Threatened butterfly species cope well in below-average butterfly year

At a time when much of the natural world is in crisis, some butterfly species are showing signs of recovery, likely the result of concentrated conservation effort and positive land management.

Results from the annual UK Butterfly Monitoring Scheme (UKBMS) led by Butterfly Conservation, the UK Centre for Ecology & Hydrology (UKCEH), British Trust for Ornithology (BTO) and Joint Nature Conservation Committee (JNCC), show that while the overall picture for butterflies in 2021 was poor, species that are the focus of targeted conservation efforts have fared well.

After three good years in a row, 2021 was a below average year for UK butterflies, and the worst since 2017. Following one of the coldest and wettest Mays on record, especially in England and Wales, many common and widespread species did poorly. The Green-veined White had its fourth worst year on record and Large Skipper its fifth worst. The Common Blue, Large White, and Small Skipper also had a poor year, and even some widespread species that have shown long-term increases fared badly in 2021, with the Ringlet recording its lowest numbers since 2012.

However, despite the context of the generally poor year, there were some promising results for many threatened species. The endangered Heath Fritillary, which has been the focus of long-term intensive conservation efforts in Kent, Essex and Somerset had a good year and has now increased 112% at monitored sites in the last decade.

Butterfly Conservation’s Associate Director of Recording and Monitoring, Dr Richard Fox, explains: “We’re delighted to be seeing some positive signs for species such as the Heath Fritillary, especially when the general long-term picture for UK butterflies is one of great decline. It reinforces the importance of managing and restoring habitat in a way that supports the survival of our butterflies. While the Heath Fritillary remains a priority for conservation, these successes demonstrate what can be achieved through dedicated long-term conservation effort.”

The meticulously gathered UKBMS data show that 2021 was also a good year for the Silver-studded Blue, a butterfly that is classed as vulnerable in Britain. It is another species that has benefitted from much conservation work on its heathland and grassland habitats. 2021 was its best year since 1996 and its numbers have increased by 70% at monitored sites since the 1970s, with it having six above average years in a row.

Dr Fox adds: “There are grounds for cautious optimism in the results of many other threatened species that are the focus of conservation action, especially given that 2021 wasn’t a good year for butterflies in general. The Black and Brown Hairstreaks both had a good year in 2021, the latter its best since 1996, as did Glanville Fritillary, Dingy Skipper, Adonis Blue and Chalk Hill Blue, all of which were above their long-term average abundance.”

The efforts of Butterfly Conservation, along with other organisations, volunteers and landowners are providing resilience for threatened butterflies in years, such as 2021, when poor weather is a problem for most species. However, there is much more to do, and the UKBMS data helps target those species most in need of conservation work. Many species still show major long-term decline. White Admiral, for example, had a terrible 2021, recording its third worst year since 1976 at monitored sites, with all three worst years taking place in the last decade. Pearl-bordered and Small Pearl-bordered Fritillary also both continued at low levels, well below their long-term averages.

Butterfly populations fluctuate naturally from year to year, largely due to the weather, but the long-term trends of UK butterflies are mainly driven by human activity, particularly the deterioration of habitats due to inappropriate management and pollution, and climate change. Conservation efforts can make a real difference to local populations and working on threatened species in key landscapes to deliver nature recovery is a priority.

Dr Marc Botham, Butterfly Ecologist at the UK Centre for Ecology & Hydrology, said: “Despite 2021 continuing to be a challenging year for data gathering and conservation activity, >we received 476,000 records from more than 2,900 sites over the year, including a record number of standard transects.

“We are incredibly grateful to the thousands of volunteers who were able to carry out monitoring and maintain this invaluable long-term dataset. This enables scientists to measure how butterflies are faring as well as assessing the health of our countryside generally. The UKBMS data are vital in assessing the effectiveness of government policies and progress towards the UK’s biodiversity targets.”

Sarah Harris, Breeding Bird Survey National Organiser at the British Trust for Ornithology, whose volunteers collect butterfly data through the Wider Countryside Butterfly Survey, said: "The results of the 2021 survey really highlight the value of long-term monitoring, since it is these data that help us understand both the long and short-term trends in butterflies. These species are indicators of the health of our natural environment and therefore the information gleaned from the UKBMS data is not just used to help understand and conserve butterflies, but also to help understand and protect the wider ecosystem on which so many birds, mammals and other species rely.”

