Warmer temperatures may be helping one species of butterfly, but many others are seriously threatened.
A towering elm tree stands 30 meters tall, somewhere near the border between England and Scotland, defying the fate that so many of its cousins met when Dutch elm disease ravaged the species in the 1970s. One of relatively few elm trees left, it is a haven for wildlife. Look closely and you can see the erratic fluttering of a small brown butterfly, with a W-shaped white streak across its wing.
This butterfly is making history: It’s crossed the border into Scotland, where it has settled happily in a native wych elm tree and been sighted in the country for the first time in 133 years. The white-letter hairstreak — Satyrium w-album — has been squeezed slowly out of its habitat over the last 40 years, but now it seems to be getting a helping hand from an unexpected source: climate change.
Although numbers were up slightly in 2017, the white-letter hairstreak isn’t doing well in the United Kingdom — the population has fallen 93 percent in the last 42 years, according to the United Kingdom Butterfly Monitoring Scheme, with a 59 percent reduction in the last decade alone.
This is largely due to severe loss of habitat. Caterpillars feed on elm; when Dutch elm disease spread through the elm population in England in the 1970s and ‘80s, the caterpillars’ source of food – the trees and their leaves – disappeared, and the butterfly declined. As a result, the white-letter hairstreak has made it onto various priority lists of species that need to be conserved. Volunteers across the country have been keeping an eye out for it.
It was one such volunteer, butterfly recorder Iain Cowe, who made the exciting new discovery in the summer of 2017, in a field near Paxton, Berwickshire, about 100 meters from the English border.
“It is not every day that something as special as this is found when out and about on a regular butterfly foray,” Cowe told T he Guardian . “It was a very ragged and worn individual found feeding on ragwort in the grassy edge of an arable field.”
“Iain is indefatigable — he had an eye to look for it, and he came across it by accident,” said Paul Kirkland, director of Butterfly Conservation in Scotland. “A couple of other volunteers found some eggs in the autumn, and Iain’s been back this year and found caterpillars, so we now have the full life cycle recorded in Scotland.”
The white-letter hairstreak’s northward journey is thought to be a response to the warming climate; it’s one of about 15 different butterflies heading north. Other species spotted for the first time in Scotland include the small and Essex skippers and the comma butterfly, which moved 220 kilometers from central England to Edinburgh in just 20 years.
“We assume this is related to a warming climate,” said Kirkland. “It’s hard to prove anything in relation to climate change, but the fact is that, certainly in the UK, Europe and North America, scientists are recording the northward movement of species that were formerly confined to southern areas. We don’t know exactly which aspect is important — more sunshine, warmer winters, drier winters — but the core data shows us that many species in the UK are moving northward at different speeds.”
We’ve seen this evidence for some time: In 2011, a team at the University of York in the UK analyzed data from previous studies about animal and plant species. They estimated that, on average, species have moved 12.
