Accredited official statistics

Butterflies in the United Kingdom and in England: 2023

Updated 22 October 2024

© Crown copyright 2024

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This publication is available at https://www.gov.uk/government/statistics/butterflies-in-the-wider-countryside-uk/butterflies-in-the-united-kingdom-habitat-specialists-and-species-of-the-wider-countryside-1976-to-2021

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

Responsible 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

What’s new in this release

Accreditation as official statistics

Since last years publication, this release has achieved accredited official statistics status, by the Office for Statistics Regulation (OSR). This recognition confirms the data meets the OSR’s high standards of trustworthiness, quality, and value, as set out in the Code of Practice for Statistics.

In obtaining this status, Defra made several improvements to enhance data quality and methods. We have also merged the previously separate UK and England releases into a single, combined report. If you have any feedback, please email enviro.statistics@defra.gov.uk.

New butterfly indicators

We have included new indicators in this release, including indices for specialist and generalist species in England, indices for all woodland and farmland species as well as woodland and farmland specialists.

Shift from 5-Year to 10-Year trends

Previously, short-term trends were based on 5 years of data, but expert opinion is that this period is too brief to reflect true butterfly population changes. Therefore, we have extended this to 10 years and the trend period remains ‘short term’.

Increase in monitoring sites since 1976

To support the accreditation, we have included new data (Figure 4.3) that highlights the increasing number of monitoring sites since 1976, demonstrating the schemes’ expanding uptake.

Key messages

  • Overall, 2023 was a slightly above-average year for butterflies in the UK and England. In 2023, the Meteorological Office recorded significant weather extremes, including cold snaps, a warm summer, named storms, and above-average rainfall. Wales and Northern Ireland had their warmest years on record, and the year was among the wettest for Northern Ireland and England.

  • The index for the abundance of all-species of butterfly in the UK has declined in the long and short term, whereas in England the same trend periods have remained stable. This is largely due to greater variability in the England index than the UK index.

  • The indices for habitat specialist butterflies in both the UK and England have declined over the long term. Although habitat specialists in the UK have remained stable over the short term, this contrasts with the short-term trend in England where numbers have declined.

  • The generalist butterfly species indices for the UK and England have not changed in either the long-term or the short-term.

  • Though the Farmland and Woodland habitat specialists have declined, their decline is more severe in woodland habitats.

  • Butterfly populations typically experience large annual fluctuations due to weather conditions, so statistical assessments are based on long term smoothed trends rather than unsmoothed ones.

Table 1: Trend assessments for the UK and England - Long- and short-term

Measure Assessment Time period Result
UK: All-species Long term 1976 to 2023 Deteriorating
UK: All-species Short term 2013 to 2023 Deteriorating
England: All-species Long term 1976 to 2023 Little or no overall change
England: All-species Short term 2013 to 2023 Little or no overall change
UK: Farmland butterflies Long term 1990 to 2023 Deteriorating
UK: Farmland butterflies Short term 2013 to 2023 Little or no overall change
England: Farmland butterflies Long term 1990 to 2023 Deteriorating
England: Farmland butterflies Short term 2013 to 2023 Little or no change
UK: Woodland butterflies Long term 1990 to 2023 Deteriorating
UK: Woodland butterflies Short term 2013 to 2023 Little or no overall change
England: Woodland butterflies Long term 1990 to 2023 Deteriorating
England: Woodland butterflies Short term 2013 to 2023 Little or no overall change

Introduction

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.

This release covers three measures of annual butterfly population abundance in the UK and England. First, in all-species, we analyse all-species of butterflies, which is further divided into indicators for ‘habitat specialists’ and ‘generalist species’ of butterflies. These indicators all begin in 1976, when the monitoring scheme started. The release also covers measures for farmland butterflies and woodland butterflies. These measures are also divided into indicators for habitat specialists and generalist species of butterflies. These were included to present a more complete picture of butterfly abundance. These two habitat indicators cover data from 1990 onward, as earlier data are insufficient at this scale to be considered representative of farmland and woodland.

Habitat specialists are strongly associated with semi-natural habitats such as chalk downland and generalist species are found in a wide range of habitats including both semi-natural habitats and the wider countryside habitats.

