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C6. Insects of the wider countryside (butterflies)

1. Habitat specialists

2. Species of the wider countryside

Type: State Indicator

Introduction

This indicator consists of 2 measures of annual butterfly population abundance: 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. 

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 of these groups.

Key results

Methodological Note

Improvements were made to the analytical techniques in 2020 to better account for the colonisation of sites. The change has been to add pre-colonisation zero abundance counts for species at sites they have colonised, where the site was being monitored prior to colonisation. In general, the effect of these changes has been most notable for expanding species whereby there has been a slight reduction in their population indices for the earlier years, relative to the latter years. This analysis improvement has coincided with relatively favourable recent years for butterflies. The combination of the relative reductions in the indices of earlier years for colonising species with the relatively high indices in recent years have resulted in the current indicator assessment differing from previous assessments to a greater extent than in previous updates. Further details can be found in the technical background document.

Butterfly icon Since 1976, the unsmoothed habitat specialist butterflies index has fallen by 59% (Figure C6a).

Butterfly icon Over the same period, the unsmoothed index for species of the wider countryside has fallen by 20% (Figure C6b).

Butterfly icon Large fluctuations in numbers between years are a typical feature of butterfly populations, principally in response to weather conditions. With warmer than average spring and summer temperatures, 2019 was a good year for butterflies in the UK; with more than half of species (53%) increasing in annual abundance.

Butterfly icon The statistical assessment of change is made on an analysis of the underlying smoothed trends. Since 1976, populations of habitat specialists have declined significantly though species of the wider countryside show no change. Since 2014, both trends show no significant change.

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Habitat Specialists

Left part is a line graph showing changes to the UK habitat specialist butterflies index. Right part is two 100% stacked bar charts showing the percentage of individual species within the UK habitat specialist butterflies index that have shown a statistically significant increase, a statistically significant decrease or no statistically significant change (little change) over both the long term (since 1976) and short term (latest 5 years).

Notes:

  1. The line graph shows the unsmoothed trend (dashed line) and the smoothed trend (solid line) together with its 95% confidence interval (shaded).
  2. The figure in brackets shows the number of species included in the index.
  3. The bar chart 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 change).
  4. Since 2017, an improved analysis method has been used to derive the species indices (see ‘Background’ section for further information).
  5. Further improvements were made to the analytical techniques in 2020 to better account for the colonisation of sites. The change has been to add pre-colonisation zero abundance counts for species at sites they have colonised, where the site was monitoring prior to colonisation. In general, the effect of these changes has been most notable for expanding species whereby there has been a slight reduction in their population indices for the earlier years, relative to the latter years. This analysis improvement has coincided with relatively favourable recent years for butterflies. The combination of the relative reductions in the indices of earlier years for colonising species with the relatively high indices in recent years have resulted in the current indicator assessment differing from previous assessments to a greater extent than in previous updates. Further details can be found in the technical background document (see ‘Background’ section for further information.

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

Habitat specialist species, 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. The habitat specialist index declined by 59% between 1976 and 2019 (Figure 1). Underlying analysis of the smoothed trend shows a statistically significant reduction in relative abundance over the period 1976 to 1979. The index showed an increase over the period 2014 to 2019 from 37% to 41% of the 1976 level, however, analysis showed no statistically significant short-term change. 

Individual butterfly species fare differently within the overall trend. Habitat specialists showing the greatest decline since 1976 include: heath fritillary; wood white; Lulworth skipper; grayling; and small pearl-bordered fritillary. No species show a short-term decline since 2014. Silver-spotted skipper, large heath, black hairstreak, silver-washed fritillary and dark green fritillary show the largest significant increases over the long term, whilst silver-studded blue and black hairstreak show a statistically significant increase since 2014.

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Species of the wider countryside

Left part is a line graph showing changes to the UK butterflies in the wider countryside index. Right part is two 100% stacked bar charts showing percentage of species within the UK butterflies of the wider countryside index that have shown statistically significant increase, statistically significant decrease or no statistically significant change (little change) over both long term (since 1976) and short term (latest 5 years).

