C6. Insects of the wider countryside (butterflies)

a. Semi-natural habitat specialists

b. Species of the wider countryside

 

Type: State Indicator

 

Indicator Description

The indicator consists of two measures of annual butterfly population abundance: the first for specialist butterflies (species strongly associated with semi-natural habitats such as unimproved grassland) and the second for 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.

Summary

 

 Since 1976 the habitat specialists butterflies index has fallen by 74%.

 Over the same period the index for species of the wider countryside has fallen by 57%.

 Large fluctuations in numbers between years are typical features of butterfly populations, principally in response to weather conditions.2016 was a bad year for butterflies; short term changes in butterfly populations are affected heavily by weather and although the summer of 2016 was mostly warm and dry in many parts of the UK, there was a very cold spell in spring following a mild winter, which may have negatively influenced populations.

 The statistical assessment of change is made on an analysis of the underlying smoothed trends due to the large fluctuations from year to year.  Since 1976, populations of habitat specialists and species of the wider countryside have declined significantly but both trends show no significant change since 2011.

 

Figure C6ai.  Trends in butterfly populations in the UK: habitat specialists, 1976 to 2016.

Notes:

  1. The figure in brackets shows the number of species included in the index.
  2. The line graph shows the unsmoothed trend (dashed line) and smoothed trend (solid line) with it’s 95% confidence interval (shaded).
  3. The bar chart shows the percentage of species within the indicator that have shown a statistically significant increase, statistically significant decrease, or no change.
  4. The chart is not directly comparable to previous versions of this publication, improvements in the modelling technique have allowed the inclusion of more data, resulting in slight alternations in the trends for individual species.

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

Species fare differently within the overall trend.  Habitat specialist species showing the greatest decline since 1976 include: wood white, Lulworth skipper, pearl-bordered fritillary and heath fritillary. Wood white and Lulworth skipper both show no change in the short term since 2011, whilst pearl-bordered fritillary and health fritillary both continued to decrease. Silver-spotted skipper, dark green fritillary, and large heath show the greatest increases over the long term with all showing no change since 2011.

 

 

Figure C6bi. Trends in butterfly populations in the UK: species of the wider countryside, 1976 to 2016.

Notes:

  1. The figure in brackets shows the number of species included in the index.
  2. The line graph shows the unsmoothed trend (dashed line) and smoothed trend (solid line) with it’s 95 per cent confidence interval (shaded).
  3. The bar chart shows the percentage of species within the indicator that have shown a statistically significant increase, statistically significant decrease, or no change.
  4. Since 2013, an improved analysis method has been applied to the measure for species of the wider countryside (see ‘Background’ section for further information).
  5. The chart is not directly comparable to previous versions of this publication, improvements in the modelling technique have allowed the inclusion of more data, resulting in slight alternations in the trends for individual species.

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

Species of the wider countryside showing greatest decline since 1976 include: white-letter hairstreak, wall, and small tortoiseshell, all of which show no change in the short term since 2011. Ringlet and scotch argus have both increased 3-fold since 1976. Comma also show large increases over the same time period.

 

Assessment of change in butterfly populations

 

Long term

Short term

Latest year

Semi-natural habitat specialists


1976–2016


2011–2016

 

Decreased (2016)

 

Species of the wider countryside


1976–2016


2011–2016

Decreased (2016)

 

Notes: While percentage changes in these indices are reported based on the most recent unsmoothed data point (2016), the formal long-term and short-term assessments of the statistical significance of these changes are made using the smoothed data to 2016.  Analysis of the underlying trends is undertaken by the data providers. 

The species of the wider countryside index decreased by 57% between 1976 and 2016; the underlying analysis indicates that this decrease was chiefly due to a statistically significant reduction in relative abundance over the period 1976 to 2005.  The index showed a decrease over the period 2011 to 2016, from 59% of the 1976 level to 43% in 2016.  However, this change is not statistically significant and an amber assessment is assigned to the measure in the short term.

Habitat specialist species, which are vulnerable to semi-natural habitat loss and fragmentation, have not recovered from population declines experienced in the late 1970s, attributed mainly to drought conditions in 1976.  The habitat specialist index has declined by 74% between 1976 and 2016.  The underlying analysis shows that this decrease was due to a statistically significant reduction in abundance over the period 1976 to 2002.   The index showed a decrease over the period 2011 to 2016, from 48% to 26% of the 1976 level.  However, the change since 2011 is not statistically significant and an amber assessment is assigned to the measure in the short term. 

 

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Last updated: August 2017

Latest data available: 2016

 

 

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