Together, these composite indices provide a comprehensive view of how populations of butterflies in the UK and England 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 four 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 and 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). The WCBS was established in 2009 and generates important data on the abundance of widespread butterfly species from under-recorded habitats such as farmland, plantation woodland, uplands and urban green spaces and these data are included with all other UKBMS data in the calculation of these indicators (Figure 4.3).

These statistics contribute to the Biodiversity Indicators, a suite of indicators due to be updated on 10 December 2024.

A list of the species included in each indicator and all of the data presented in this release can be accessed through the accompanying published data set.

Butterfly indicators

General notes about this statistical release

  • In this statistical release, we focus on the habitats used by butterflies. We acknowledge that some generalist species can be host plant specialists and therefore not strictly ‘generalists’. However, for simplicity, we have referred to butterflies of the wider countryside as generalists.

  • The image above ‘Butterfly indicators’: Although there are 50 species in the UK and 49 species in England for the all-species index, two species are counted as one. This is because an aggregate trend is used for the small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris). These two species have been combined due to historical difficulties in distinguishing between them in the field.

  • Scotch Argus does not have long-term trend data for England.

  • Resident species: When we refer to resident species this will not cover all resident species but those resident species for which there is sufficient data for robust analyses.

All-species of butterflies

Indicator trend

The all-species of butterflies index for the UK has declined in both the long (1976 to 2023) and short term (2013 to 2023). The index for the abundance of butterflies in England has shown greater variation over the time series, which means there is less statistical certainty that there has been a meaningful change (Figure 1.1).

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

Note for Figure 1.1:

  • The smoothed index is presented together with the 95% confidence interval (a measure of uncertainty) represented by the shaded area around the trend line.

Habitat breakdown

All-species of butterflies are further divided into groups: habitat specialist and generalist species. Habitat specialists have fared worse than the generalists. Generalist species in both the UK and England have shown little change over the long- and short-term. Analysis of the smoothed trends shows habitat specialists have declined over the long term (1976 to 2023) in the UK and England. Whereas, the short-term trend (2013 to 2023) for the UK shows no overall change, this is in contrast to the trend for England where habitat specialist species have declined over this period.

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

Note for Figure 1.2:

  • The smoothed index is presented together with the 95% confidence interval (a measure of uncertainty) represented by the shaded area around the trend line.

The bar chart in Figure 1.3 shows, for those butterflies included in the ‘all-species’ indicator, the proportion of that are increasing, declining, or remaining stable. Over the long-term period (1976 to 2023), 38% of species have declined and 30% have increased in the UK. There is a very similar pattern when looking at the England long-term trend for all-species of butterflies. Over the short-term period (2013 to 2023), in both the UK and England, 6% of species have increased with the vast majority not showing significant change.

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

Notes for Figure 1.3:

  • The bar chart above shows the percentage of species within the indicator that have increased, decreased or shown no change in abundance over the long-term and short-term assessment periods.
  • Totals may not add upto 100 percent due to rounding.

Habitat specialist butterflies

Indicator trend

Habitat specialist butterflies are restricted to particular habitats such as flower-rich grassland, heathland and woodland clearings. They are vulnerable to semi-natural habitat loss and fragmentation and many have not recovered from declines experienced in the late 1970s. Some of these early declines are likely to be at least partly a result of the knock-on effects of the drought conditions experienced in 1976 but the lack of suitable habitat thereafter is regarded to be the main driver behind the persistent decline and lack of recovery.

In both the UK and England, analysis of the smoothed trend for habitat specialist butterflies shows a significant reduction in relative abundance over the long-term period, driven by decreases from 1976 to 1979. In the short term, since 2013, the smoothed trend for habitat specialist butterflies in the UK has remained stable, whereas the smoothed trend in England has declined (Figure 1.2).

The bar chart in Figure 1.4 shows the proportion of species that are increasing, in decline, or remaining stable. Over the long-term period (1976 to 2023), 46% of species have declined and 31% have increased in the UK. There is a very similar pattern when looking at the England long-term trend for habitat specialist butterflies. Over the short-term period (2013 to 2023), in both the UK and England, 12% of species have increased with the vast majority not showing change.

