British swallowtail butterfly is much older subspecies than thought, study finds

Older than expected

The endangered swallowtail butterfly Papilio machaon britannicus, which now breeds only in the Norfolk Broads in Britain, has been a distinct subspecies for at least 200,000 years, according to a new study. Previously, scientists thought it developed its unique characteristics – smaller size, darker colour, and wetland specialization – during the last 8,000 years after being confined to wetlands in eastern England following the flooding of Doggerland.

However, whole-genome sequencing of swallowtail populations across Europe, published in Insect Conservation and Diversity, indicates that britannicus is a wetland specialist that once ranged widely across northern European wetlands. It split from its continental relatives between 200,000 and 1.7 million years ago. The study also found some evidence of inbreeding but concluded that the surviving populations are not suffering from damaging mutations.

Conservation implications

The findings are likely to change conservation approaches. Some butterfly experts have recently argued that the continental swallowtail, Papilio machaon gorganus, could be introduced to Britain. This subspecies is more common because its caterpillars feed on a variety of plants including fennel and wild carrot, and could potentially hybridise britannicus out of existence.

But Mark Collins, president of the Swallowtail and Birdwing Butterfly Trust and a co-author of the paper, stresses the genetic distinctiveness of britannicus. “We’re looking at a relict population in the Norfolk Broads that’s not just a relict for Britain but a relict from a once much wider distribution in wetlands across Europe,” he said. “Britannicus is part of our own natural heritage, it’s protected by law for good reason because it’s a unique thing, and we should not allow it to be wiped out.”

Threats and future

In the wild, the caterpillars of britannicus only reliably eat milk parsley, a rare wetland plant, which is why the butterfly is so scarce in Britain. Both plant and butterfly are threatened by rising sea levels causing salination of the Norfolk Broads, England’s largest freshwater wetland. With most populations living at or below sea level, and saltwater rapidly killing milk parsley, the species will need to be translocated to new wetland sites protected from rising seas.

Meanwhile, global heating is enabling the continental swallowtail to reach Britain more regularly. It is now commonly seen in Kent and Sussex and often breeds there during fine summers. While some lepidopterists believe hybridisation is inevitable, Collins is confident that britannicus can survive in wetlands if given a helping hand. “There’s every chance that the specialist wetland subspecies could survive into the future with the general swallowtail species, gorganus, flying around the wider countryside,” he said. “Hybridisation may occur on the fringes but britannicus could endure if we preserve our wetlands.” The next step is to identify the best sites to grow milk parsley and introduce the butterfly, such as Lakenheath in Suffolk, Shapwick in Somerset, or sites in Yorkshire.