The dramatic decline of one of Britain’s butterflies may be because climate change is creating a “lost generation” according to research by Belgian scientists.
The disappearance of the wall brown (Lasiommata megera) from swathes of southern England has mystified conservationists for two decades but new evidence suggests that the butterfly is dying out because warmer weather is causing generations to hatch out too late in the year to survive.
In recent years, instead of the offspring of the wall butterflies found flying in July and August spending winter as a caterpillar before emerging as a butterfly the following year, warm conditions encourage the caterpillars to quickly turn into a butterfly by September and October.
By emerging so late in the year, these butterflies fall into what researchers, led by Professor Hans Van Dyck of Louvain University, call a “developmental trap”. By autumn, it is too cold and there are not suitable plants for their offspring to eat before winter. In effect, these autumn butterflies are a lost generation, leaving no caterpillars that can survive to become butterflies the following spring.
Similarly doomed generations of other species have also been found flying in the autumn in recent years, including the white admiral, Duke of Burgundy and orange tip, which traditionally only fly in spring or summer.
In Britain, the wall has decreased by 86% since 1976 – vanishing from much of central and southern England – but it has survived at coastal sites.
In research published in the international journal Oikos, enclosed pots of captive-bred wall caterpillars were placed at coastal and inland sites in Belgium of similar latitude during the summer. All the caterpillars placed at inland sites quickly developed to become a third generation of wall butterflies which emerged in the autumn. At coastal sites, however, just 42.5% developed into a third generation, with the majority following the species’ traditional life-cycle – spending winter as a caterpillar before emerging as a butterfly the following year.
The scientists found that the micro-climate at the inland sites was on average 1.2C warmer than at coastal sites, suggesting that cooler conditions by the sea enabled the butterfly to maintain its traditional life-cycle.
While climate change should benefit many sun-loving butterfly species, the fate of the wall could be shared by many other insects struggling to adapt their lifestyles quickly enough to rapid changes in temperature.
The white admiral is another butterfly once only spotted in July which is now producing a small extra generation which flies in September, when scientists doubt it can produce offspring capable of surviving the winter.
“This year there were a lot of unusual sightings of moths in the autumn – late spring species suddenly being seen again,” said Richard Fox of Butterfly Conservation. “It probably is an effect that will be seen across species that use temperature and other environmental cues that relate to temperature as part of the decision-making process driving their development.”
Fox said he hoped that natural selection would ensure that the genotypes of wall browns that try to squeeze an extra generation into the year are selected out of existence “but whether the butterfly can adapt and not fall into this trap is a big question – some species adapt and thrive but lots of species become extinct.”
Other butterfly species have surprised scientists by the speed in which they have adapted to changing conditions.
In North America, a subspecies of the quino checkerspot butterfly defied expectations it would become extinct by moving to higher altitudes and choosing a new plant on which to feed.
In Britain, brown argus caterpillars switched to a new food plant in warmer conditions, enabling the butterfly to rapidly expand its range northwards.
The Belgian scientists stressed that their findings are not the “smoking gun” and further research is required. Fox warned that there may be other factors apart from climate change causing the wall’s decline. Another theory is that increased nitrogen pollution is changing vegetation and stopping its caterpillars from feeding successfully.
“It’s a very elegant theory but there may be more to it,” said Fox. “There often is with butterflies.”
Comments (…)
Sign in or create your Guardian account to join the discussion