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

Notes for Figure 1.4:

  • The bar chart above shows the percentage of species within the indicator that have increased, decreased or shown no change in abundance over the long-term and short-term assessment periods.
  • Totals may not add upto 100 percent due to rounding.

Species breakdown

Individual butterfly species fare differently within the UK long-term trends (Figure 1.5). The main causes for decline in habitat specialists are widely considered to be the loss, fragmentation and degradation of their habitats, with agricultural intensification also likely to be contributing. The following list shows species exhibiting the greatest long-term decline:

  • heath fritillary; wood white; small-pearl bordered fritillary; grayling; pearl-bordered fritillary; Lulworth skipper; high brown fritillary; white admiral; Northern brown argus; and grizzled skipper.

Some recent increases in habitat specialists can be attributed to targeted conservation action, as well as the beneficial side of climate change enabling some species to expand their range. Habitat specialists showing the largest long-term increases include the following:

  • black hairstreak; silver-spotted skipper; large heath; dark-green fritillary; silver-washed fritillary; and purple emperor.

Percentage change for these species in England are similar, except for large heath which has shown little or no change in England.

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

Generalist butterfly species

Indicator trend

Generalist butterfly species can breed in a range of habitats across the countryside landscape and in more urbanised areas.

The generalist butterfly indices for the UK and England have not changed in either the long-term (1976 to 2023) or the short-term (2013 to 2023) (Figure 1.2).

The bar chart in Figure 1.6 shows the proportion of species that are increasing, in decline, or remaining stable. Over the long-term period (1976 to 2023), 29% of species have declined and 29% have increased in the UK, with very similar patterns in England. Over the short-term period (2013 to 2023), the vast majority of generalist butterflies in both the UK and England showed no change.

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

Notes for Figure 1.6:

  • The bar chart above shows the percentage of species within the indicator that have increased, decreased or shown no change in abundance over the long-term and short-term assessment periods.
  • Totals may not add upto 100 percent due to rounding.

Species breakdown

Individual butterfly species fare differently within the UK long-term trends (Figure 1.7), however, the following list shows generalist species of butterfly exhibiting the greatest long-term declines:

  • wall; small tortoiseshell; and white-letter hairstreak.

Generalist species showing the largest increases over the long term in the UK include:

  • ringlet; comma; holly blue; speckled wood; and marbled white.

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

Farmland butterflies

Indicator trend

Over the long-term period (1990 to 2023), analysis of the smoothed trends for the abundance of farmland butterflies in the UK and England have declined. There has been no change in the abundance of farmland butterflies in the UK and England since 2013 (Figure 2.1).

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

Note for Figure 2.1:

  • The smoothed index is presented together with the 95% confidence interval (a measure of uncertainty) represented by the shaded area around the trend line.

Habitat breakdown

The farmland butterfly indicator contains data for 44 species in the UK and 43 species in England (Scotch Argus does not have long term trend data for England). Within the indicator there are 22 ‘habitat specialist’ butterflies in both the UK and England, and 22 ‘generalist’ species’ of butterflies in the UK (21 species in England as there is no long term trend data for Scotch Argus).

Figure 2.2 shows that in both the UK and England, farmland habitat specialists fared worse than the farmland generalists, which have less reliance on specific habitats. The abundance of farmland habitat specialist butterflies has declined in both the UK and in England, over the long term, whereas there was no change in the short term (2013 to 2023). Although each species has individual and regional circumstances, the main causes for decline in the farmland habitat specialists are the loss, fragmentation and degradation of their habitats, with agricultural intensification also contributing. In contrast, the smoothed trends for the farmland generalists in both the UK and England have remained stable over the long- and short term.

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

Note for Figure 2.2:

  • The smoothed index is presented together with the 95% confidence interval (a measure of uncertainty) represented by the shaded area around the trend line.

The bar chart in Figure 2.3 shows the proportion of species that are increasing, in decline, or remaining stable. Over the long-term period (1990 to 2023), 39% of species have declined and 18% have increased in the UK. There is a very similar pattern when looking at the England long-term trend for farmland butterflies. Over the short-term period (2013 to 2023), in both the UK and England, 9% of species have increased with the vast majority not showing change.

