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1b. Trends in populations of selected species (butterflies)

i) semi-natural habitat specialists

ii) species of the wider countryside

Focal Area: Status and trends in the components of biological diversity

Type: State Indicator

Summary

Figure 1b (i). Trends in butterfly populations for habitat specialists and generalist (wider countryside) species, 1976 to 2010

Assessment of change in butterfly populations
	Long term	Since 2000	Latest year
Semi-natural habitat specialists	1976-2010		Increased (2010)
Generalist species of the wider countryside	1976-2010		No change (2010)

Since 1976, the indices for butterflies associated strongly with semi-natural habitats (specialists) and for those found in the wider countryside (generalists) show apparent declines of 65 per cent and 23 per cent respectively.
Large fluctuations in numbers between years are typical features of butterfly populations. The assessment of change is therefore made on an analysis of the underlying trends undertaken by Butterfly Conservation and the Centre for Ecology & Hydrology.
This analysis shows that since 1976 specialists have declined significantly but for generalists there has been little or no overall long-term change.
Since 2000, specialists have shown a small increase from 29 per cent to 35 per cent of the 1976 level. Generalists have shown a small decrease from 79 per cent to 77 per cent of the 1976 level. However, the underlying analysis shows that there was little or no overall change for these measures.
In 2010, specialists increased by 30 per cent over the previous year, whilst there was no change for generalists.

Indicator description

The indicator shows two measures of the annual populations of specialist butterflies (strongly associated with semi-natural habitats such as unimproved grassland) and generalist butterflies found in the wider countryside. The measures show marked fluctuations from year to year, principally in response to weather conditions. For this reason, the assessment of change is based on analysis of underlying trends undertaken by Butterfly Conservation and the Centre for Ecology & Hydrology⁷.

The generalist measure decreased by 23 per cent between 1976 and 2010, although the analysis of the underlying trend indicates that this change could be due to natural variation. Consequently, an assessment of little or no overall change is given.

Habitat specialist species, declined by 65 per cent between 1976 and 2010. These species have not recovered from population declines experienced in the late 1970s, attributed mainly to drought conditions in 1976.

There is some evidence of a recent recovery in habitat specialists, with an increase from 29 per cent to 35 per cent of the 1976 level between 2000 and 2010. The generalist measure decreased slightly over the same period, from 79 per cent to 77 per cent of the 1976 level. However, neither of these changes were statistically significant and an amber assessment is assigned to both measures. Due to large fluctuations in butterfly numbers between years, short-term assessments of trends are often uncertain.

⁷See accompanying technical background paper.

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, which collectively account for more than 50 per cent of the UK's terrestrial wildlife species. Butterflies are complementary to birds and bats as an indicator because they use resources in the landscape at a much finer spatial scale.

Background and data sources

This indicator is a multi-species index compiled by Butterfly Conservation (BC) and the Centre for Ecology & Hydrology (CEH) on behalf of the Department for Environment Food and Rural Affairs (Defra) and the Joint Nature Conservation Committee (JNCC), chiefly from data collated through the UK Butterfly Monitoring Scheme (UKBMS).

The indicator shows individual measures for 26 habitat specialist butterflies (low mobility species restricted to semi-natural habitats) and 25 generalist (wider countryside) butterflies (Table 1b) using data collected at 1,859 sites (Figure 1b(ii)).

The year-to-year fluctuations are often linked to natural environmental variation, especially weather conditions. To identify underlying patterns in the changes, the assessment of trends is based on indices smoothed by structural time series modelling with a measure of precision (confidence intervals) estimated by applying the Kalman filter. For each measure a statistical test has been applied to compare the difference between the current year index value and the index value for each year of the preceding years, and to quantify long-term (1976-2010) and short-term (2000-2010) changes.

Within the measures, each species 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. Further details of the methods used can be found on the UK Butterfly Monitoring Scheme website.

The indicator has been revised to include one additional habitat specialist and one additional generalist, for which data are now available.

Figure 1b (ii). Location of the 1859 UK Butterfly Monitoring Scheme sites

Figure 1b (iii). Habitat specialists indicator (26 species): unsmoothed (black line) and smoothed (red line, red dashed lines give 95 per cent confidence intervals)

Figure 1b (iv). Wider countryside butterfly indicator (25 species): unsmoothed (black line) and smoothed (red line, red dashed lines give 95 per cent confidence intervals)

Table 1b (i). Species used in each of the measures for the UK butterfly indicator

Generalist butterflies	Habitat specialist butterflies
Aglais io (peacock)	Apatura iris (purple emperor)
Aglais urticae (small tortoiseshell)	Argynnis adippe (high brown fritillary)
Anthocharis cardamines (orange-tip)	Argynnis aglaja (dark green fritillary)
Aphantopus hyperantus (ringlet)	Argynnis paphia (silver-washed fritillary)
Aricia agestis (brown argus)	Aricia artaxerxes (northern brown argus)
Celastrina argiolus (holly blue)	Boloria euphrosyne (pearl-bordered fritillary)
Coenonympha pamphilus (small heath)	Boloria selene (small pearl-bordered fritillary)
Erebia aethiops (scotch argus)	Callophrys rubi (green hairstreak)
Favonius quercus (purple hairstreak)	Coenonympha tullia (large heath)
Gonepteryx rhamni (brimstone)	Cupido minimus (small blue)
Lasiommata megera (wall)	Erynnis tages (dingy skipper)
Lycaena phlaeas (small copper)	Euphydryas aurinia (marsh fritillary)
Maniola jurtina (meadow brown)	Hamearis lucina (Duke of burgundy)
Melanargia galathea (marbled white)	Hesperia comma (silver-spotted skipper)
Ochlodes sylvanus (large skipper)	Hipparchia semele (grayling)
Pararge aegeria (speckled wood)	Leptidea sinapis (wood white)
Pieris brassicae (large white)	Limenitis camilla (white admiral)
Pieris napi (green-veined white)	Melitaea athalia (heath fritillary)
Pieris rapae (small white)	Papilio machaon (swallowtail)
Polygonia c-album (comma)	Plebejus argus (silver-studded blue)
Polyommatus icarus (common blue)	Polyommatus bellargus (adonis blue)
Pyronia tithonus (gatekeeper)	Polyommatus coridon (chalkhill blue)
Satyrium w-album (white-letter hairstreak)	Pyrgus malvae (grizzled skipper)
Thymelicus lineola (Essex skipper) ^new	Satyrium pruni (black hairstreak) ^new
Thymelicus sylvestris (small skipper)	Thecla betulae (brown hairstreak)
	Thymelicus acteon (Lulworth skipper)

Further development planned

Annual updates are planned. Development of country and regional breakdowns and further climate change indicators is on-going. There are also plans to test data smoothing methods to account for the high degree of variability in the data and provide more accurate assessments based on underlying trends. The indicator may be revised if improved methodologies are developed and applied retrospectively to earlier years.

Web links for further information

Reference	Title	Web site
Defra Statistics	Populations of butterflies: 1976-2007	http://archive.defra.gov.uk/evidence/statistics/environment/wildlife/kf/wdkf10.htm
UK Butterfly Monitoring Scheme	Butterflies as indicators	http://www.ukbms.org/butterflies_as_indicators.htm

Download Datasheet

Download Technical background paper

Last updated: May 2011

Latest data available: 2010