NORFOLK BROADS — A new study published in the journal Insect Conservation and Diversity indicates that the British swallowtail subspecies Papilio machaon britannicus has maintained its distinct genetic identity for at least 200,000 years. Whole-genome sequencing revealed that the subspecies separated from continental swallowtails between 200,000 and 1.7 million years ago. Earlier theories had proposed that the subspecies developed its unique characteristics over the last 8,000 years, following the flooding of Doggerland. The research also found some evidence of inbreeding within populations across Europe, but concluded that surviving populations were not experiencing damaging mutations.

Papilio machaon britannicus is smaller and darker in color compared to the continental swallowtail, Papilio machaon gorganus. The British subspecies is vulnerable to extinction and is exclusively found in Britain, particularly breeding in the Norfolk Broads, which is England's largest freshwater wetland. In the wild, the caterpillars feed solely on the rare wetland plant milk parsley. Rising sea levels threaten these freshwater wetland habitats. The salination of the Norfolk Broads is occurring as a result of rising seas, and salty water rapidly kills milk parsley. Most of Britain's breeding populations reside at or below sea level.

Mark Collins, president of the Swallowtail and Birdwing Butterfly Trust and a co-author of the study, stated that the British swallowtail populations are a relict from a once wider distribution across European wetlands. "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," Collins said. He remarked, "We know now we are dealing with a distinctive butterfly that must be conserved." Collins also noted that the continental swallowtail is increasingly reaching Britain, often breeding successfully in fine summers in areas like Kent and Sussex.

Collins addressed the potential for hybridisation and the need for habitat protection. "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." Collins emphasized the need to identify the best sites for growing milk parsley and introducing the butterfly to ensure its survival. "We need to urgently identify the best sites to make sure we can grow milk parsley there and introduce the butterfly and it will thrive, whether that's Lakenheath in Suffolk, Shapwick in Somerset or sites in Yorkshire," Collins 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."

Why It Matters

This study redefines the understanding of the British swallowtail's evolutionary timeline, establishing its genetic distinctiveness over a much longer period than previously thought. The subspecies, unique to Britain and primarily found in the Norfolk Broads, faces extinction threats from habitat loss due to rising sea levels and the salination of its freshwater wetland environments. Its survival is directly linked to the availability of milk parsley, its sole food source, which is sensitive to saltwater. The findings also highlight the broader conservation challenge posed by global climate change, as increased temperatures facilitate the arrival of continental swallowtails, potentially leading to hybridisation. Protecting the specialized wetland habitats of Papilio machaon britannicus is crucial for the long-term survival of this legally protected subspecies.