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

Notes for Figures 2.3:

  • The bar chart above shows the percentage of species within the indicator that have increased, decreased or shown no change in abundance over the long-term and short-term assessment periods.
  • Totals may not add upto 100 percent due to rounding.

Species breakdown

Individual butterfly species fare differently within the overall declining long-term trend (Figure 2.4). The farmland habitat specialist species showing the greatest significant declines in the ‘farmland butterfly’ index over the long term across the UK are:

  • wood white: heath fritillary; pearl-bordered fritillary; Northern brown argus; high brown fritillary; Lulworth skipper; small pearl-bordered fritillary; and grayling.

The farmland habitat specialist species showing significant increases are:

  • large heath and dark green fritillary.

Percentage change for these species in England are similar, except for large heath which has shown little or no change in England.

The farmland generalists showing the greatest significant declines in the farmland butterfly index over the long term across the UK are:

  • small tortoiseshell; Scotch argus; wall; and large skipper.

Farmland generalists showing significant increases include:

  • white letter hairstreak; ringlet; and brimstone.

Percentage change for these species in England are similar.

Individual species trends for the UK and England can be found in the accompanying data set.

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

Notes for Figure 2.4:

  • G in brackets on the y axis next to the species name stands for ‘generalist species’ of butterflies.
  • HS in brackets on the y axis next to the species name stands for ‘habitat specialist’ butterflies.
  • The transparency of the coloured bars indicates whether a species is classified as a ‘generalist’ or ‘habitat specialist’. ‘Habitat specialists’ species are represented by solid coloured bars, while ‘generalist species’ are shown with translucent bars.

Woodland butterflies

Indicator trend

The indicator showing the abundance of woodland butterflies, over the long-term period (1990 to 2023), has declined in the UK and in England. There has been no change in the abundance of woodland butterflies in the UK and England since 2013 (Figure 3.1).

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

Note for Figure 3.1:

  • The smoothed index is presented together with the 95% confidence interval (a measure of uncertainty) represented by the shaded area around the trend line.

Habitat breakdown

The woodland butterfly indicator contains data for 39 species in the UK and 38 species in England (Scotch Argus does not have long term trend data for England). Within the indicator there are 15 woodland habitat specialist butterflies in both the UK and England, and 24 woodland generalist species of butterflies in the UK (23 species in England due to Scotch Argus having no long term trend data for England).

Figure 3.2 shows that in both the UK and England, woodland habitat specialist butterflies fared worse than the woodland generalist species, which require less reliance on specific habitats. Each species has individual and regional circumstances, however, the main causes for decline in woodland habitat specialists are the loss, fragmentation and degradation of their habitats, with agricultural intensification also contributing. The abundance of woodland generalist species of butterflies over the long-term period (1990 to 2023) has declined in the UK and in England. The short-term trend for woodland generalists has shown no change in both the UK and England. Whereas woodland habitat specialist butterflies over the same long-term period have declined in both the UK and England; with declines continuing in the short-term (2013 to 2023).

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

Note for Figure 3.2:

  • The smoothed index is presented together with the 95% confidence interval (a measure of uncertainty) represented by the shaded area around the trend line.

The bar chart in Figure 3.3 shows the proportion of species that are increasing, in decline, or remaining stable. Over the long-term period (1990 to 2023), 46% of species have declined and 8% have increased in the UK. There is a very similar pattern when looking at the England long-term trend for the woodland butterflies indicator. Over the short-term period (2013 to 2023), in the UK, 3% of species have increased with the vast majority not showing change. There is a very similar pattern for woodland butterflies in England, in both the long- and short-term.

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

Notes about Figure 3.3

  • The bar chart above shows the percentage of species within the indicator that have increased, decreased or shown no change in abundance over the long-term and short-term assessment periods.
  • Totals may not add upto 100 percent due to rounding.

Species breakdown

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. Individual butterfly species fare differently within the UK and England long-term trends.

Individual butterfly species fare differently within the overall declining long-term trend (Figure 3.4). The woodland habitat specialist species showing the greatest significant declines in the woodland butterfly index over the long term across the UK are:

  • high brown fritillary; Duke of Burgundy; grizzled skipper; pearl-bordered fritillary; dingy skipper; and green hairstreak.

Woodland habitat specialist species showing significant increases are:

  • purple emperor and silver-washed fritillary.

