As Spring ushers in warm weather and sunshine, butterflies will be emerging - Small Tortoiseshell from hibernation, Speckled Wood and Orange Tip from their over-wintered chrysalides. While we look forward to this year’s butterflies, the United Kingdom Butterfly Monitoring Scheme (UKBMS) has released its Official Statistic for 2021, revealing patterns and trends for 56 species that regularly occur across the UK. 

The UKBMS is one of the longest-running insect monitoring schemes in the world, with annual data reaching back to 1976. Since 2009, the Wider Countryside Butterfly Survey - undertaken by many BTO/JNCC/RSPB Breeding Bird Survey volunteers - has boosted the data for under-recorded areas like uplands and plantation woodlands. The information collected improves our understanding of the ways in which butterfly populations are affected both year-to-year, by annual changes in weather, and over many years through long-term trends visible through the annual fluctuations. 

Despite following three excellent years for butterflies, 2021 proved to be below-average for these insects. Common and widespread species such as the Green-veined White, Common Blue and Small Skipper all suffered some of their worst years on record, while Holly Blue, Clouded Yellow and Purple Hairstreak were recorded in numbers around half of those in 2020. Several species that have been steadily declining for many years continued in this trend, with White Admiral, Pearl-bordered Fritillary and Small Pearl-bordered Fritillary being recorded in particularly low numbers. The poor numbers were likely caused by an unusually wet Spring in England and Wales. 

UKBMS data also reveals the causes of the long-term declines. The Official Statistic for 2021 describes the reduction and degradation of habitats as key drivers of dwindling populations, as agricultural intensification, changes in forestry practices, urban development and pollution impact our landscape to a greater and greater extent. Climate change is also likely to play a key role in changing butterfly populations, but its effect is hard to quantify due to the interwoven nature of its impacts.

In the face of these declines, the Official Statistic offers good news for several species of conservation concern. UKBMS data shows the Heath Fritillary increasing by 112% in the last ten years, likely due to long-term, targeted conservation efforts. The Silver-studded Blue, which has also been the focus of conservation, had its best year since 1996, demonstrating that conservation can act as a buffer for vulnerable species even in years when common and widespread species struggle. These stories are powerful evidence for implementing targeted conservation measures for our vulnerable species. 

The data collected by UKBMS volunteers help inform conservation measures such as these on a national and international scale. The Official Statistic, for example, informs UK biodiversity indicators as well as supporting progress targets such as those set by the Convention on Biological Diversity. The value of the data is in large part due to the number of sites that are sampled, something which would not be possible without the time and skill of the citizen scientists who contribute to the scheme. 

Help survey butterflies in 2022

Join the Wider Countryside Butterfly Survey on your BBS square, or sign up to survey alternative sites, and contribute to one of the world’s longest-running monitoring programmes.

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