Hot, dry summer impacts UK butterfly populations

The heatwave and drought of summer 2022 has had a major negative impact on some UK butterfly species, a study has confirmed.

The latest 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 the extreme weather experienced across much of the UK in the summer of 2022 has had a significant impact on some butterfly species.

The Green-veined White, Small White, Small Tortoiseshell, Peacock and Brimstone, all familiar garden and countryside butterflies, appeared in good or average numbers during the spring and early summer, but numbers in the subsequent generations were greatly reduced following the widespread drought conditions, and scientists fear that there will be similar negative impacts on other butterfly species when they start to emerge in 2023.

Drought impacts the offspring of the butterflies that are flying during the hot dry weather by causing the plants that caterpillars rely on for food to wither and die. Without sufficient food, many caterpillars will fail to survive, leading to lower numbers of butterflies in the next generation.

For some of the UK species that have more than one generation in a year, the resulting major decline in numbers has already been seen. However, for others, the next generation isn’t on the wing until this summer, meaning there could be noticeably fewer butterflies around in 2023.

It’s not the first time butterflies have been affected by drought in the UK. Data gathered by the UKBMS has shown serious negative impacts of droughts on butterflies in 1976 and 1995. Some species have never recovered their former abundance levels after the 1976 drought, although habitat destruction is likely to be a major factor in their failure to bounce back.

The difference in 2022’s drought compared to 1976 is that nowadays most UK butterfly species are already in decline. Butterfly Conservation’s recently published State of the UK’s Butterflies 2022 report revealed that 80% of butterflies have declined in abundance, distribution or both since the 1970s.

With droughts predicted to become more frequent and severe in the UK as climate change continues, some butterfly species may be pushed towards extinction.

Butterfly Conservation’s Head of Science, Dr Richard Fox, said: “Overall, the data for 2022 tells us that it was an average year for butterflies. However, it was a year of two halves with butterflies seen early and in about average total numbers (compared to the last 10 years) from April – July but then in greatly reduced abundance after the summer heatwave and drought.

“In general, warm, sunny weather is good for butterflies as they can be active, finding food, mating and laying eggs. But drought is a major problem as plants wither and die, meaning female butterflies may struggle to find anywhere to lay their eggs, or there is not enough food for the caterpillars when they hatch.

“The knock-on effect is fewer butterflies in the following generation. We have already seen an indication of this in the 2022 data for some of those species with a generation that flies in late summer and autumn, and sadly we can expect to see a decline in numbers of other species in 2023 too.”

The meticulously gathered UKBMS data show that, despite these concerns, 2022 was a good year for some species, including Purple Emperor, Large Blue, Chequered Skipper and Dark Green Fritillary, all of which have been the focus of targeted conservation work over the last few years. However, scientists issued a word of caution, as we have yet to see the result of the drought on these species because the next generation will not emerge until this summer.

There is much work to be done to conserve threatened species of butterfly in the UK, and the UKBMS data helps target those species most in need of conservation work.

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: “In 2022 we collected data from over 3,000 sites and we are incredibly grateful to the thousands of volunteers who 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.”

Nature Minister Trudy Harrison said: “Our beautiful butterflies are at risk from a changing climate and human activity. We have set out clear steps to enable nature’s recovery through our Environmental Improvement Plan and Pollinator Action Plan, which will help us meet our legal target to halt the decline in nature by 2030.

“The impacts of warm and dry weather last year are cause for concern, but it is encouraging that the last decade has seen most species stabilise. In particular, I welcome the findings that the Large Blue and the Purple Emperor, two wonderful and unique species people may recognise from Wild Isles, both experienced their second best year on record in 2022.”

Dr James Heywood, Breeding Bird Survey National Organiser at the British Trust for Ornithology, whose volunteers collect butterfly data through the Wider Countryside Butterfly Survey, said: “These data are incredibly valuable as butterflies 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.”