Percentage change for these species in England are similar.

The woodland generalists showing the greatest significant declines in the woodland butterfly index over the long term across the UK are:

  • wall; small tortoiseshell; small copper; Essex/small skipper; Scotch argus (not England); common blue; and gatekeeper.

Percentage change for these species in England are similar.

Individual species trends for the UK and England can be found in the accompanying data set.

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

Notes for Figure 3.4:

  • G in brackets on the y axis next to the species name stands for ‘generalist species’ of butterflies.
  • HS in brackets on the y axis next to the species name stands for ‘habitat specialist’ butterflies.
  • The transparency of the coloured bars indicates whether a species is classified as a ‘generalist’ or ‘habitat specialist’. ‘Habitat specialists’ species are represented by solid coloured bars, while ‘generalist species’ are shown with translucent bars.

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 the following years were primarily attributed to the knock-on effects of the drought. However, the additional analysis revealed that 1976 was not an exceptional year (Figure 4.1).

Figure 4.1 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 two 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.

Source: Butterfly Conservation, UK Centre for Ecology & Hydrology

Notes for Figure 4.1:

  • Figure 4.1 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

Source data

All data are 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 annual ‘all-species’ index is compiled for butterflies that are resident in the UK and in England. The UK ‘all-species’ index includes 50 resident species out of the 58 regularly occurring species of butterflies (26 habitat specialists and 24 generalist species) for England, the ‘all-species’ index includes 49 resident species out of the 57 regularly occurring of butterflies (26 habitat specialists and 23 generalist species). Data for the UK ‘all-species’ index uses data collected at 6,827 locations across the UK. Data for the England ‘all-species’ index uses data collected at 5,446 sample locations across England – 3,580 UKBMS butterfly transect and reduced effort sites and 1,866 randomly selected 1-kilometre squares of the WCBS. The resident species do not include regular migrant species or rarer species with less complete data runs as this would not be representative.

The ‘habitat specialists and ‘generalist species of butterflies’ in the UK and England indicators include individual measures for 26 habitat specialist butterflies (low mobility species restricted to semi-natural habitats) and 24 (23 species in England) generalist species of butterflies (which use both semi-natural and general countryside habitats). It uses butterfly count data collected at 6,827 sample locations across the UK – 4,501 UKBMS butterfly transect and reduced effort sites and 2,326 randomly selected 1-kilometre squares of the WCBS (see the interactive map through the UKBMS sites details map or, see Figure 4.2 for further information).

The two habitat indices include 44 species in the UK (43 species in England) surveyed on farmland and 39 species (38 species in England) in woodland. The two habitat-specific measures for the UK and England are ‘farmland butterflies’, and ‘woodland butterflies’. Each habitat measure is split into ‘habitat specialists’ or ‘generalist species’ of butterflies. It uses butterfly count data collected at UKBMS butterfly transect sites on farmland and in woodland, along with additional data from randomly selected 1-kilometre squares of the WCBS, primarily comprised of farmland and woodland in the UK and England. This totals 5,131 farmland sites in the UK and 4,059 in England, and 3,796 woodland sample locations in the UK and 3,137 in England. See the interactive map through the UKBMS sites details map or see, Figure 4.2 for further information. The base year for the two habitat indices is 1990 rather than 1976. This is because prior to this date, the data for these two indices is not considered to be representative at this scale of farmland and woodland.

For all the indices presented an aggregate trend is used for small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris). These two species have been combined due to historical difficulties with distinguishing between them in the field.

Figure 4.3 shows the number of monitored sites in the schemes each year since the start of the UKBMS in 1976. The Wider Countryside Butterfly Survey was incorporated into the dataset in 2006. In 1976, 39 sites were being monitored. By 2023, this number had increased to 3,316 sites. The introduction of the WCBS and the increase in the number of sites monitored have contributed to an improvement in spatial coverage. Figure 4.3 shows how data collection by volunteers was significantly impacted by the COVID-19 pandemic and associated restrictions.

UKCEH, who are part of the UKBMS are currently exploring how to improve the dataset regarding regional and habitat biases. The aim is to improve the quality of the data and better represent numbers of butterflies.