Notes:

  1. The line graph shows the unsmoothed trend (dashed line) and the smoothed trend (solid line) together with its 95% confidence interval (shaded).
  2. The figure in brackets shows the number of species included in the index.
  3. This indicator includes individual measures for 25 species of butterflies; the wider countryside index, however, only includes 24 This is because an aggregate trend is used for small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris) and has been retained for indicator consistency; these 2 species were combined due to historical difficulties with distinguishing between them in the field.
  4. The bar chart 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 change).
  5. Since 2017, an improved analysis method has been used to derive the species indices (see ‘Background’ section for further information).
  6. Further improvements were made to the analytical techniques in 2020 to better account for the colonisation of sites. The change has been to add pre-colonisation zero abundance counts for species at sites they have colonised, where the site was monitoring prior to colonisation. In general, the effect of these changes has been most notable for expanding species whereby there has been a slight reduction in their population indices for the earlier years, relative to the latter years. This analysis improvement has coincided with relatively favourable recent years for butterflies. The combination of the relative reductions in the indices of earlier years for colonising species with the relatively high indices in recent years have resulted in the current indicator assessment differing from previous assessments to a greater extent than in previous updates. Further details can be found in the technical background document (see ‘Background’ section for further information).

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

The unsmoothed species of the wider countryside index showed a decrease of 20% between 1976 and 2019 (Figure C6b); though the underlying analysis shows the change was not statistically significant. The index showed an increase over the period 2014 to 2019, from 73% to 80% of the 1976 level, however, this short-term change is not statistically significant.

Individual butterfly species again fare differently within the overall trend. Species of the wider countryside showing the greatest declines since 1976 include: wall; small tortoiseshell; white-letter hairstreak and small heath; small tortoiseshell shows a short-term decline since 2014. Ringlet, comma, speckled wood, marbled white and orange-tip all show increases over the long term; though no wider countryside butterfly species show a short-term increase since 2014.

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Assessment of change in butterfly populations

  Long term Short term Latest year
Habitat specialists

Deteriorating
1976–2019

Little or no overall change
2014–2019

Increased (2019)
Species of the wider countryside

Little or no overall change
1976–2019

Little or no overall change
2014–2019

Decreased (2019)

Note:

While percentage changes in these indices are reported based on the most recent unsmoothed data point (2019), the formal long-term and short-term assessments of the statistical significance of these changes are made using the smoothed data to 2019. Analysis of the underlying trends is undertaken by the data providers.

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Relevance

Butterflies 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 of those groups.

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Background

This indicator is comprised of 2 multi-species 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) using data collected at 5,737 sample locations (see Figure 3). The wider countryside measure, however, only includes 24 trends 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 multi-species indicator is assessed by structural time-series analysis. A statistical test is performed using the software ‘TrendSpotter’ 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 measure may be increasing or decreasing, irrespective of the overall trends. The bar chart provided alongside each trend graph above shows the percentage of species within that 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 index, 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 dataset.

The method for compiling species annual indices was improved in 2017 and used again here. Indices are calculated for all species using the Generalised Abundance Index (GAI) method developed in 2016 (Dennis et al. (2016) BIOMETRICS: DOI: 10.1111/biom.12506) with an additional modification that the data from each site in each year is weighted in the final stage relative to the proportion of the species flight period surveyed that year for that site. This weighting is necessary as the GAI extrapolates from observed data to estimate the total count across the season, accounting for gaps in the recording, and ensures that the observed data have a stronger effect upon the final indices than the extrapolated data.

The new method uses all butterfly counts collected at both UKBMS sites (3,649 compared with 3,443 in 2018) and randomly selected 1km squares of the Wider Countryside Butterfly Survey (2,088 compared with 2,037 in 2018) (see Figure 3 for the locations of these sites). The method uses all butterfly counts in a season to estimate the seasonal pattern of butterfly counts for that year, using a concentrated likelihood method (see Dennis et al. (2016)); the resulting indices and species trends are similar to those generated through previous analysis methods. In 2020, further improvements were made to better model trends for species that have expanded in range and colonised new UKBMS sites, changing the trend for a small number of species including Essex skipper and purple hairstreak.

