C6. Insects of the wider countryside (butterflies)

The UK butterfly indicator was published on 29 June at the following weblink: https://www.gov.uk/government/statistics/butterflies-in-the-wider-countryside-uk. This webpage will be edited as part of the process of updating the UK biodiversity indicators and will be updated on 3 August 2017.

 

a. Semi-natural habitat specialists

b. Species of the wider countryside

 

Type: State Indicator

 

Summary

Figure C6ai.  Trends in butterfly populations in the UK: habitat specialists, 1976 to 2014.

Figure C6ai. Trends in butterfly populations in the UK: habitat specialists, 1976 to 2014

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 its 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.

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

 

 

Figure C6bi. Trends in butterfly populations in the UK: species of the wider countryside, 1976 to 2014.

Figure C6bi. Trends in butterfly populations in the UK: species of the wider countryside, 1976 to 2014.

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 its 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).

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

 

Assessment of change in butterfly populations

 

Long term

Short term

Latest year

Semi-natural habitat specialists

indicator declining
1976–2014

indicator stable
2009–2014

Increased (2014)

Species of the wider countryside

indicator declining
1976–2014

indicator stable
2009–2014

Decreased (2014)

Notes: While percentage changes in these indices are reported based on the most recent unsmoothed data point (2014), the formal long-term and short-term assessments of the statistical significance of these changes are made using the smoothed data to 2014.  Analysis of the underlying trends is undertaken by the data providers. 

 

  • Since 1976, the indices for butterflies strongly associated with semi-natural habitats (habitat specialists) and for those found in the wider countryside have decreased by 61 per cent and 41 per cent respectively.
  • Large fluctuations in numbers between years are typical features of butterfly populations.  The statistical assessment of change is therefore made on an analysis of the underlying smoothed trends.
  • This analysis shows that, since 1976, populations of habitat specialists and species of the wider countryside have declined significantly.
  • The unsmoothed data for habitat specialists show an increase between 2009 and 2014, whilst species of the wider countryside show a decrease over the same period.  However, the underlying analysis of the smoothed trend shows that these increases are not significant; there has been no overall change in either of the two indices in the five years to 2014.
  • In the most recent year, 2014, habitat specialist butterflies increased by 6.7 per cent from the previous year, whilst wider countryside species decreased by 8.0 per cent.
 

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.  Both measures show marked fluctuations from year to year, principally in response to weather conditions.  For this reason, the assessment of change is based on an analysis of underlying smoothed trends and undertaken by Butterfly Conservation and the Centre for Ecology & Hydrology.

The species of the wider countryside index decreased by 41 per cent between 1976 and 2014; the underlying analysis indicates that this decrease was chiefly due to a statistically significant reduction in relative abundance over the period 1976 to 2002.  The index showed a decrease over the period 2009 to 2014, from 61 per cent to 59 per cent of the 1976 level.  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 61 per cent between 1976 and 2014.  The underlying analysis shows that this decrease was due to a statistically significant reduction in abundance over the period 1976 to 1980.  There is some suggestion of recent recovery in habitat specialists from 2009–2014, with an increase from 35 per cent to 39 per cent of the 1976 level.  However, the change since 2009 is not statistically significant and an amber assessment is assigned to the measure in the short term. 

 

 

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 an indicator, especially the habitat specialists, because they use resources in the landscape at a much finer spatial scale than either of these groups.

 

Background

This indicator is a multi-species index compiled by Butterfly Conservation (BC) and the Centre for Ecology & Hydrology (CEH) from data collated through the UK Butterfly Monitoring Scheme (UKBMS) and, for the wider countryside measure, the Wider Countryside Butterfly Monitoring Scheme (WCBS).

The indicator shows individual measures for 26 habitat specialist butterflies (low mobility species restricted to semi-natural habitats) and 25 butterflies which use habitats across the wider countryside using data collected at 4,040 sites (Figure C6i).  The wider countryside measure, however, includes 24 trends, as an aggregate trend is used for Small Skipper (Thymelicus lineola) and Essex skipper (Thymelicus sylvestris), which have been combined due to difficulties in distinguishing them in the field.

