UK butterflies continue to decline

The population of the once-common small tortoiseshell fell by 25% in three years

The British butterfly population is continuing a marked downward trend.

This is according to a national survey which revealed that numbers of the insects fell by more than 20% between 2010 and 2011.

The results, announced by the charity Butterfly Conservation, appear to contrast with a recent study revealing a boom in numbers of rare UK species.

But while rare species may thrive in Britain's "pollinator hot spots", the more general outlook appears bleak.

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Top five butterflies

• The meadow brown was the most widespread species, found in more than 80% of the areas surveyed in 2011
• The small white was the second most commonly seen species; in just over 75% of the squares. This is fewer than 2010, however, when it was observed in 80% of survey areas
• The green-veined white (above) increased in occurrence, appearing in 73% of sampled areas, compared to 69% in 2010
• The large white was also found in 73% of survey squares, but its range contracted slightly between 2010 and 2011
• The distribution of gatekeeper butterflies seemed to be stable. It was spotted across 70% of the countryside in 2010 and 2011

• See the results of the 2011 Wider Countryside Butterfly Survey
• How caterpillars master the art of camouflage
• Springwatch: Britain's most troubled and beautiful butterfly species

According to its co-ordinator, Dr Zoe Randle, the Wider Countryside Butterfly Survey (WCBS), gives a "scientifically robust" picture of the butterfly population throughout the country.

"It's carried out in randomly-generated one-kilometre squares," she told BBC Nature. "Because it's random, you can get a better idea of what's happening throughout the countryside."

In July and August 2011, more than 500 volunteers counted butterflies on these patches of countryside. Each person counted an average of 47 butterflies and saw seven different species.

This is a reduction of more than 20% in the number of butterflies per survey compared to 2010. It is also a 40% reduction compared with 2009, when each recorder saw an average of 80 butterflies and eight different species.

Butterfly Conservation has blamed the decline on "last year's record-breaking cold summer", but also said there was a long-term and "ongoing deterioration of suitable butterfly habitat across the countryside".

The once ubiquitous small tortoiseshell was one of the species badly-affected, with less than one seen per kilometre walked, on average.

Butterfly Conservation says that, less than a decade ago, this species was "likely to be seen in almost every garden and flowery place through the summer months".

Dr Randle explained that butterfly habitat throughout the UK was being lost and destroyed.

"The spread in modern agricultural practices, like use of pesticides and herbicides, makes it very difficult for butterflies," she explained.

The common blue was found in only a third of squares compared with more than half in 2010

"And it doesn't help that many people are ripping up lawns and putting out decking; butterflies can't live without a nectar source."

But the conservationist insisted it "wasn't all doom and gloom".

"Targeted agri-environment schemes can really benefit species," she said.

She also encouraged the public to plant "caterpillar food plants, and butterfly nectar plants, and added that it was a good idea to "leave a patch of your garden long with lots of flowers."

"We want to halt butterfly declines by 2020," Dr Randle said.

"And I think with the right targeted habitat management, we can do it."

Butterfly Conservation run this survey with the British Trust for Ornithology, and the UK Centre for Ecology and Hydrology.