Since 2015, the site index only data has been incorporated into the models; these data are most prevalent in earlier years and thus the graphs are slightly different to those previously presented. As there are delays in data submission, data for previous years are also updated retrospectively; in 2019 extra data were added for 2017 and 2018, for example. 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 background document for this indicator. 

Since 2019, 2 additional multi-species indices were 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 new 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 new indices include individual trends for 22 species of wider countryside butterflies surveyed on farmland and 24 species surveyed in woodland; they use data collected at 4,027 locations with farmland habitat and 2,768 locations with woodland habitat (these farmland and woodland locations are subsets of the 5,737 locations shown in Figure 3). A list of species included within each index, 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 dataset.

Figure C6i. Locations of the 3,649 UK Butterfly Monitoring Scheme sites (blue) and the 2,088 Wider Countryside Butterfly Survey squares (red) as at 09 June 2020.

Map of UK showing the locations of the 3,649 UK Butterfly Monitoring Scheme sites and the 2,088 Wider Countryside Butterfly Monitoring Survey squares as at 9 June 2020.

The assessment of change for these additional measures is made on an analysis of the underlying smoothed trends, however, the base year for the long-term trends is 1990. This is because prior to this date, there are insufficient data for a number of species included within the 2 indices. Since 1990, UK populations of wider countryside butterflies on farmland show no significant change, whilst in woodland butterflies have declined significantly. Since 2014, the trend for farmland and woodland butterflies show no statistically significant change.

Butterflies of the wider countryside on farmland

The unsmoothed butterflies of the wider countryside on farmland index for the UK increased by 18% between 1990 and 2019. The index also increased in the 5 years to 2019, from 10% above its 1990 level in 2014 to 18% above this level in 2019 (Figure C6ii).

Left part is a line graph showing changes to the index of UK butterflies of the wider countryside on farmland. Smoothed index has been stable throughout the period. Unsmoothed index shows year-on-year variation. Right part is two 100% stacked bar charts showing percentage of species within index that have shown statistically significant increase, statistically significant decrease or no statistically significant change (little change) over long term (since 1990) and short term (latest 5 years).

Notes:

  1. The line graph shows the unsmoothed trend (dashed line) and the smoothed trend (solid line) together with its 95% confidence interval (shaded).
  2. The figure in brackets shows the number of species included in the index.
  3. This indicator includes individual measures for 23 species of butterflies; the UK farmland index, however, only includes 22 This is because an aggregate trend is used for small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris) and has been retained for indicator consistency; these 2 species were combined due to historical difficulties with distinguishing between them in the field.
  4. The bar chart 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 change).
  5. Improvements were made to the analytical techniques in 2020 to better account for the colonisation of sites. The change has been to add pre-colonisation zero abundance counts for species at sites they have colonised, where the site was monitoring prior to colonisation. In general, the effect of these changes has been most notable for expanding species whereby there has been a slight reduction in their population indices for the earlier years, relative to the latter years. This analysis improvement has coincided with relatively favourable recent years for butterflies. The combination of the relative reductions in the indices of earlier years for colonising species with the relatively high indices in recent years have resulted in the current indicator assessment differing from previous assessments to a greater extent than in previous updates. Further details can be found in the technical background document (see ‘Background’ section for further information.

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

Individual butterfly species fare differently within the overall stable trend. Species in significant long-term decline on farmland are: small tortoiseshell, wall, gatekeeper, small copper, Essex/small skipper and large skipper. Of these, small tortoiseshell has also decreased over the short term since 2014. Three species, the ringlet, speckled wood and brimstone, increased over the long term, whilst Holly blue and small copper have increased over the short-term (since 2014).

