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C6. Butterflies

a. Habitat specialists

b. Species of the wider countryside

Type: State indicator

Indicator Description

This indicator consists of two 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.

Contents

  1. Summary
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Summary

 Since 1976, the unsmoothed habitat specialist butterflies index has fallen by 68% (Figure C6a).

 Over the same period, the unsmoothed index for species of the wider countryside has fallen by 30% (Figure C6b).

 Large fluctuations in numbers between years are typical features of butterfly populations, principally in response to weather conditions. The summer heatwave of 2018 meant that last year was a better year for butterflies in the UK; ranked 18th in the 43-year series, with more than two-thirds of species increasing in annual abundance.

 The statistical assessment of change is made on an analysis of the underlying smoothed trends. Since 1976, populations of habitat specialists and species of the wider countryside have declined significantly but since 2013, both trends show no significant change.

Habitat Specialists

Figure C6a. Trends for habitat specialist butterflies in the UK, 1976 to 2018.A line graph showing how the trend for the UK habitat specialist butterflies index has changed between 1976 and 2018. The smoothed index fell sharply between 1976 and the early 1980s but has since leveled out. The unsmoothed index shows considerable year-on-year variation. A 100% stacked bar chart 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 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.
  4. Since 2017, an improved analysis method has been used to derive the species indices (see ‘Background’ section for further information).
  5. The line graph is not directly comparable to those appearing in previous versions of this publication. Improvements in the modelling technique have allowed the inclusion of more data; this has resulted in slight alterations to the trends for individual species and the composite trend.

Source: Butterfly Conservation, Centre for Ecology & Hydrology, Defra, 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 unsmoothed habitat specialist index declined by 68% between 1976 and 2018 (Figure C6a). Underlying analysis shows that this decrease was due to a statistically significant reduction in relative abundance over the period 1976 to 1996 that was more pronounced in the late 1970s and early 1990s. The index showed an increase over the period 2013 to 2018, from 28% to 32% of the 1976 level, however, this short-term change is not statistically significant. 

Individual butterfly species fare differently within the overall trend. Habitat specialists showing the greatest decline since 1976 include: heath fritillary, wood white, Lulworth skipper, pearl-bordered fritillary and grayling. Lulworth skipper and chalkhill blue show a short-term decline since 2013. Silver-spotted skipper, large heath, dark green fritillary, adonis blue and silver-washed fritillary show significant increases over the long term, whilst black hairstreak is the only habitat specialist species to show a statistically significant increase since 2013.

Species of the wider countryside

Figure C6b. Trends for butterflies of the wider countryside in the UK, 1976 to 2018.

A line graph showing how the trend for the UK butterflies in the wider countryside index has changed between 1976 and 2018. The smoothed index remained relatively stable between 1976 and the early 1990s but has fallen steadily since then. The unsmoothed index shows considerable year-on-year variation. A 100% stacked bar chart showing the percentage of individual species within the UK butterflies of the wider countryside index that have shown a statistically significant increase, a statistically significant decrease or no statistically significant 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. This indicator includes individual measures for 25 species of butterflies; the wider countryside index, however, only includes 24 trends. This is because an aggregate trend is used for small skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris); these 2 species have been combined due to historical difficulties with distinguishing 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.
  5. Since 2017, an improved analysis method has been used to derive the species indices (see ‘Background’ section for further information).
  6. The line graph is not directly comparable to those appearing in previous versions of this publication. Improvements in the modelling technique have allowed the inclusion of more data; this has resulted in slight alternations to the trends for individual species and the composite trend.

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

The unsmoothed species of the wider countryside index decreased by 30% between 1976 and 2018 (Figure C6b); the underlying analysis indicates that this decrease was chiefly due to a statistically significant reduction in abundance over the period 1976 to 2001. The index showed an increase over the period 2013 to 2018, from 67% to 70% 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: white-letter hairstreak, wall, small tortoiseshell, purple hairstreak and small heath; small tortoiseshell and peacock both show a short-term decline since 2013. Ringlet, comma, speckled wood and marbled white all show increases over the long term; holly blue is the only species of the wider countryside to show a short-term increase since 2013.

Assessment of change in butterfly populations

 

Long term

Short term

Latest year

Habitat specialists


1976–2018


2013–2018

Increased (2018)

Species of the wider countryside


1976–2018


2013–2018

Increased (2018)

Note:

While percentage changes in these indices are reported based on the most recent unsmoothed data point (2018), the formal long-term and short-term assessments of the statistical significance of these changes are made using the smoothed data to 2018. Analysis of the underlying trends is undertaken by the data providers. 

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Downloads

Download the Fiche, Datasheet and Technical background document from the JNCC Resource Hub

Last updated: September 2019

Latest data available: 2018

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Categories:

UK Biodiversity Indicators 2019

Published: .

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