A Rare Butterfly Success Story
And now for some good news.
While British butterflies are set for one of their worst ever years, one species is bucking the trend. After decades of hard work by conservationists, volunteers and scientists, the large blue Maculinia arion has had its best season since it was reintroduced to the country in 1984, reaching its highest numbers in Britain in 80 years.
I have been following the fortunes of Britain’s large blues with some interest for the last couple of years, ever since I actually managed to see one. It was on a college trip to Collard Hill, near Glastonbury, where the National Trust manages the only population of British large blues that is fully accessible to the public.
It was a glorious day. When they are at their best, England’s wildflower meadows are among the most beautiful habitats in the world, and on this day Collard Hill was full of life. Butterfly orchids and bee orchids were out in profusion, and the air was filled with marbled white butterflies. To my slightly nerdy eye, one thing was very noticeable: half of the reserve had pretty long grass, while the other half had exceedingly short grass.
Our group spread out, in a fairly orderly line, and started scouring a closely-cropped hill for newly emerged large blues. We weren’t the only ones – several other groups were taking advantage of the good weather to have a look for the butterflies. One chap from our course stayed at the top of the hill, smoking a cigarette, bemused by the fuss we were all making.
After about half an hour he called out to us, ‘is this one?’ We rushed over, and there one was, next to his foot.
It was a lovely creature – bright blue, with delicate dark patches on its wings. The name is perhaps a little misleading. It is not really all that much larger than, say a holly blue.
It is also a globally endangered species.
The large blue is famous for two reasons. The first reason is that it is the highest profile species to go extinct in Britain in recent years.
The second reason it is famous is that, like many butterflies in the family Lycaenidae (which includes blues, coppers and hairstreaks), the large blue has formed a bizarre life-cycle. To survive, it depends on a single species of ant, Myrmica sabuleti.
The adult large blue first lays its eggs on a patch of wild thyme or marjoram. After three weeks of feeding, the caterpillar mimics the grubs of Myrmica sabuleti, by producing scents and songs. At this point it even looks like the ant grubs. The caterpillar is carried underground into the ants’ nest by a concerned worker ant, and placed with the ant grubs. The caterpillar spends the next 10 months underground, feasting on the grubs, before pupating in the nest and finally emerging above ground as a butterfly the following year.
It’s a tricky balancing act. Too many caterpillars in the same ant nest will cause the nest to collapse. The ants themselves are also pretty picky creatures, needing a relatively warm microclimate to survive. In England, this means south-facing slopes and short grass, which creates a warmer microclimate than long grass.
To survive, then, the large blue needs three things; access to a significant number of colonies of Myrmica sabuleti, prodigious quantities of wild thyme, and short grass.
While this may have been a useful niche at some point in the distant past, in today’s highly cultivated, fragmented, post-industrial landscape, it really is rather a hindrance.
Large blues went extinct in Britain in 1979. The reasons are fairly predictable – habitat destruction and changes in farming techniques. The final nail in the coffin was the onset of myxomatosis, which all but destroyed Britain’s rabbit population. Until then, the rabbits had been doing an excellent job of keeping the sward of Britain’s countryside nice and low. As the rabbits died off, the grass shot up.
Nowadays, the specific habitat requirements of the large blue are exceedingly unlikely to occur ‘in the wild’ without some assistance from humans.
All of Britain’s current large blue colonies (there are about 11) occur on reserves which are managed specifically for them. The reintroduction work was slow to begin with. Suitable sites were located and managed for several years before any new large blues were released. New butterflies were sourced from Sweden. They are a different subspecies to the original ‘British large blue’, and so look ever-so-slightly different to the butterflies that originally lived here. Bizarrely, this has proved useful on occasion; a man was recently caught selling ‘British large blue’ butterfly specimens online, claiming that they were from a historical collection. It was easy enough for wildlife crime officers to prove that they were collected from the newly reintroduced populations, and so the man was prosecuted.
After over 30 years of dedicated conservation effort, Britain now hosts the largest concentration of the species anywhere in the world. Populations are found in Devon, Somerset and Gloucestershire. Prof Jeremy Thomas, chairman of the Joint Committee for the Restoration of the Large Blue Butterfly, said “The success of this project is testimony to what large scale collaboration between conservationists, scientists and volunteers can achieve … its greatest legacy is that it demonstrates that we can reverse the decline of globally-threatened species once we understand the driving factors.”
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