THE long-tailed blue butterfly, named for the stringy “tails” on its wings and its iridescent azure colour, is common across Africa and southern Europe. It has rarely ventured as far north as the UK. In fact, for most of the time since one was first recorded on the south coast of England in 1859, only a few dozen more had been spotted there. But over the past decade or so, the long-tailed blue (pictured above) has been arriving in the country during the late summer in greater and greater numbers. Last year, eagle-eyed gardeners saw at least 50 adults and hundreds of eggs.
The sight of such exotic visitors is a thrill for butterfly enthusiasts. For conservationists, it is something else entirely: a worrying confirmation that butterflies are already feeling the effects of human-induced global warming. “This isn’t something that’s 50 years ahead. This is happening right now,” says Dan Hoare, an ecologist and director at UK-based charity Butterfly Conservation. Although the long-tailed blue is taking advantage of rising temperatures by expanding its range, most wildlife won’t have it so good. “The long-tailed blue should make us think twice about how nature is going to be able to adapt,” says Hoare.
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As it happens, watching butterflies is one of the best ways to answer that question because these fragile, transient creatures are bellwethers. When it comes to understanding how climate change is affecting wildlife and ecosystems, their lifestyles and famous visibility make them uniquely revealing. So, as temperatures rise, what is becoming of butterflies? And what do their various fates tell us about how climate change will affect biodiversity in general?
Butterflies aren’t facing global warming on a firm footing. Around the world, populations are experiencing steep downturns. In the UK, as the long-tailed blues have moved in, there have also been serious long-term declines in the majority of butterfly species. It is a similar story wherever there are reliable figures. Butterfly numbers in the Netherlands have dropped by more than 80 per cent since 1890, for instance, while all 26 species listed as threatened or endangered in the US have dwindled to the point of extinction.
Much of that decline has been attributed to habitat loss and the use of pesticides. But now the effects of global warming are beginning to bite too. The role of human-induced climate change has, so far, been difficult to disentangle. The intricate and interwoven effects that global warming can trigger, from severe weather to shifts in the timing of plant life cycles, make it challenging to decipher the cause of any given species decline. Yet butterflies offer a unique chance to see through the fog.
As cold-blooded animals that depend on the climate to regulate their temperature, they are sensitive to warming. Their short but complex lifespans also mean that knock-on effects quickly become apparent, although that fleetingness should also give them the capacity to adapt faster than most. “Many species of butterfly have more than one generation a year and are thus subject to rapid selection pressure compared to longer-lived mammals or birds,” says Hoare. “Their diversity, with varying lifestyles and ecological niches, offers lots of opportunities to respond to environmental change.”
“Emerging earlier can throw butterflies out of sync with their host plants”
The same could be said of nearly all insects. The problem is that there is precious little data on many of them. Butterflies, on the other hand, are well-documented, having long been a favourite of keen-eyed amateur biologists who collect information about their comings and goings.
As a good proxy for other insects, butterflies serve as an early indicator for the impact climate change will have more generally. By studying them, we get the first indication of how human-induced warming is playing out, telling us which species might have the capacity to adapt, what it takes to do so and the extent to which adaptation can keep pace with rising temperatures.
One of the most obvious responses to global warming is the drift of species away from the equator and towards the cooler climes of the poles. Long-tailed blues heading northwards are far from the only example. A recent study showed that the area in which Canadian and eastern tiger swallowtail butterflies meet and interbreed – a strip of land stretching from Minnesota to New England – has since the 1980s. Other butterflies are moving upwards too, with higher elevations offering optimum temperatures as the climate warms.
Nowhere to go
Yet for those cold-adapted species of butterfly that had already made higher altitudes home, there is quite literally nowhere left to go. In some cases, they are being pushed to the brink of extinction. Take the mountain ringlet, an upland species found in the north of England and Scotland that is beautifully adapted for cold and damp conditions. It is a specialist butterfly, meaning it can only thrive in very specific circumstances. Previously, it could be found as low as 200 metres above sea level. “Now it’s been pushed to 500, 600 metres and up,” says Andrew Bladon at the University of Cambridge. “The highest mountains in the UK don’t go much above 1000 metres. There’s very little space left for that species. At some point, it will disappear because of climate change.”