Butterflies of the wider countryside in woodland

The unsmoothed butterflies of the wider countryside in woodland index for the UK decreased by 41% between 1990 and 2019 but increased in the 5 years to 2019, from 53% of its 1990 level in 2014 to 59% of its 1990 level in 2019 (Figure C6iii).

Left part is a line graph showing changes to the index for UK butterflies of the wider countryside in woodland. Smoothed index fell steadily between 1990 and 1998 but has since levelled out. Unsmoothed index has shown year-on-year variation. Right part is two 100% stacked bar charts showing percentage of species within the index that have shown statistically significant increase, decrease or no significant change (little change) over the long term (since 1990) and short term (latest 5 years).

Notes:

  1. The line graph shows the unsmoothed trend (dashed line) and the smoothed trend (solid line) together with its 95% confidence interval (shaded).
  2. The figure in brackets shows the number of species included in the index.
  3. This indicator includes individual measures for 25 species of butterflies; the UK woodland index, however, only includes 24 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.
  4. The bar chart 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 change).
  5. Improvements were made to the analytical techniques in 2020 to better account for the colonisation of sites. The change has been to add pre-colonisation zero abundance counts for species at sites they have colonised, where the site was monitoring prior to colonisation. In general, the effect of these changes has been most notable for expanding species whereby there has been a slight reduction in their population indices for the earlier years, relative to the latter years. This analysis improvement has coincided with relatively favourable recent years for butterflies. The combination of the relative reductions in the indices of earlier years for colonising species with the relatively high indices in recent years have resulted in the current indicator assessment differing from previous assessments to a greater extent than in previous updates. Further details can be found in the technical background document (see ‘Background’ section for further information.

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

 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 largest long-term declines in woodland include: wall, small tortoiseshell, small copper, Essex/small skipper and gatekeeper, with the small tortoiseshell also undergoing a significant decline in the short term. Ringlet displayed the same trends in woodland as in farmland, increasing significantly over the long term, but showing no significant change since 2014.

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Goals and Targets

Aichi Targets for which this is a primary indicator

Strategic Goal B. Reduce the direct pressures on biodiversity and promote sustainable use.

Aichi Target 7 icon

Target 7: By 2020, areas under agriculture, aquaculture and forestry are managed sustainably, ensuring conservation of biodiversity.

 

Strategic Goal C. To improve the status of biodiversity by safeguarding ecosystems, species and genetic diversity.

Aichi Target 12 icon

Target 12: By 2020, the extinction of known threatened species has been prevented and their conservation status, particularly of those most in decline, has been improved and sustained.

Aichi Targets for which this is a relevant indicator

Strategic Goal B. Reduce the direct pressures on biodiversity and promote sustainable use.

Aichi Target 5 icon

Target 5: By 2020, the rate of loss of all natural habitats, including forests, is at least halved and where feasible brought close to zero, and degradation and fragmentation is significantly reduced.

Strategic Goal C. To improve the status of biodiversity by safeguarding ecosystems, species and genetic diversity.

Aichi Target 11 icon

Target 11: By 2020, at least 17 per cent of terrestrial and inland water, and 10 per cent of coastal and marine areas, especially areas of particular importance for biodiversity and ecosystem services, are conserved through effectively and equitably managed, ecologically representative and well connected systems of protected areas and other effective area-based conservation measures, and integrated into the wider landscape and seascapes.

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Reference Title 

Butterfly Conservation

The state of Britain’s butterflies

UK Butterfly Monitoring Scheme

Butterflies as indicators

 

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References

Dennis, E. B., Morgan, B. J. T., Freeman, S. N., Brereton, T. & Roy, D. B. (2016). A generalized abundance index for seasonal invertebrates. Biometrics, 72(4), 1305–1314. https://doi.org/10.1111/biom.12506

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Downloads

Download the Datasheet and Technical background document from JNCC's Resource Hub.

A more general technical document on UKBMS official statistics is available.   

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Last updated: October 2020

Latest data available: 2019

 

This content is available on request as a pdf in non-accessible format. If you wish for a copy please go to the enquiries page.

 

Categories:

UK Biodiversity Indicators 2020

Published: .

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