Some of the UK’s most threatened butterflies weathered a poor year in 2021 thanks to conservation efforts, annual survey results have shown.
The woodland-loving heath fritillary has doubled in abundance in the past decade, although it is 90% down on 1980 levels. The silver-studded blue also did well, recording its best year since 1996.
The restoration of lost habitats enabled these species to do well despite bad weather, including a cold, drenching May in England. The UK Butterfly Monitoring Scheme, comprising almost half a million records, showed 2021 was a difficult year for overall butterfly abundance, ranking 28th of 46 years in records dating to 1976.
Many common species struggled, including the green-veined white, which had its fourth worst year on record, and the large skipper, which had its fifth worst. Even some widespread species that have increased over the long term fared badly, with the ringlet recording its lowest numbers since 2012.
“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,” said Richard Fox, an ecologist at Butterfly Conservation. “It reinforces the importance of managing and restoring habitat. The heath fritillary is a good example of a species that almost certainly would have gone extinct in Britain by now, if it wasn’t for conservation efforts.”
These efforts include recreating woodland clearings that were once common when people practised coppicing. “There are also grounds for cautious optimism in the results of many other threatened species that are the focus of conservation action,” Fox said. “The black and brown hairstreaks both had a good year in 2021, as did the glanville fritillary, dingy skipper, adonis blue and chalk hill blue.”
The UK Butterfly Monitoring Scheme uses data collected by volunteers from more than 2,900 sites across the UK. When the weather is suitable, the butterfly spotters walk a specified route each week from April to September.
Butterfly populations fluctuate naturally from year to year, largely owing to the weather. The adult insects need warm, dry weather to fly, while caterpillars need to avoid drought, which can kill the plants they feed on.
“They are cold-blooded creatures and rely on the warmth of the sun to warm up to become active,” said Fox. “So if it is really cold, they can’t fly, feed or find a mate. The incredibly bad May will have impacted some butterflies that were in the adult stage then, such as the holly blue.”
The long-term trends for British butterflies are mainly driven by human activity, in particular destruction and degradation of natural habitats by intensive farming. Of the 54 species with long-term data in England, 20 show declines and 12 are more abundant. The pearl-bordered and small pearl-bordered fritillary, for example, are at just a third of their 1970s levels.
“The wholesale destruction of natural habitat has largely stopped now in the UK,” Fox said. “But there is ongoing deterioration of habitat, even on nature reserves.” That was caused by lack of funding as well as pollution and pesticides from farming, he said.
Scotland is bucking the trend, with 12 monitored species showing long-term increases in abundance compared with three showing long-term declines. “The main reason is because there are a whole bunch of butterfly species that are spreading northwards with climate change,” Fox said.
Sarah Harris, at the British Trust for Ornithology, whose volunteers also collect butterfly data, said: “Butterfly species are indicators of the health of our natural environment and therefore can also be used to help understand and protect the wider ecosystem on which so many birds, mammals and other species rely.”