Official Statistics

Butterflies in England: species of the wider countryside on farmland and in woodland, 1990 to 2022

Updated 2 February 2024

Applies to England

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

Butterflies can be categorised as habitat specialists and species of the wider countryside. This release covers 2 measures of annual butterfly population abundance for the species of the wider countryside in England since 1990: the first on farmland and the second in woodland. Data before 1990 are not sufficient at this scale to be considered representative for inclusion. These statistics contribute to the Biodiversity Indicators, a suite of indicators due to be updated on 14 November 2023.

The release also includes an additional England ‘all-species’ index that does not form part of the formal assessment. This index covers data back to 1976 as this is when the monitoring scheme started.

These composite indices are sourced by the UK Butterfly Monitoring Scheme (UKBMS) and considered representative of butterfly trends across England. 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 England, with 46% of all species contributing to the England indicators (23 out of 50) falling in abundance from the previous year. 2022 was the warmest year on record, 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 butterflies of the wider countryside on farmland in England shows little or no overall change since 1990, whereas the long-term smoothed trend for butterflies in woodland is significantly downward.
• Assessments of the short-term smoothed trends show no significant change for butterflies of the wider countryside in either habitat in England since 2017.

3. Butterflies of the wider countryside on farmland

The abundance of butterflies on farmland has remained largely unchanged from the start of 1990 (Figure 3.1). In the short term, since 2017, the abundance of butterflies has also shown little or no change.

Individual butterfly species fare differently within the overall stable trend. Some species on farmland show a significant long-term decline including small tortoiseshell; wall; gatekeeper; large skipper; small copper and small or Essex skipper. Large skipper also decreased significantly on farmland over the short term (since 2017). Five species on farmland, brimstone; marbled white; the ringlet; speckled wood and white-letter hairstreak, increased significantly over the long term, but no species of butterfly have increased significantly on farmland since 2017.

Figure 3.1: Trends for the abundance of butterflies of the wider countryside on farmland in England, 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 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 butterflies of the wider countryside on farmland in England, 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 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. Butterflies of the wider countryside in woodland

The abundance of butterflies in woodland has significantly declined since the start of 1990 (Figure 4.1). In the short term, since 2017, the abundance of butterflies has shown little or no change.

The long-term decline of woodland butterflies is thought to be chiefly due to a lack of woodland management and loss of open spaces in woods. Species of the wider countryside showing the largest long-term decline in woodland include common blue; large skipper; marbled white; peacock; wall; small tortoiseshell; small copper; small or Essex skipper; and gatekeeper. No species of butterfly have undergone a significant short-term decline in woodland (since 2017). The ringlet displayed the same trends in woodland as on farmland, increasing significantly over the long term, but showing no significant short-term change.

Figure 4.1: Trends for the abundance of butterflies of the wider countryside in woodland in England, 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 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 woodland in England, 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 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. England ‘all-species’ index

The index for the abundance of ‘all-species’ of butterflies resident in England has shown little or no change over both the long term (1976 to 2022) and the short term (2017 to 2022) (Figure 5.1).

Individual butterfly species fare differently within the overall declining long-term trend. Species in significant long-term decline across England include heath fritillary; wall; wood white; small tortoiseshell; white-letter hairstreak; and pearl-bordered fritillary. Two species, silver-washed fritillary and small pearl-bordered fritillary showed a significant decline over the short term.

Species displaying the greatest statistically significant increases over the long term include dark-green fritillary; silver-spotted skipper; black hairstreak; the ringlet; silver-washed fritillary; comma; holly blue and speckled wood. Species displaying the greatest statistically significant increases over the short term include chalk hill blue and the dingy skipper. Dark green fritillary showed both long and short term statistically significant increases. The Lulworth skipper showed a statistically significant decrease over the long term but is displaying a short term statistically significant increase.

Figure 5.1: Trends for the abundance of 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 5.1

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.

6. Background and methodology

Assessment

Long and short-term assessments of the statistical significance of change over time (since the time series began in 1990 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 of change is given as an acknowledgement of very recent trends and as a possible early indication of emerging trends. No formal assessment is conducted on the ‘all-species’ index as it has been introduced recently and is not a headline indicator.

Trend assessments for butterflies of the wider countryside on farmland:

• Long term (1990 to 2022) – little or no overall change
• Short term (2017 to 2022) – little or no overall change
• Latest year (2022) – decreased

Trend assessments for butterflies of the wider countryside in woodland:

• Long term (1990 to 2022) – deteriorating
• Short term (2017 to 2022) – little or no overall change
• Latest year (2022) – increased

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 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 subsequent declines in butterfly populations were primarily attributed to the knock-on effects of the drought. The additional analysis revealed that 1976 was not an exceptional year (Figure 6.1).

Figure 6.1 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 6.1: 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 6.1:

• Figure 6.1 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 and the UK Centre for Ecology and Hydrology (UKCEH) from data collated through the UK Butterfly Monitoring Scheme (UKBMS) including the Wider Countryside Butterfly Survey (WCBS). The indicator includes 22 species of butterflies associated with farmland and 24 species associated with woodland. It uses butterfly count data collected at UKBMS butterfly transect sites on farmland and in woodland together with additional data from randomly selected 1 kilometre squares of the WCBS primarily comprised of farmland and woodland (totalling 3,287 farmland and 2,369 woodland sample locations across England) – see the interactive map through the UKBMS sites details map or see, Figure 6.2 for further information.

An annual ‘all-species’ index is also compiled for butterflies that are resident in England. This measure in not formally assessed but it is included here in order to give a higher-level picture of the trend for butterfly populations in England. The ‘all-species’ index includes 50 resident species out of the 58 regularly occurring species of butterflies in England (26 habitat specialists and 24 species of the wider countryside) and uses data collected at 5,186 sample locations across England – 3,358 UKBMS butterfly transect and reduced effort sites and 1,828 randomly selected 1-kilometre squares of the WCBS. These 50 resident species of England do not include regular migrant species or rarer species with less complete data runs.

While the farmland and woodland butterfly measures and the ‘all-species’ measure include 22, 24 and 50 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 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 Figure 5.2 for the additional ‘all-species’ measure) 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 the species included within each of the 3 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 6.2: 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.

7. 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.

8. 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 farmland and woodland species indicators are presented in the England Biodiversity Indicators

Next publication date

Autumn 2024