includes 49 trends. This is because an aggregate trend is used for small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris); these 2 species have been combined due to historical difficulties with distinguishing between them in the field.
• Figure 5.2 shows the percentage of species within the indicator that have shown a statistically significant increase, a statistically significant decrease or no statistically significant change (little or no change) over the long-term and short-term assessment periods.

Butterflies of the wider countryside in woodland

The smoothed trend for butterflies of the wider countryside in woodland shows a significant long-term decline (1990 to 2022). However, in the short term between 2017 and 2022, there was no change in abundance of butterflies of the wider countryside in woodland (Figure 5.3).

The long-term decline of woodland butterflies is thought to be due to a lack of woodland management and loss of open spaces in woods. Species showing the greatest significant long-term declines in woodland include wall; small copper; small tortoiseshell; small or Essex skipper; and gatekeeper. No species show a significant decline in the short term. The ringlet displayed the same trends in woodland as on farmland, increasing significantly in both habitats over the long term, but showing no significant change since 2017. No other woodland butterflies show a significant long-term increase, and none show a significant increase over the short term.

Figure 5.3: Trends for the abundance of butterflies of the wider countryside in UK woodland, 1990 to 2022

Source: Butterfly Conservation, UK Centre for Ecology and Hydrology, British Trust for Ornithology, Defra, Joint Nature Conservation Committee.

View the data for Figure 5.3

Download the data for Figure 5.3 in csv format

Figure 5.4: Long-term and short-term changes in individual species trends for butterflies of the wider countryside in UK woodland, 1990 to 2022

Source: Butterfly Conservation, UK Centre for Ecology and Hydrology, British Trust for Ornithology, Defra, Joint Nature Conservation Committee.

View the data for Figure 5.4

Download the data for Figure 5.4 in csv format

Notes for Figures 5.3 and 5.4:

• This indicator includes individual measures for 25 species of butterflies; the UK woodland butterfly index, however, only includes 24 trends. This is because an aggregate trend is used for small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris); these 2 species have been combined due to historical difficulties with distinguishing between them in the field.
• Figure 5.4 shows the percentage of species within the indicator that have shown a statistically significant increase, a statistically significant decrease or no statistically significant change (little or no change) over the long-term and short-term assessment periods.

6. UK ‘all-species’ index

The ‘all-species’ butterflies index for the UK shows significant declines in both the long and short term. This assessment of change is made on an analysis of the smoothed trend; long-term (1976 to 2022) and short-term (2017 to 2022) (Figure 6.1).

Individual butterfly species fare differently within the overall declining long-term trend. Species in significant long-term decline across the UK include heath fritillary; wall; wood white; small tortoiseshell; white-letter hairstreak; Lulworth skipper; grayling; small-pearl-bordered fritillary; pearl-bordered fritillary; high brown fritillary and white admiral. One species, silver-washed fritillary showed a significant decline over the short term. Species displaying the greatest statistically significant increases over the long term include silver-spotted skipper; black hairstreak; large heath; the ringlet; dark green fritillary; and silver-washed fritillary. Dark green fritillary also increased significantly over the short term. Chalk hill blue and dingy skipper both increased significantly over the short term but showed no long-term change.

Figure 6.1: Trends for the abundance of all species of butterflies resident in the UK, 1976 to 2022

Source: Butterfly Conservation, UK Centre for Ecology & Hydrology, Defra, British Trust for Ornithology, Joint Nature Conservation Committee.

View the data for Figure 6.1

Download the data for Figure 6.1 in csv format

Figure 6.2: Long-term and short-term changes in individual species trends for all species of butterflies resident in the UK, 1976 to 2022

Source: Butterfly Conservation, UK Centre for Ecology & Hydrology, Defra, British Trust for Ornithology, Joint Nature Conservation Committee.

View the data for Figure 6.2

Download the data for Figure 6.2 in csv format

Notes for Figures 6.1 and 6.2:

• This indicator includes individual measures for 51 species of butterflies; the UK ’all-species butterflies index, however, only includes 50 trends. This is because an aggregate trend is used for small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris); these 2 species have been combined due to historical difficulties with distinguishing between them in the field.
• Figure 6.2 shows the percentage of species within the indicator that have shown a statistically significant increase, a statistically significant decrease or no statistically significant change (little or no change) over the long-term and short-term assessment periods.

