Accredited official statistics

Insects of the wider countryside (butterflies)

Updated 10 December 2024

Applies to England

Last updated: 2024

Latest data available: 2023

Introduction

This indicator covers three measures of annual butterfly population abundance in England. First, we analyse all-species of butterflies, which is further divided into indicators for ‘habitat specialists’ and ‘generalist species’ of butterflies. There are also individual indicators for ‘farmland butterflies’ and ‘woodland butterflies’. These measures are also divided into indicators for habitat specialists and generalist species of butterflies.

Butterflies are complementary to birds and bats as an indicator, especially the habitat specialists, because they use resources in the landscape at a much finer spatial scale than either birds or bats.

Throughout this indicator, you should be aware that 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.

Data for this indicator can be found in the published datafile. We also have a similar indicator at the UK level. For more detailed information, including individual species trends, see the in the Butterflies in the United Kingdom and in England: 2023 statistical release.

Type of indicator

State indicator

Type of official statistics

Accredited official statistic

Assessment of change

Assessment of change in butterfly populations

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

Notes on the indicator assessment

• Long-term and short-term assessments of the statistical significance of these changes are made using the smoothed data to 2023. Analysis of the underlying trends is performed by the data providers.
• Please note that short-term trends for these indicators 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 of data and the trend period remains ‘short term’.

Key results

• All-species in England have shown variation over the time series and little or no meaningful change over both the long term and the short term (Figure 1).
• The indices for habitat specialist butterflies in England have declined in both the long and short term (Figure 3).
• The generalist butterfly species indices in England have remained stable in both the long- and short term (Figure 3).
• Though the farmland habitat specialists and the woodland habitat specialists have declined over the long term, in the short term, habitat specialists remained stable on farmland whereas they declined in woodland habitats (Figures 5 and 7).

All-species of resident butterflies

The ‘all-species’ butterflies index in England has shown variation over the time series and little or no meaningful change over both the long term and the short term (Figure 1).

Overall, 2023 was a slightly above average year for butterflies in England. In 2023, the Meteorological Office recorded significant weather extremes, including cold snaps, a warm summer, named storms, and above-average rainfall.

Figure 1: Trends for the abundance of all-species of butterflies resident in England (49 species), 1976 to 2023

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

Note about Figure 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.

Figure 2: Long-term and short-term changes in individual species trends for all-species of butterflies resident in England (49 species), 1976 to 2023

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

Notes about Figure 2

• 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 up to 100 percent due to rounding.

The ‘all-species’ index can be further divided into groupings for ‘habitat specialists’ and ‘generalist’ butterfly species.

Habitat breakdown

‘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.

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

‘Habitat specialists’ have fared worse than the ‘generalists’. ‘Generalist’ species in England have shown little change over the long- and short-term. Analysis of the smoothed trends shows ‘habitat specialist’ species have shown a significant reduction in relative abundance over the long term period (1976 to 2023), driven by decreases from 1976 to 1979. In the short-term period the ‘habitat specialists’ have continued to decline in England (Figure 3).

The ‘generalist’ butterfly species index for England has not changed in either the long-term (1976 to 2023) or the short term (2013 to 2023) (Figure 3).

Figure 3: Trends for the abundance of habitat specialist (26 species) and generalist butterfly species (23 species) in England, 1976 to 2023

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

Note about Figure 3

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

Figure 4: Long-term and short-term changes in individual species trends for habitat specialist (26 species) and generalist (23 species) butterflies in England, 1976 to 2023

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

Notes about Figure 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 up to 100 percent due to rounding.

For detailed information, including individual species trends, see the ‘all-species’ section in the Butterflies in the United Kingdom and in England: 2023.

Farmland butterflies

On farmland in England, habitat specialist butterflies fared worse than generalist species, which require less reliance on specific habitats (Figure 5). Each species has individual and regional circumstances, however, the main causes for decline in farmland habitat specialists are the loss, fragmentation and degradation of their habitats, with agricultural intensification also contributing. The abundance of farmland habitat specialist butterflies have declined in England, over the long term, whereas there was no change in the short term (2013 to 2023). In contrast, the smoothed trends for farmland generalists in England have remained stable over the long- and short term.

Figure 5: Trends for the abundance of generalist and specialist farmland butterflies in England (43 species), 1990 to 2023

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

Note about Figure 5

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

Figure 6: Long-term and short-term changes in individual species trends for generalist and specialist farmland butterflies in England (43 species), 1990 to 2023

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

Notes about Figure 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 up to 100 percent due to rounding.

For detailed information, including individual species trends, see the ‘farmland butterflies’ section in the Butterflies in the United Kingdom and in England: 2023.

Woodland butterflies

In woodland in England, woodland habitat specialist butterflies fared worse than woodland generalists, which require less reliance on specific habitats (Figure 7). 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 England and shown no change in the short term (2013 to 2023). Whereas woodland habitat specialist butterflies in England have declined over both the long- and short-term (2013 to 2023).

Figure 7: Trends for the abundance of generalist and specialist woodland butterflies in England (38 species), 1990 to 2023

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

Note about Figure 7

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

Figure 8: Long-term and short-term changes in individual species trends for generalist and specialist woodland butterflies in England (38 species), 1990 to 2023

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

Notes about Figure 8

• 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 up to 100 percent due to rounding.

For detailed information, including individual species trends, see the ‘woodland butterflies’ section in the Butterflies in the United Kingdom and in England: 2023.

Further detail

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.

Web links for further information

• Butterflies in the United Kingdom and in England: 2023

• 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

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

Background

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 England. The England ‘all-species’ index includes 49 resident species out of the 57 regularly occurring species of butterflies (26 habitat specialists and 23 generalist species). 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 England indicators include individual measures for 26 habitat specialist butterflies (low mobility species restricted to semi-natural habitats) and 23 generalist species of butterflies (which use both semi-natural and general countryside habitats).

The two habitat indices include 43 species surveyed on farmland in England and 38 species in England’s woodland. The two habitat-specific measures for 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 England. This totals 4,059 farmland sites in England and 3,137 woodland sample locations in England. See the interactive map through the UKBMS sites details map. 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.

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.

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.

Methodology

Data collection

The core indicator 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 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.

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. 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.

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 undergoes a verification process through which automated checks are applied and the data are reviewed by experts. More detail on the data collection, processing and verification process can be found in the published statistical release.

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

More details for which can be found in the published statistical release.

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.

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.

The UK Biodiversity Indicators use 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.

TrendSpotter software (Visser, 2004) is used to identify periods of significant change in butterfly abundance. 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 1 (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).

Table 1: Classification of composite trends on the basis of the 95% confidence intervals of the yearly change rates in TrendSpotter smoothed indices (see Soldaat et al., 2007 for explanation).

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

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.

Development plan

Future developments to be implemented over the coming years for the Butterflies in the United Kingdom and in England: 2023 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.

References

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