Last year was the third good summer in a row for butterflies and the 10th best since records began, but one-third of Britain’s species are still in long-term decline.
Conservation scientists warned against overstating the butterfly boom, saying perceptions of a “good” year have lowered in the light of plummeting insect numbers.
“Perhaps because of the sunny spring weather last year and the fact that more people were enjoying nature as part of their day-to-day activities, butterflies seemed more numerous,” said Richard Fox, of Butterfly Conservation. “But in fact our baseline experience of the nature around us has changed over time.”
The UK Butterfly Monitoring Scheme (UKBMS), the most comprehensive scientific insect survey in the world, showed that 2020 had the 10th best summer since 1976, continuing a run of above-average summers. The summer of 2019 was the seventh best, and 2018 the 15th best.
Even so, almost half (27) Britain’s 58 resident species had below average numbers last year.
Common species such as the brimstone and orange tip enjoyed good years, as did the rapidly expanding marbled white. Rare butterflies that have been the target of specific conservation action also thrived, including the large blue (its joint second best year), silver-spotted skipper (third best year), silver-studded blue (joint fourth best year) and Duke of Burgundy (joint sixth best year).
The once common small tortoiseshell rallied after four very poor years but still showed a 79% decrease in abundance compared with 1976. Populations of wall, grayling and small skipper all remained low and the small pearl-bordered fritillary had its third worst year on record, with populations down by 68% since records began.
Butterfly populations fluctuate with the weather but conservation scientists expect numbers to build up during a run of sunny and warmer-than-average summers.
“It is worrying that, even after three good years, population levels of so many butterfly species continue to be down compared to 40 years ago, with just under a third (31%) of butterfly species assessed in the UK showing long-term declines,” said Fox.
“We need to be wary of shifting baseline syndrome, whereby we forget (or never experienced) the greater biodiversity that occurred in the UK in former decades and therefore lower our expectations and aspirations for conservation. Here the UKBMS has a vital role to play in showing how insect populations have declined over time.”