The Common Blue is one of our more abundant and widespread butterflies of unimproved grasslands, chalk downs, and in urban areas, so-called urban commons ‘meadows’.
These are spontaneous grasslands and herbaceous communities that spring up on derelict sites
following demolition of houses or factories for example. The butterfly chooses these habitats because they need the abundant flowers found in open, sunny sites, and for their larvae, the food-plants too. In the case of the Common Blue, the preferred plant on which to lay their eggs is a small member of the pea family, the Bird’s-foot Trefoil.
Like all the peas the trefoil can ‘fix’ nitrogen from the air and in essence produce its own fertiliser to allow it to grow on poorer soils; so it thrives on ancient meadows and other ‘unimproved’ grasslands, and on the rubble of former derelict sites. If there are grasses around, especially the common Cocksfoot Grass, then the habitat may also be rich in other butterfly species particularly the ‘Browns’ and the ‘Skippers’.
I still recall the first ever Common Blue I found on ‘waste’ ground near derelict allotment gardens at Norton Lees, on the only patch of Bird’s-foot Trefoil for miles around! I was probably about ten years’ old and was very excited by my discovery! Within a few years the site, like so many others, had gone for new housing. In many ways this is one half of the butterfly story for these species, as the scattered habitat patches are lost, one-by-one under urban development.
The other half of the story of course is the inexorable loss of wildflower meadows and other grasslands since the 1950s. As habitats go, then the diversity and numbers of butterflies go down as well.
If we have long hot summers like this year, then see increases in both diversity (I had around twenty different species in my garden for example), and in sheer numbers, though the drought didn’t help.
However, many garden visitors are species catholic in their needs and not dependent on the habitats that are still declining. To keep the butterflies we need to conserve the sites.