UK country differences (*note that not all species have trends in each country and the time periods of trends vary)

England

2021 was a poor year for butterflies in England, ranking 33^rd out of the 46 years since the UKBMS began.

It was a particularly terrible year for Green-veined White, is one of our most widespread butterfly species, which suffered its joint third worst year since 1976. White Admiral also had its third worst year on record and its populations have now decreased by 62% in England since the UKBMS began.

After four consecutive years of increase, Large Skipper slumped to its worst numbers since 2012. Gatekeeper had another disappointing year, and this species has now not reached above average numbers since 2007.

It was a below average year for many other common butterflies such as Large White, Small Copper, Common Blue and Holly Blue. Even Ringlet numbers, which have increased by 335% in England since 1976, had its worst year since 2012.

In contrast, Dark Green and Glanville Fritillaries both had their best year on record in England. Brown Hairstreak, Black Hairstreak and Chalk Hill Blue all had their third best year, and Silver-studded Blue and Red Admiral has their fourth best year since monitoring began.

Of the 54 butterfly species for which English trends could be produced, 12 show long-term increases in abundance and 20 show long-term declines.

Scotland

Scotland was the only UK country in which butterflies fared well overall in 2021, with more species increasing than decreasing from 2020 levels. The year ranked 10^th out of 43 years with UKBMS monitoring data in Scotland.

Small Pearl-bordered Fritillary and Pearl-bordered Fritillary did very well, having their best and second-best years on record respectively. Both have significantly increased in Scotland since monitoring began (although that was only in 2002 for Pearl-bordered Fritillary), in contrast to severe long-term declines in England and Wales. Northern Brown Argus also had a good year and Marsh Fritillary numbers showed the largest year-on-year increase in Scotland since 2013.

Three species that have expanded their ranges in Scotland over recent decades enjoyed another good year: Wall had its best year on record in Scotland, while both Speckled Wood and Ringlet had their 3rd best years. In contrast to England and Wales, Large White had a good year in Scotland in 2021 and Green-veined White and Ringlet numbers showed little change from 2020.

Small Copper registered another poor year in Scotland and the long-term trend for this species is now a significant 67% decrease. Small Tortoiseshell numbers dipped back below average after a very good year in 2020 but remained well above the low counts reported in 2016-2018. Red Admiral numbers were lower than in recent years, in contrast to a bumper year in England.

Of the 24 butterfly species for which Scottish trends could be produced, 12 show long-term increases in abundance and three show long-term declines.

Wales

The year 2021 was an average year for butterflies in Wales, ranking 24th out of 46 years since monitoring began.

After two above average years, Dingy Skipper recorded its worst year on record in Wales in 2021 (although the trend for this species only starts in 2004). Small Skipper has its worst year since 1989 and its second worst ever, and Large Skipper also fared poorly. Peacock and Small Tortoiseshell numbers both dropped by more than 50% in relation to the good counts in 2020. Although populations of Green-veined White dropped to their lowest levels since 2007, the species still had a better year in Wales than it did in England. Ringlet also had its worst year in Wales since 2007.

Small Pearl-bordered Fritillary and Small Heath bounded back from very poor counts in 2020 to reach above average numbers in 2021 and Pearl-bordered Fritillary also showed a small year-on-year increase.

Of the 31 butterfly species for which Welsh trends could be produced, eight show long-term increases in abundance and seven show long-term declines.

Northern Ireland

Overall, 2021 was an average year for butterflies in Northern Ireland, ranking 10^th of the 18 years for which trends can be calculated.

Cryptic Wood White and Orange-tip both had their third worst years on record in Northern Ireland but, in stark contrast to England and Wales, numbers of Large, Small and Green-veined Whites all increased in numbers after a couple of poor years. Ringlet numbers in Northern Ireland also bucked the trend seen in England and Wales, with an increase compared with 2020. Speckled Wood had its joint second-best year and Small Tortoiseshell its third best, despite a substantial fall in numbers from the record levels in 2020.