Why are swallowtails so restricted in the UK?

Entomologist and BBC Wildlife contributor Richard Jones explains why this beautiful butterfly is only found in Norfolk.

A
a
-
Swallowtail feeding on thistles in Norfolk

Swallowtail butterfly feeding on thistles in Norfolk © David Martin / Getty

 

The British swallowtail Papilio machaon, subspecies britannicus, is limited to the fens of the Norfolk Broads. Its caterpillars feed solely on milk parsley, Peucidanum palustre, which only grows in the East Anglian wetlands.

Yet in France and the Low Countries, the subspecies gorganus is common and widespread, availing itself to a range of host plants.

Like many organisms at the edge of a range, our swallowtail has a narrow set of very precise habitat requirements. Genetic diversity is the key factor: in southern Europe, intermingling populations keep genetic variability high, resulting in a greater diversity of habits, host plants and ranges.

Outlier populations, such as those in Norfolk, were founded by a small number of individuals – possibly just one – following the last Ice Age. With a shallower gene pool, their members are more sedentary, less experimental when it comes to egg-laying and less capable of colonising new ecological niches.

There are fears that rising numbers of gorganus migrants into Britain may well establish colonies here and interbreed with the Fenland race, diluting it to the point of non-existence.        

 

Click here to read more of our Wildlife Q&As.

Do you have a wildlife question you’d like answered? Email your question to wildquestions@immediate.co.uk or post it to Q&A, BBC Wildlife Magazine, Immediate Media Company, 2nd Floor, Tower House, Fairfax Street, Bristol BS1 3BN

 

We use cookies to improve your experience of our website. Cookies perform functions like recognising you each time you visit and delivering advertising messages that are relevant to you. Read more here