New charity warns Britain’s largest butterfly could be lost within four decades as rising seas turn its habitat into saltmarsh

Britain’s biggest butterfly, the swallowtail, could become extinct within four decades because of rising sea levels, a new charity has warned.

New inland habitat needs to be created for the swallowtail because rising seas are predicted to turn much of its current home, the Norfolk Broads, into saltmarshes later this century.

The British swallowtail caterpillar’s only food plant, milk parsley, cannot survive in saltwater, and so the plant and the butterfly will need to be translocated to the Cambridgeshire fens, according to butterfly experts.

“At least 90% of the current swallowtail breeding sites will become salt marsh with a sea level rise of 50cm,” said Mark Collins, chair of the Swallowtail and Birdwing Butterfly Trust, a new charity working to save the 500 swallowtail and birdwing butterfly species worldwide. “We could be looking at 30 or 40 years and these sites will be gone, given the rate of sea-level rise and also tidal surges and ‘salination events’, where saltwater comes rushing up the Broads’ rivers.”

A conference of conservation scientists from 25 different organisations including Butterfly Conservation and the Royal Entomological Society this week heard that the Norfolk and Suffolk Broads contain 1,500 different species of conservation concern, including 66 species, such as the Norfolk hawker dragonfly, that rely on the area’s six rivers and dozens of freshwater lakes for their future.

Climate change projections indicate that part and possibly all of the Broads will become saltwater marshes and estuaries later this century, which would imperil freshwater species including the milk parsley on which the swallowtail depends.

The swallowtail was once found on marshland across southern England but the draining of marshes in Victorian times saw it confined to the Broads, where it has become slightly smaller and is now a unique subspecies, britannicus.

The swallowtail was reintroduced on to Wicken Fen in Cambridgeshire in the 1990s but that project failed, and experts believe the butterfly requires a much larger area. One possible destination is the Great Fen, a 50-year project to restore 3,700 hectares of fen and marsh between Woodwalton and Holme Fen nature reserves in Cambridgeshire.

But Andy Brazil, butterfly recorder for Norfolk, told the conference the British swallowtail is almost never seen more than 30kms from the Broads, so the butterfly cannot reach Cambridgeshire by itself.

The government’s 25-year plan for nature includes a commitment to creating a “nature recovery network” and reintroducing species.

Facebook Twitter Pinterest The British swallowtail is almost never seen far from the Norfolk Broads. Photograph: Thomas Hanahoe/Alamy

According to Collins, moving the swallowtail “is the sort of thing these government resources should be put towards” but he warned that before individual swallowtails were relocated, ecologists would need to understand better how to grow milk parsley in the wild and whether the British swallowtail required a unique microclimate created by Broadland reedbeds to survive.

He added: “We’re changing the landscape and seascape at a pace at which these animals cannot move. The swallowtail can be leading the way for a broader discussion about these problems with dozens of other species. We can’t stick our heads in the sand about this. The strategy has got to be a managed retreat so that we can keep this incredibly important species – our largest and most spectacular British butterfly.”

While the British swallowtail is threatened, a warming climate could help the continental swallowtail become a permanent resident on Britain’s south coast. Continental swallowtail caterpillars have been found on carrots and fennel in gardens in Sussex in recent summers, but the butterfly has yet to permanently establish itself.