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

Figure 4.2: Map showing locations of all UKBMS sites that have contributed to the 2023 indicators, including standard transects and targeted surveys (red dots) and WCBS squares (blue dots)

Source: UK Butterfly Monitoring Scheme.

Figure 4.3: The number of monitoring sites per year, 1976 to 2023

Source: UK Butterfly Monitoring Scheme.

Assessment of change

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 1.3, 1.4, 1.6, 2.3, 3.3) show the percentage of species within each indicator that have shown a statistically significant increase, a statistically significant decrease or no statistically significant change (no change). A list of species included within each of the nine 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 by volunteers, data for previous years are also updated retrospectively. This means that the species indices for individual years may vary from previous publications.

Long and short-term assessments of the statistical significance of changes over time 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 technical annex below for more information.

National Statistics Accreditation

These statistics were awarded as accredited official statistics in August 2024. They comply with the standards of trustworthiness, quality and value in the Code of Practice for Statistics. We will continue to comply with the Code of Practice for Statistics and to update the development plan for this release annually.

We seek feedback from users of the data and we would be interested to hear your thoughts on the amalgmation of this release. Please email enviro.statistics@defra.gov.uk

Development plan

Future developments to be implemented over the coming years are:

  • Review the species to be included in all measures and incorporate any additional species where data availability allows.
  • Review the method for producing the composite indicators alongside recent research developments and consider how to adapt it further if appropriate.
  • Consider alternative methods for assessing change in the indices. For instance, consider basing assessments on the annual average rate of change.
  • Carry out some targeted user engagement to gauge the impact of our developments on the value of the statistics for their users.

The State of UK’s Butterflies in 2022 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

Acknowledgements

Thank you to all citizen scientists contributing data to the UK Butterfly Monitoring Scheme. And thank you to Butterfly Conservation, UK Centre for Ecology and Hydrology and the UKBMS who provide the data.

Technical annex

Data collection

This publication is comprised of multi-species 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). Through the UKBMS, data are collected from around 3,000 sample locations distributed across the UK by around 3,000 skilled volunteers each year. The scheme comprises three survey components:

  • traditional butterfly transects (Pollard Walks);
  • Wider Countryside Butterfly Survey;
  • targeted surveys in which non-transect methods are used to survey specific priority species.

The sampling locations are largely chosen by the recorder, and so are not evenly distributed across the UK. Sites are recorded repeatedly throughout any given year. Volunteer recorders are well supported and receive training and guidance on collecting and submitting data by Butterfly Conservation (see Figures 4.2 and 4.3 for monitoring sites).

Data capture

The primary method for capturing UK Butterfly Monitoring Scheme (UKBMS) data, including the Wider Countryside Butterfly Survey (WCBS), is through the UKBMS online data capture system. This includes site details (for example, location, habitat and management information), species counts through transect walks and other survey methods (for example, timed counts and egg/larval counts).

A proportion of data are also captured via the Transect Walker software package or via spreadsheets.

Data are processed on an annual basis. The majority of data are from surveys conducted in the previous summer, but data from previous years are also often collated. All data are processed in the same way.

Standardisation and harmonisation of the UKBMS data set

All UKBMS data are collated into a single data set to enable analysis and reporting. As of 2023, the data set comprises over 10 million butterfly counts. Data are standardised to conform with the UKBMS database structure, including: standardised species nomenclature, data integrity checks to ensure that all mandatory information is captured, valid date and time information and accurate geographic location information.

Data verification

The UKBMS online data capture system is built using the Indicia software tools and links to the iRecord verification system to enable review of the data by experts approved by Butterfly Conservation or other National Recording Schemes (for records for non-lepidoptera). To support verification, iRecord applies automated data checks against known species distributions (for example, derived from the Butterflies for the New Millennium recording scheme) and timing of adult flight periods. Experts can use these checks and other information to confirm observations.

The UKBMS online data capture system also provides data summaries to enable UKBMS Branch Co-ordinators to review all transect data for their area and make corrections.

