C6. Insects of the wider countryside (butterflies)

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.

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.

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).

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).

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.