7. Background and methodology

Assessment

Long and short-term assessments of the statistical significance of changes over time (since the time series began in 1976 and over the latest 5 years – 2017 to 2022) are based on smoothed data, with the analysis of the underlying trend performed by the UKBMS. Confidence intervals around the trend line are calculated and a statistical test is used to compare the difference in the smoothed index between the most recent year and the previous years in a series. See the accompanying technical document for more information. Latest-year changes are based on unsmoothed data. While this time period is too short to make any meaningful assessment, when the change exceeds a 1% threshold, the direction is given simply as an acknowledgement of very recent trends and as a possible early indication of emerging trends. No formal assessment is conducted on the 2 ‘additional wider countryside’ or the ‘all-species’ indices as they have been introduced recently and are not headline indicators.

Trend assessments for habitat specialist butterflies:

• Long term (1976 to 2022) – deteriorating
• Short term (2017 to 2022) – little or no overall change
• Latest year (2022) – little or no overall change

Trend assessments for butterflies of the wider countryside:

• Long term (1976 to 2022) – little or no overall change
• Short term (2017 to 2022) – little or no overall change
• Latest year (2022) – little or no overall change

Relevance

Butterflies are considered to provide a good indication of the broad state of the environment because they respond rapidly to changes in environmental conditions and habitat management, occur in a wide range of habitats, and are representative of many other insects, in that they utilise areas with abundant plant food resources. Butterflies are complementary to birds and bats as indicator species, especially the habitat specialists, because they use resources in the landscape at a much finer spatial scale than either birds or bats. There are also long-term data available on changes in populations of butterflies which help in the interpretation of shorter-term fluctuations.

Background

Although the UKBMS started in 1976, Butterfly Conservation, one of the main partners of this monitoring scheme, conducted additional analysis to examine butterfly occurrence before this year. The UK experienced a severe drought in 1976 and declines in butterfly populations were primarily attributed to the knock-on effects of the drought. However, the additional analysis revealed that 1976 was not an exceptional year (Figure 7.1).

Figure 7.1 shows the combined indices of occurrence (with confidence intervals) for habitat specialists (blue line) and wider countryside species (red line) derived from the Butterflies for the New Millennium (BNM). There are 2 notable features. First, there has been a clear long-term decrease for both groups in the UK from 1976 onwards with little sign of recovery to earlier levels. Second, the occurrence values for the first five years (1970 to 1975) show that overall butterfly occurrence was similar to that in 1976. This demonstrates that 1976 was not an atypical year for butterflies and, therefore, is an appropriate start point for trends from the UKBMS.

Figure 7.1: Long term trends in the occurrence of habitat specialist and wider countryside species, in the UK

Source: Butterfly Conservation, UK Centre for Ecology & Hydrology

Notes for Figure 7.1:

• Figure 7.1 shows the occurrence of habitat specialist (blue line) and wider countryside species (red line).
• 1976 is indicated by a vertical dashed line. There was a severe drought in 1976 which precipitated a decline in the distribution of butterfly species across the UK.
• This figure is reproduced by kind permission of the Butterfly Conservation from The State of the UK’s Butterflies 2015, Butterfly Conservation and the Centre for Ecology & Hydrology.

Methodology

The core indicator is comprised of 2 composite indices compiled by Butterfly Conservation (BC) and the UK Centre for Ecology & Hydrology (UKCEH) from data collated through the UK Butterfly Monitoring Scheme (UKBMS) including from the Wider Countryside Butterfly Survey (WCBS). The indicator includes individual measures for 26 habitat specialist butterflies (low mobility species restricted to semi-natural habitats) and 25 more widespread butterflies (which use both semi-natural and general countryside habitats). It uses butterfly count data collected at 6,509 sample locations across the UK – 4,227 UKBMS butterfly transect and reduced effort sites and 2,282 randomly selected 1-kilometre squares of the WCBS (see the interactive map through the UKBMS sites details map or see, Figure 7.2 for further information.