The year-to-year fluctuations of butterflies are often linked to natural environmental variation, especially weather conditions.  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 habitat chart above shows the percentage of species within that indicator that have shown a statistically significant increase, a statistically significant decrease or no change.

A table listing the species that are included in each index can be found in the Datasheet. For each species, an estimate of change in the long-term and in the short-term is given, as well as an assessment of whether the species trend is increasing or decreasing.

Since 2013, an improved analytical method has been applied to the measure for species of the wider countryside.  The new method makes more efficient use of available data to generate more precise and representative indices and trends.  The new method uses all butterfly counts collected at both UKBMS sites (2,383 compared with 2,330 in 2013) and randomly selected 1km squares of the Wider Countryside Butterfly Survey (1,657).  The method uses all butterfly counts in a season to estimate the seasonal pattern of butterfly counts for that year, and uses this to weight observed counts when calculating annual population indices.  Further details of the method can be found in Dennis et al. (2013).  The new trend for species of the wider countryside is similar to that generated through the old analysis, though population indices are at times lower as on average additional wider countryside sample sites support lower densities of butterflies.  In 2015, the site index only data was 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 2014 extra data were added for 2012 and 2013, 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 UK Butterfly Monitoring Scheme website and in the technical background paper provided at the end of this indicator.

 

Figure C6i.  Locations of the 2,383 UK Butterfly Monitoring Scheme sites (blue) and 1,657 Wider Countryside Butterfly Monitoring Scheme squares (red).

Figure C6i. Locations of Butterfly Monitoring Scheme sites

 

 

In 2015-16, Defra commissioned a project (reference BE0112), to provide evidence statements to accompany a number of species trend indicators and an overview of the causes of biodiversity change. The output from this project is contained in a methods report, drivers of change report, and summary of evidence.  These are summarised indicator by indicator in a set of Evidence Statements, which aim to ensure that interpretation of trends, casual factors and relationship to policy interventions is rigorous, objective and reflects scientific consensus.

In parallel with the Evidence Statements, Defra also commissioned a Quality Assurance Panel to provide advice on improvements that could be considered to the species based indicators in the UK and England biodiversity indicator sets. The report of the review has led to an action plan of changes to be made as resources allow.

 

Further development planned

Development of country and regional breakdowns is ongoing.  There are also plans to implement smoothing by using Generalised Additive Models, the technique adopted for birds and bats, to make the assessments more comparable between indicators.  The indicator may be further revised if improved methodologies are developed and applied retrospectively to earlier years.

 

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 icon 7Target 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 icon 12Target 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 icon 5Target 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 icon 11Target 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.

 

 

Web links for further information

Reference

Title

Website

UK Butterfly Monitoring Scheme

Butterflies as indicators

http://www.ukbms.org/indicators.aspx

Defra Evidence Statements project: Method report http://sciencesearch.defra.gov.uk/Document.aspx?Document=13933_BE0112FinalReport.pdf
Defra Evidence Statements project: Drivers of Change http://sciencesearch.defra.gov.uk/Document.aspx?Document=13945_Annex2_DriversOfChange.pdf
Defra Evidence Statements project: Summary of Evidence http://sciencesearch.defra.gov.uk/Document.aspx?Document=13947_Annex3_SummaryOfEvidence.pdf
Defra Quality Assurance Science Panel report View QASP report

 

References

Dennis, E.B., Freeman, S.N., Brereton, T. & Roy, D.B. 2013. Indexing butterfly abundance whilst accounting for missing counts and variability in seasonal pattern. Methods in Ecology and Evolution, 4(7), 637–645.

 

 

Download Datasheet

Download Technical background paper

Download Evidence statements for

 

Last updated: November 2016

Latest data available: 2014