Conservation groups report that numbers have fallen across more than half of Britain's butterfly species over the last 35 years. Were this pattern to persist, the effects on other members of the ecosystem would be difficult to predict. When a butterfly egg hatches it turns into a caterpillar - the second stage in the creature's development - and it is at this point that the insect both strips away large quantities of vegetation and becomes a meal for birds, bats and an assortment of small mammals. Halting the losses, let alone reversing them, will depend on conservationists first understanding what is driving the numbers down.
What butterflies want from the weather is a comfortable middle ground, neither too warm nor too chilly. As the climate shifts and summer temperatures edge upward almost everywhere, the insects are left to work out how to cope. Rescheduling the part of the year in which they emerge and breed is among the chief strategies they use to keep their conditions bearable. The timing of such life-stage events is what biologists mean by 'phenology'; whenever a plant or animal takes up some activity earlier in the year than usual, it is said to be 'advancing its phenology'.
None of this is new - the same advances have already shown up across a wide variety of butterflies, and virtually every species is bringing its phenology forward to some degree. Britain's mean spring temperature has risen by around 0.7C across the past 25 years, and over that period butterflies have shifted their activity earlier by an average of four to nine days, staying in step with the cooler weather they prefer. Should we read this as evidence that the insects can take climate change in their stride, retuning themselves to new temperatures with ease? Or are their populations in fact under considerable strain, hauled along unwillingly by change that is arriving unnaturally quickly? For now the answer remains uncertain, but a fresh study has set out to resolve it.
The researchers began by drawing together several million sightings that hobbyist observers - people who spend their leisure hours watching what the different species get up to - had contributed over the years. Taken together, this covered 115 butterfly species found throughout Great Britain, recorded year on year across a quarter of a century. Using it, the team gauged how common each species was and how widely it ranged over that time, together with the distance it had travelled northward. Critically, the same material also exposed slight shifts in the moment each year when a given species matured into an adult butterfly.
Examining how each of these measures had changed, the team found that the species best able to profit from an earlier, climate-driven emergence were those with the most adaptable life cycles. A number of butterflies can pass from caterpillar to adult two or more times in a single year, which means the individuals on the wing this spring might be the grandchildren, or even great-grandchildren, of the ones seen a year before.
Within this group, the butterflies that had pushed their phenology forward the furthest over the 25 years of the study also showed the strongest upward trends in numbers, range and northward spread. Take Britain's smallest butterfly, the delicate Dwarf Skipper, which gathers in clusters of as many as a hundred: some individuals reach the adult stage early in spring, so that their summer offspring can squeeze in a further breeding cycle by autumn, and the population grows as a result.
A second set of species, by contrast, is far less adaptable, confined to just one breeding cycle a year, and for these the researchers found no sign that emerging sooner did any good at all. More troublingly, the members of this group that are fussy about where they live - often because their caterpillars will feed on only certain plants - turned out to suffer most from an advancing phenology. The lovely Amber Fritillary, widely called Britain's most threatened butterfly, belongs here: it survives now only in fragments of limestone pavement and open chalk grassland, raises a single generation each year, and has advanced its phenology. Climate change, though certainly not the whole story, may therefore have had a hand in this butterfly's steep decline.
Yet the picture is not entirely bleak. On the European continent, a good many of Britain's single-brood species prove capable of squeezing in a second generation whenever a year turns warm enough. So, as warming continues, butterflies such as the Heath Argus may likewise begin producing several generations within the UK, drawing benefit from the extra heat and perhaps climbing in number as they do.
In the nearer term, this body of knowledge equips conservationists to read the early warning signs in species heading for trouble. Take the Barred Admiral of eastern England: its numbers swelled markedly from the 1930s onward, yet the past 25 years have seen them drop sharply. One reason may be that its caterpillar draws all of its food from a single climbing plant; another, in all likelihood, is climate change.
