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.
Return to the highlands
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 the 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 datafrom previous studies about animal and plant species. They estimated that, on average, species have moved 12.2 meters higher in altitude and 17.6 kilometers northward every decade.
Chris Thomas, a professor of conservation biology at York, led the project. He commented that, “These changes are equivalent to animals and plants shifting away from the Equator at around 20 cm per hour, for every hour of the day, for every day of the year. This has been going on for the last 40 years and is set to continue for at least the rest of this century.”
Climate change: A double-edged sword
The newly comfortable climate in Scotland opens up a whole new world for the butterfly: The country is full of healthy wych elm trees, which have evaded Dutch elm disease and are still prevalent there. According to Kirkland, this is good news for the white-letter hairstreak.“They’ve only been spotted just over the border, but we’re confident that there’s so much elm in that area that they’ll spread. The upshot is if the white-letter hairstreak can struggle north into Scotland, it’ll be a very happy butterfly,” he said.
Even though climate change now appears to be bringing at least one of the butterflies back, it’s still bad news. “Although some of us are seeing a short-term benefit, the overall prognosis is pretty gloomy,” said Kirkland. Climate change is threatening many species, but it seems to be giving the white-letter hairstreak a boost.
Species aren’t just moving across the border, they’re also moving north within Scotland — there is evidence that the northern species of butterfly are suffering due to climate change. Species like the orange-tip, peacockand ringlethave moved further and further north, away from the rising warmth and humidity.
“It’s much harder to prove something’s disappearing than appearing, but there are some hints that northern species are retreating northward,” Kirkland said.
But there is hope for other species. Those struggling to survive in England because of intensive agriculture and development might now find a suitable climate — and unspoiled land — in Scotland. For Kirkland, there’s one species, in particular, that might do well to move.
“There’s a lovely butterfly called the Duke of Burgundy,” he said. “In recent years, it’s really been in trouble in England, but I think the climate might be suitable in Scotland now, and there’s plenty of habitat here. It won’t get here on its own — it’s a fussy species, even though it breeds on primrose and pansies.”
Kirkland might be “flying a kite” as he puts it, but it’s possible that as the climate continues to change, conservation organizations like his could even have a role in relocating butterflies to give them a helping hand.
A boost for some means a bust for others
Climate change is also giving some less desirable species a helping hand. In the US, the Asian needle ant (Pachycondyla chinensis) is causing mayhem — invading ecosystems, threatening native species and even damaging human health with its potentially deadly sting. In a study in PLOS ONE, researchers from France and Japan modeled the climate in 2020, 2050 and 2080 to work out where the ant could potentially spread to next. Their findings suggest that the ant’s habitat could grow by almost 65 percent worldwide.
They wrote: “Our models suggest that the species currently has a far greater potential distribution than its current exotic range, including large parts of the world landmass, including Northeast America, Southeast Asia and Southeast America. ... The results of our study suggest P. chinensisdeserves increased attention, especially in the light of on-going climate change.”
There’s something strange happening in the water, too. The proliferation of “rock snot” — a kind of algae — in eastern Canada was baffling (and angering) the local fishing community, and many thought it was an invasive species brought in by dirty fishing gear. But a2014 study revealedthat the algae had actually been there for decades, and was just growing more due to the changing climate.
The blooms, made up of the diatom Didymosphenia geminata, had been undetectable in the 1970s, but likely there since the 1800s. “We suspect that climate change is favoring this species in several ways,” said the study’s lead author Michelle Lavery, currently a Ph.D. candidate in animal behavior at the University of Guelph. For example, warmer air means less ice and therefore less disruption to the flow of water in the rivers; the algal blooms survive better when the water moves less. “Instead of having to start over every summer, they can build on themselves and get bigger and bigger,” she explained to Scientific American.
It’s not just about warmer temperatures; climate change is also altering the ocean’s pH balance, impacting different species in different ways. We know more acidic waters are bad news for coral, but the pH is having an indirect impact too, by favoring a predator.
Adult crown-of-thorns starfish (Acanthaster planci) eat coral — an increase in their number means a decrease in coral. Researchers in Australia wanted to know if the pH of the water affected how much of the coral the young of the starfish eat, determining how many of them survive into their coral-feeding adulthood. In a 2017 study, the team grew juvenile crown-of-thorns and their food, coralline algae, at three different pH levels, going down to 7.6, the level expected in the next few decades.
The results showed that the young starfish ate more in water with a lower pH, possibly because the pH altered the chemical composition and therefore the “taste” of the food. They wrote: “These results indicate that near future acidification will increase the success of early juvenile [starfish] and boost recruitment into the coral-eating life stage.”
There’s still a lot we don’t yet understand about how climate change will impact ecosystems, and indirect effects like these could be surprising. The emergence of the field of climate change ecology is bringing many of these issues to light, but we must continue to monitor individual species and mitigate climate change in order to protect ecosystems.
For now, the white-letter hairstreak is enjoying its new home among Scotland’s wych elms, as its cousins are being pushed further and further toward the northern Atlantic coast.
Lucy Goodchild van Hilten is a writing fellow for Earth | Food | Life, a project of the Independent Media Institute. She is a writer with a Master of Science degree in the history of science, medicine and technology from Imperial College London. She has served as assistant editor of Microbiology Today and senior marketing communications manager for Life Sciences at Elsevier. She set up Tell Lucyin 2014, with a mission to make the complex understandable. Lucy also runs Bucket List Bookshop— home of the world’s best writing.