Butterfly Numbers: A Mixed Story of Rise and Fall

Recent data from one of the world's largest insect monitoring programs shows a complicated story about butterflies in the United Kingdom. Warmer and sunnier weather over the last fifty years, partly driven by climate change, has helped some butterfly species flourish. However, the overall picture remains troubling. Many of the UK's most distinctive butterflies are in steep decline, meaning their numbers are dropping very fast.

These findings come from the UK Butterfly Monitoring Scheme, known as UKBMS. Since 1976, volunteers have contributed over 44 million records from 782,000 surveys. This makes the scheme one of the largest and longest-running citizen science projects of its kind. The data covers 59 native species. Of these, 33 have declined in number, 25 have improved, and data is too limited to assess one mountain species.

Several butterfly species are doing well. The Red Admiral is a prime example. It is now spending its winters in the UK as the climate warms. The Comma butterfly, known for the ragged edges of its wings, has recovered in numbers since the survey began. Orange tip butterfly numbers are up by more than 40% since 1976. Conservation work has also helped the Black Hairstreak, one of the UK's rarest butterflies, to recover. Even the Large Blue butterfly has done well thanks to conservation efforts. It was declared extinct in 1979 but has since come back.

The charity Butterfly Conservation says the results highlight a growing divide. This divide exists between adaptable species and specialists. Butterflies that can thrive in many environments, like farmland, parks, and gardens, are generally coping better. Some are even increasing in number.

Warmer conditions linked to climate change are helping this trend. The warm weather boosts survival rates for flexible species. It also allows them to extend their geographic range and lengthen their breeding seasons. Professor Jane Hill, a butterfly expert at the University of York, calls the data collected over five decades "extraordinary." She says it represents a gold standard for wildlife surveys worldwide. She explains that because butterflies are cold-blooded insects, they generally do better in warmer conditions.

"Most British butterflies reach their northern range limit in the UK," she adds. "This means they have opportunities to expand further north into northern England and Scotland."

Butterflies whose lifecycles depend on specific habitats are struggling. These habitats include woodland clearings or chalk grasslands. Many of these specialist butterflies are declining at alarming rates. This happens as their special environments come under pressure. They cannot expand their ranges because there are not enough suitable new habitats for them to colonize.

Some of the losses have been dramatic. The white-letter hairstreak butterfly has fallen by 80% since the monitoring scheme began. Its caterpillars glow under ultraviolet light. The pearl-bordered fritillary, a striking orange-and-black butterfly, has declined by 70%. Its caterpillars feed only on violets. Even among more adaptable butterflies, the picture is mixed. The once-common small tortoiseshell butterfly, for example, has declined by 87%.

Professor Richard Fox is head of science at Butterfly Conservation. He said, "Just as we have lost family-run shops and traditional skills from the nation's high streets, we've lost variety and diversity in the butterfly communities that can exist in our damaged and simplified landscapes."

The huge dataset reflects a massive public effort. Volunteers have walked more than 932,000 miles in total. They have surveyed over 7,600 sites. Their work provides the evidence needed to understand what is happening to nature.

Steve Wilkinson is the director of the Joint Nature Conservation Committee. The committee advises the UK governments and helps run the UKBMS. He said, "Without this evidence timeline, we would be flying blind."

He added, "Understanding where conservation efforts are making a real difference and where we need to strengthen efforts depends entirely on the quality and continuity of data that our volunteers make possible."

Much conservation work focuses on protecting and enlarging the habitats butterflies need. This is especially important in the face of land-use changes. These changes include the intensification of farming and environmental degradation. The work is made even more challenging by how picky some butterfly species and their caterpillars are about food. Many species have evolved to rely on just one or two specific plant species. For example, the Duke of Burgundy butterfly relies on primroses and cowslips. The purple emperor butterfly relies on goat or grey willow.

This is why Butterfly Conservation's Magdalen Hill Downs reserve tries to sustain a range of different habitats. Fiona Scully is the charity's reserves officer. She gestured across the chalky fields covered with blooming cowslips. She listed just a few of the native plants that thrive there.

"Lady's bedstraw, toadflax, betony, scabious, knapweed – we've just got so many," she said.

She says it is this variety of plants that makes the site such a stronghold for butterflies. Recent results from the UK Butterfly Monitoring Scheme highlight the scale of the challenge. In 2025, the UK experienced its sunniest year on record. Sunny conditions are typically favorable for butterflies. Yet 2025 ranked only as an average year, the 20th best out of the past 50. No butterfly species recorded its best year.

This pattern matches findings from Butterfly Conservation's Big Butterfly Count. That project saw record participation from more than 125,000 people. Yet it reported only average butterfly numbers per count. This shows that even with good weather, the recovery of butterfly populations remains a significant challenge. The complex needs of these insects mean that warm weather alone cannot fix the problems caused by habitat loss and farming changes.