Indeed, specialists everywhere are particularly vulnerable to climate change. As average temperatures rise, many butterflies are emerging earlier in the year. This is fine for species with multiple breeding cycles per year and some level of flexibility in their preferred habitat. But it is bad news for single-generation butterflies that rely on a restricted number of plant species to feed their caterpillars, according to an analysis of 130 different species carried out last year by Callum Macgregor at the University of York, UK, and his colleagues. tends to throw them out of synchrony with their host plants.
It is possible to find a new niche, as the northern brown argus demonstrates. In mainland Europe, this species feeds mostly on geraniums, but in the UK, the caterpillars prefer rock roses – or at least they did. “It was assumed that the British population was a subspecies that had specialised for some reason,” says Bladon. As the climate has warmed up, however, the caterpillars of the northern brown argus have switched to geraniums.
“Just like shopping malls, ecosystems will looks increasingly homogenous”
For Bladon, this suggests that the plant itself could be less important than the microclimate it generates – geraniums provide cooler spots than rock roses – and raises interesting questions about what makes species specialise in the first place. For some, it might not be the characteristics of the host plant that matter so much as the habitat that it provides, in which case some species will be more robust to climate change than they first appear.
Whether butterflies can actually evolve to cope with climate change, as opposed to changing their habits and habitats, is another question. “We know that evolution can happen rapidly if selection pressure is strong enough, and we have lots of examples in insects, so the expectation is that butterflies should be able to,” says Joel Kingsolver at the University of North Carolina at Chapel Hill.
This is difficult to demonstrate because you need to look at traits over time, and that requires data from experiments carried out in previous decades to compare with data from today. But Kingsolver and his colleagues have done that on several occasions in their work with sulphur butterflies in the Rocky mountains in North America. Earlier this year, they recreated an experiment from 1971 that looked at pigmentation in adult wings. They found that over the past 47 years, these butterflies have , but also the ability to calibrate wing colour in response to the temperature during their pupal stage – both of which are known to have fitness advantages in a warming world. “So that trait has evolved, and the plasticity too,” says Kingsolver.
Climactic lag
That isn’t to say that butterflies will always evolve fast enough to keep pace with the effects of climate change, saving species or populations that would otherwise go extinct. “I’m kind of dubious about that,” says Kingsolver. But if it happens at all, it will be in species that can reproduce quickly, he says.
Overall, what we are learning from how butterflies are responding to the first throes of climate change is that generalists, those species that aren’t fussy about habitats and host plants, are best suited to adjust in the short-term – and especially those that can reproduce more than once a year. Specialists, meanwhile, will suffer as climate change shifts their life cycles out of sync with the plants and habitats on which they rely, even if some may be able to find new microhabitats.
Ultimately, the result is likely to be what Anne Magurran at the University of St Andrews in the UK has dubbed the “shopping mall” effect. Just as shopping malls and high streets are losing distinct, independent shops while a few chain stores dominate everywhere, so ecosystems that previously hosted all manner of species will look more and more homogenous. “We end up with increasing populations of common and widespread species, but lose treasured rarities,” says Hoare.
In the long term, even the generalists may not be able to adapt quickly enough to keep pace with climate change. Researchers are already observing a gap, known as “climactic lag”, between where species should be if they were tracking rising temperatures and where they actually are, geograpically speaking. One key reason they are falling behind is because there is little suitable habitat to host them.
The best thing we can do, beyond drastically reducing carbon emissions to slow the pace of global warming, is to make sure we preserve and expand a diverse range of nature reserves. This will help provide a range of habitats for those species that are able to quickly up sticks and shift towards the poles. “The only response,” says Hoare, “is to build large, robust networks of habitats with lots of variety that run through fragmented landscapes, and let butterflies find their way.”