Since 2019, 2 additional multi-species indices have also been compiled for butterflies in the UK: the first for butterflies of the wider countryside on farmland, the second for butterflies of the wider countryside in woodland. Although these habitat-specific measures are not included within the formal assessment for ‘Butterflies in the United Kingdom: habitat specialists and species of the wider countryside’, they are presented here in order to give a more complete picture of the trends for UK butterfly populations.

The 2 habitat indices include 23 species of wider countryside butterflies surveyed on farmland and 25 species surveyed in woodland using data collected at 4,372 locations with farmland habitat and 3,079 locations with woodland habitat (these farmland and woodland locations are subsets of the 6,509 locations used in the core wider countryside indicator). While these habitat-specific measures are compiled from a subset of the species in the UK wider countryside butterflies indicator, the base year for the long-term trends is 1990 rather than 1976. This is because prior to this date, the data for these 2 indices is not considered to be representative at this scale.

An annual ‘all-species’ index is also compiled for butterflies that are resident in the UK. As with the additional wider countryside indices, this measure in not formally assessed but it is included here in order to give a higher-level picture of the trend for UK butterfly populations. The ‘all-species’ index includes 51 resident species out of the 59 regularly occurring species of butterflies in the UK (26 habitat specialists and 25 species of the wider countryside) and uses data collected at the same 6,509 locations as the 2 core measures. These 51 resident species of the UK do not include regular migrant species or rarer species with less complete data runs.

While the UK wider countryside butterfly measure, the additional wider countryside farmland and woodland measures and the UK ‘all-species’ measure include 25, 23, 25 and 25 species of wider countryside butterflies respectively, they all include one less trend each. This is because an aggregate trend is used for small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris). These 2 species have been combined due to historical difficulties with distinguishing between them in the field.

The year-to-year fluctuations in butterfly numbers are often linked to natural environmental variation, especially weather conditions. Therefore, in order to identify underlying patterns in population trends, the assessment of change is based on smoothed indices. The smoothed trend in the composite (multi-species) indicator is assessed by structural time-series analysis. A statistical test is used to compare the difference in the smoothed index in the latest year versus other years in the series. Within the measures, each individual species trend is given equal weight, and the annual figure is the geometric mean of the component species indices for that year.

Populations of individual species within each composite measure may be increasing or decreasing, irrespective of the overall trends. The bar charts (Figures 3.2 and 4.2 for the core measures and Figures 5.2, 5.4 and 6.2 for the additional measures) show the percentage of species within each indicator that have shown a statistically significant increase, a statistically significant decrease or no statistically significant change (little change). A list of species included within each of the 5 indices, together with a summary of the estimated long-term and short-term changes for each species and an assessment of the individual species trends can be found in the accompanying data set.

As there are delays in data submission, data for previous years are also updated retrospectively. This means that the species indices for individual years may vary from previous publications.

Further details of the methods used can be found on the UKBMS website and in the technical annex for this indicator.

Figure 7.2: Map showing locations of UKBMS transects that produced a site index (red dots) and WCBS squares (blue dots) that were monitored in 2022

Source: UK Butterfly Monitoring Scheme.

8. National Statistics Accreditation

Defra is currently undergoing the process of obtaining National Statistics accreditation for these statistics: National Statistics are accredited official statistics following an independent review by the Office for Statistics Regulation (OSR). To receive this accreditation, Defra has to fulfil 4 requirements as set out in OSR’s assessment report.

Given this feedback, in this current release, Defra has made some progress toward fulfilling requirements 1 and 2. We have included more detail on the methods used to produce these statistics in the methods section above, and in the technical document accompanying these publications to help users understand the context and strengths and limitations of the statistics.

We have also considered where alternative trends might help users understand changes in the data and included an extra chart showing the trend in butterfly occurrence prior the start of the UKBMS in 1976.

Defra will continue to work towards improved engagement with users and enhance the public value of these statistics.

9. Web links

The State of UK’s Butterflies in 2015 is available on the UKBMS website

Further information about the UK Butterfly Monitoring Scheme is available on the UKBMS website

Further information about the state of Britain’s butterflies can be found on the Butterfly Conservation website

The butterfly indicators included within this release together with additional species indicators are presented in the UK Biodiversity Indicators

Next publication date

Autumn 2024