Two species of butterfly are bucking the trend of decline in the UK by dramatically increasing their numbers on Studland Heath in Dorset – and it’s partly thanks to ants!
As the East Lulworth based Butterfly Conservation charity launches the Big Butterfly Count – which has shown an 80 percent decline in UK numbers in the past 50 years – Purbeck is celebrating a remarkable success story.
Good news for graylings in Studland
And even better news for the silver studded blue butterfly in Studland
A blaze of blue on Purbeck heather
On lowland heath managed by the National Trust in Studland, populations of two very different butterflies – the silver studded blue and the grayling – have risen steeply and are continuing to reverse the national trend.
The silver studded blue butterfly may be tiny, but brings out the Purbeck heather in a blaze of bright blue, while the grayling is a master of disguise, and uses its drab colouring to blend in perfectly with bare earth, stones, and dry vegetation.
Volunteers have monitored butterflies at Studland in spring and summer for almost 50 years. Figures for one transect of land showed that silver studded blues had risen from 21 butterflies in 1976 to 376 in 2023.
Graylings, which have declined across the UK by 70 percent in the last 50 years, have risen in Studland from 50 in 1976 to 134 in 2023.
Insect expert Martin Warren has led butterfly counts at Studland for years
“Ants protect them from predators”
Martin Warren of Corfe Castle, one of the UK’s leading butterfly experts and a National Trust volunteer, said:
“These figures really are something to celebrate, at a time when many butterflies aren’t faring well at all. It’s especially good to see that the graylings are increasing, when they are in serious decline elsewhere in the country.
“The silver studded blue lives on this heathland and has done incredibly well here for reasons that we don’t fully understand. It’s a rare butterfly and as most butterflies are doing really badly, it is nice to have one that seems to be bucking the trend.
“The males are blue and the females are brown. They have miniscule studs on the underside, which is where they get their name from, and have been out for a few weeks – and already along this transit there are amazing numbers.
“It’s a very specialist habitat and what they do, which I think is quite remarkable, is to live as caterpillars in the nests of one species of ant, which protects them from predators.”
Pointing out one of the tiny silver studded blues – on a warm, still day there may be dozens of them together
Closer up, the beauty of the silver studded blue is plain to see
“Evolution honing itself”
Martin Warren added:
“Normally when any insect reaches big population numbers, something will come along and eat it, but here you have the ants protecting the caterpillars.
“When they are young, the ants actually transport the caterpillars from the nest to the food plant – the heathers – which they eat and then effectively milk them like we do cows for a nectar they produce from their honey gland.
“As the ants like the taste of it, they will protect the caterpillars – so the adult butterflies will only lay their eggs where they can detect the presence of these ants, which I think is really clever, evolution honing itself to a remarkable relationship.
“The ants nest underground and the caterpillars are ferried about when they are young and are protected while they are feeding, so it’s a very close relationship.”
Firebreaks and cycle tracks may have provided the perfect environment for Studland’s butterflies
The female silver studded blue is less showy, and more difficult to spot
Other factors are at play
But there are also other factors at play to produce such a large increase in the numbers of silver studded blue, and in recent years the National Trust has changed the way it manages the heathland at Studland.
Traditionally, the land would have been burned and grazed by our ancestors to keep scrub out of the heathland, so the National Trust have replicated that with small patches of controlled burning to reset the vegetation which then regrows slowly because the soil is so poor.
That favours the growth of heathers, which are the favoured food of caterpillars of the silver studded blue, while their guardian ants seem to like the shorter vegetation at Studland, caused by mown areas of heath around the firebreaks.
Graylings use their drab colours to blend in perfectly on patches of bare ground
“One of many success stories for wildlife”
National Trust ecologist David Brown, added:
“We aren’t exactly sure why these two species are doing so well, but both need areas of bare ground where they can bask.
“That’s often provided by the hoof prints of grazing animals, or by pigs rooting out vegetation. At Studland, the butterflies are thriving beside a track used by walkers and cyclists – and it’s the track that provides them with the bare ground habitat.
“We are incredibly grateful to the volunteers who diligently carry out this important work, as without their consistent records we wouldn’t have a clear picture of the state of nature in Purbeck.
“It is fantastic news, one of the many success stories for wildlife across the Purbeck Heaths National Nature Reserve.”
The pearl bordered fritillary was common across England 50 years ago, but is now only found in Scotland
Join the big butterfly count
Members of the public are being asked to join the Big Butterfly Count, which started on Friday, 12th July 2024 and runs until Sunday 4th August 2024.
Launched in 2010, the Big Butterfly Count is one of the biggest projects of its kind in the world, aimed at helping to assess the health of the environment simply by counting the amount and type of butterflies seen.
Every year from mid-July to mid-August, tens of thousands of people spend time outdoors across the UK counting the country’s more common butterfly and day-flying moth species.
Emma Butler, enjoying one of Dorset’s many wild spaces
Providing space for nature to thrive
Emma Butler from Butterfly Conservation said:
“Butterflies are key biodiversity indicators as they react very quickly to changes in their environment. Therefore, if their numbers are falling – and 80 percent of butterflies have declined since the 1970s – then nature is in trouble.
“We were founded in 1968 in Dorset by a handful of dedicated naturalists who wanted to make a difference for butterflies and moths. We now have more than 100 employees, 36 nature reserves across the UK and more than 40,000 members.
“Butterfly Conservation is working to create thousands of wild spaces for people, butterflies and moths in cities, towns and villages all over the UK.
“Whether those spaces are the flowers in our window boxes, pots on our patios, patches of weeds in our gardens, or wildflower meadows in our churchyards and parks, they all provide space for nature to thrive and people to enjoy.”
Anyone can join in the Big Butterfly Count by downloading an app which comes with an ID to the most common butterflies in England, then spending just 15 minutes outside on a sunny day and recording the numbers and species spotted.
A peacock butterfly, with its distinctive ‘eye’ markings, is one of the species most likely to be spotted in Purbeck
Further information
- More about the Purbeck Heaths
- Join in the Big Butterfly Count at Butterfly Conservation’s website
