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 2007

**Assessment of change in butterfly populations**
	Long term	Since 2000	Latest year
Semi-natural habitat specialists	1976-2007		Decreased (2007)
Generalist species of the wider countryside	1976-2007		Decreased (2007)

Since 1976, the measures for populations of butterflies associated strongly with semi-natural habitats (specialists) and for those found in the wider countryside (generalists) show apparent declines of 70 per cent and 45 per cent respectively.
Large fluctuations in numbers between years are typical features of butterfly populations. The assessment of change is therefore made using analysis of the underlying trends undertaken by Butterfly Conservation.
This analysis shows that since 1976 specialists have declined significantly but for generalists there has been little or no overall change (despite the apparent decline).
Since 2000 specialists show an apparent decline from 37 per cent to 30 per cent of the 1976 level. Generalists have shown a similar apparent decline from 76 per cent to 55 per cent of the 1976 level. The underlying analysis shows that there is little or no overall change for either measure despite the apparent declines.

Indicator assessment

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. For this reason, the assessment of change is based on analysis of underlying trends undertaken by Butterfly Conservation and the Centre for Ecology and Hydrology.

Although the generalist measure has decreased by 45% between 1976 and 2007 although, the analysis of the underlying trend shows that there has been little or no overall change in the long-term (because the apparent decline may simply reflect random fluctuations - see background section). The habitat specialist measure has declined by 70% since 1976 and based on the underlying trend, the long-term change is assessed as ‘deteriorating’.

Since 2000, habitat specialists, despite an increasing trend between 2001 and 2003, have now fallen from 37% of the 1976 baseline to 30%. The generalist measure has fallen over the same period from 76% to 55% of the 1976 baseline. Large fluctuations from year to year are a typical feature of butterfly populations and this makes short term assessments particularly difficult. However, based on the assessment of the underlying trend, neither of these changes are statistically significant and an amber assessment is assigned to both measures (little or no overall change).

Description of trends

The indicator shows that butterfly populations have fluctuated markedly from year to year, principally in response to weather conditions. Despite these fluctuations, the underlying assessment performed by Butterfly Conservation and the Centre for Ecology and Hydrology indicates little or no change between 1976 and 2007. Habitat specialist species, which are vulnerable to habitat loss and fragmentation, have not recovered from population declines experienced in the late 1970s, attributed mainly to drought conditions in 1976. The measure has declined by 70% over the period 1976-2006 although underlying analysis shows that statistically significant changes are restricted to the period 1976-1978. Signs of a possible up-turn, with a 7% increase in the period 2000-2006 have been reversed by further falls in 2007.

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 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 butterfly index compiled by Butterfly Conservation (BC) and the Centre for Ecology and 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 25 habitat specialist butterflies (low mobility species restricted to semi-natural habitats) and 24 generalist (wider countryside) butterflies (Table 1b) using data collected at 1542 sites (Figure 1b (ii)).

The year-to-year fluctuations are often linked to natural environmental variation, especially weather conditions. To identify underlying patterns, 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 (1976-2007) and short-term (2000-2007) changes. These smoothed data are shown in figures 1b (iii) and 1b (iv).

Fig 1b (ii). Location of the 1542 UK Butterfly Monitoring Transects

Figure 1b (iii). Habitat specialist indicator: unsmoothed (black line) and smoothed (red line, red dashed lines give 95% confidence intervals)

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

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.

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

Generalist butterflies	Habitat specialist butterflies
Aglais urticae (Small tortoiseshell)	Apatura iris (Purple emperor)
Anthocharis cardamines (Orange-tip)	Argynnis adippe (High brown fritillary)
Aphantopus hyperantus (Ringlet)	Argynnis aglaja (Dark green fritillary)
Aricia agestis (Brown argus)	Argynnis paphia (Silver-washed fritillary)
Celastrina argiolus (Holly blue)	Aricia artaxerxes (Northern brown argus)
Coenonympha pamphilus (Small heath)	Boloria euphrosyne (Pearl-bordered fritillary)
Erebia aethiops (Scotch argus)	Boloria selene (Small pearl-bordered fritillary)
Gonepteryx rhamni (Brimstone)	Callophrys rubi (Green hairstreak)
Inachis io (Peacock)	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)
Melannargia galathea (Marbled white)	Hesperia comma (Silver-spotted skipper)
Neozephyrus quercus (Purple hairstreak)	Hipparchia semele (Grayling)
Ochlodes venata (Large skipper)	Leptidea sinapis (Wood white)
Pararge aegeria (Speckled wood)	Limenitis camilla (White admiral)
Pieris brassicae (Large white)	Lysandra bellargus (Adonis blue)
Pieris napi (Green-veined white)	Lysandra coridon (Chalkhill blue)
Pieris rapae (Small white)	Melitaea athalia (Heath fritillary)
Polygonia c-album (Comma)	Papilio glaucas (Swallowtail)
Polyommatus icarus (Common blue)	Plebeius argus (Silver-studded blue)
Pyronia tithonus (Gatekeeper)	Pteridium aquilinum (Large heath)
Satyrium w-album (White-letter hairstreak)	Pyrgus malvae (Grizzled skipper)
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 is on-going. There are also plans to further develop 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: 1976-2005	http://www.defra.gov.uk/evidence/statistics/environment/wildlife/kf/wdkf10.htm
UK Butterfly Monitoring Scheme	Butterflies as indicators	www.ukbms.org/butterflies_as_indicators.htm

Download Datasheet

Download Technical background paper

Last updated: March 2009

Latest data available: 2007