Further review and correction is undertaken by staff at Butterfly Conservation and the UK Centre for Ecology and Hydrology at the end of each field season, including the following checks that are discussed with Branch Co-ordinators and/or transect recorders:

  • counts outside of known distribution,
  • counts outside of the standard flight period for a species,
  • species newly recorded on a transect site,
  • species recorded on a transect site after being absent for more than five years, and
  • potential data input errors or misidentifications – all counts of specialist butterfly species are closely scrutinised and summary tables for wider countryside species are reviewed for anomalies.

Transect visits which are undertaken outside the criteria for butterfly activity (for example, based on weather conditions and time of day) are flagged and excluded from the main data analyses; data are retained within the database for use in other analysis.

Data analysis

The calculation of species trends from UKBMS data is not a straightforward calculation because not all transect sites in the UKBMS data set have been recorded each year and the number of weeks with transect counts varies markedly between sites and year. The analytical steps taken to produce the estimates of butterfly populations are as follows:

1. Calculation of a total abundance estimate for each species, at each site within each year, to account for missing data

2. Combining separate site level abundance into a single time series for each species

3. Calculation of multi-species (composite) indices and trends

Calculation of a total abundance estimate for each species, at each site within each year, to account for missing data

Not all transect sites in the UKBMS data set have been recorded each year and the number of weeks with transect counts varies markedly between sites and year. A statistical model is therefore needed to produce a regional or national index of how butterfly populations have changed each year. A Generalized Abundance Index (GAI) method is used which is designed for seasonal invertebrates and is applied to the UKBMS data to calculate annual indices of abundance and assess trends. This method can account for missing patchy data and so combines all UKBMS data including timed counts and data from the WCBS.

The method (Dennis et al., 2016) counts all butterflies in a season from both traditional UKBMS transects and WCBS are used to estimate the seasonal pattern (that is, flight curve) of butterfly counts for each species and year, using generalised additive models (GAMs) applied to weekly summarised data. This stage relies heavily on the traditional UKBMS transect data with good coverage throughout the season. For a given species and year, a site index, which represents an estimate of the expected total number of butterflies had a site been surveyed fully that season, is calculated by scaling the total observed count by the proportion of the species flight curve that was surveyed.

Combining separate site level abundance into a single time series for each species

The next stage of the analysis is then applied to the corrected total annual counts, accounting for where the counts occur within the flight season, to then calculate annual population indices (or time series) for each species using a statistical model to account for sites and years. Data from non-transect surveys are also incorporated at this modelling stage. In common with most butterfly and bird monitoring schemes in Europe (ter Braak et al., 1994), the statistical model uses log-linear Poisson regression to account for the fact that not all sites are sampled in every year. The national collated index is the mean (on a log scale) of the imputed and recorded site indices for each year. Long-term and decadal trends are calculated for each species at UK and country level where sufficient data are available, applying linear regression models to the collated indices.

Calculation of multi-species (composite) indices and trends

The indicators presented here are multi-species (composite) indices of abundance for butterflies in different habitats, for example, farmland and woodland. Composite indices are derived by calculating the geometric mean index across each species assemblage.

Long time series of species abundance data such as those collected through the UKBMS and used to compile Butterfly Indicators cannot always be summarised adequately by linear trend lines. These long time series may show alternating periods of increase and decrease, and it can be difficult to separate patterns of genuine change from annual fluctuations. Consequently, methods that model smoothed trend lines through abundance data are becoming increasingly popular. An extension of the linear trend approach is the application of a smoothing technique that describes the pattern by assigning a trend level (that is, a modelled abundance) to each year in the time series (similar to a moving average). There are several smoothing methods available such as polynomial regression, splines and Loess estimators. These models may be summarised as ‘flexible trend models’. The most popular flexible trend models for the analysis of wildlife populations are GAMs and these, for example, are used to produce the Wild Bird Indicators. GAMs do not however present the complete time series and do not account for serial correlation which limits their applicability to butterfly data.

TrendSpotter software (Visser, 2004) is used to identify periods of significant change in butterfly abundance. Under this approach, confidence intervals around the trend line is calculated by the application of structural time series analysis and the Kalman filter (Visser, 2004). This approach uses one observation per time point (for example, year or month) and therefore the uncertainty in the estimate of yearly index values (for example, confidence intervals around each year index) is modelled indirectly in the annual fluctuations. The main advantage of the TrendSpotter analysis however is the calculation of confidence intervals for the differences between the trend level of the last year and each of the preceding years, taking into account serial correlation which is unique for flexible trend methods. This allows short-term trends to be usefully assessed.

Periods of significant change are identified by comparing the difference in the index for the first and last year of any given time period. Thresholds for determining change are given in Table 2 (see Soldaat et al., 2007). This classification is not the same as that used for the individual species trends presented in the data set (increased, decreased and no change).

Trend class Criteria Description
Strong increase Lower confidence limit greater than 1.05 Increase greater than 5% per year (approximately equal to doubling in 15 years)
Moderate increase Lower confidence limit greater than 1.00 and less than or equal to 1.05 Increase, but unsure whether greater than 5% per year
Stable Confidence interval contains 1.00 AND lower confidence limit greater than or equal to 0.95 AND upper confidence limit less than or equal to 1.05 Population changes less than 5% per year
Moderate decrease Upper confidence limit greater than or equal to 0.95 and less than 1.00 Decrease, but unsure whether greater than 5% per year
Steep decrease Upper confidence limit less than 0.95 Decrease greater than 5% per year (approximately equal to halving in 15 years)
Uncertain Confidence interval contains 1.00 AND lower confidence limit less than 0.95 OR upper confidence limit greater than 1.05 Confidence interval too large for reliable classification

In summary, structural time series models are essentially regression models in which the explanatory variables are functions of time, and the parameters are time-varying. The Kalman filter is an efficient recursive filter that estimates the state of a dynamic system from a series of incomplete and noisy measurements. For mathematical details about structural time-series analysis and the Kalman filter please refer to Harvey (1989).

TrendSpotter is currently considered the best-available technique in the assessment of Butterfly Indicators. Regular reviews of methods to assess changes in butterfly indicators are needed; however, techniques to model trends are an active area of statistical development.

Data checks carried out by Defra

Defra statisticians review the current year’s data with the data provided in previous years. Defra statisticians chart the data for the composite indicators to highlight anomalies. These anomalies are then queried with the UKBMS (data provider).

References

  • Brakefield, P. M., (1987). Geographical variability in, and temperature effects on, the phenology of Maniola jurtina and Pyronia tithonus (Lepidoptera, Satyrinae) in England and Wales. Ecological entomology, 12(2), pp.139-148.
  • Brereton, T. M., Roy D. B., Middlebrook, I., Botham, M. & Warren, M., (2011). The development of butterfly indicators in the United Kingdom and assessments in 2010. Journal of Insect Conservation, 15, 139-151.
  • Dennis, E. B., Morgan, B. J., Freeman, S. N., Brereton, T. M. & Roy, D. B., (2016). A generalized abundance index for seasonal invertebrates. Biometrics, 72(4), pp.1305-1314.
  • Gregory, R. D., Vorisek, P., van Strien, A. J., Gmelig Meyling, A. W., Jiguet, F., Fornasari, L., Jiri, R., Chylarecki, P. & Burfield, I. J., (2007). Population trends of widespread woodland birds in Europe. Ibis: 149 (Suppl. 2), 78–97.
  • Harvey, A. C., (1989). Forecasting structural time series models and the Kalman filter. Cambridge University Press, London.
  • Rothery, P. & Roy, D. B., (2001). Application of generalized additive models to butterfly transect count data. Journal of Applied Statistics, 28(7), pp.897-909.
  • Soldaat, L. L., Visser, P., van Roomen, M. & van Strien, A. (2007). Smoothing and trend detection in waterbird monitoring data using structural time-series analysis and the Kalman filter. Journal of Ornithology. Vol. 148 suppl. 2. Dec. 2007.
  • ter Braak, C. J. F., van Strien, A. J., Meijer, R., & Verstrael, T. J., (1994). Analysis of monitoring data with many missing values: which method? In Bird Numbers 1992: Distribution, monitoring and ecological aspects. (eds W. Hagemeijer & T. Verstrael), pp. 663-673. SOVON, Beek-Ubbergen, Netherlands.
  • Visser, H., (2004). Estimation and detection of flexible trends. Atm Environment 38: 4135-4145.
  • Visser, H., (2005). The significance of climate change in the Netherlands. An analysis of historical and future trends (1901-2020). MNP report 55000200.
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