It is noticeable that the Common Blue is rare locally, yet Birds Foot Trefoil the food plant for the caterpillar is, if not common, certainly widespread. But the following story suggests why this might be so and also suggests that in the natural world the answers may often be less obvious and more complicated than you might think.

The Large Blue butterfly went extinct in southern England due to the price of wool and myxamatosis. The LB lays its eggs on Wild Thyme, but after feeding up the caterpillar seeks out the nest of a specific species of ant and using pheromones convinces the ants into taking it in (well usually !). The caterpillar thanks it`s host by predating the ant lava and suitably fattened turns into a chrysalis. Emerging from the transformation the LB emerges in all it`s splendour (the largest of the Blues) and leaves the nest to start the cycle again.

But here`s the problem, reduction in sheep and rabbits in areas of the South Downs led to the grass getting longer. Not a problem for the LB, or the Wild Thyme, but for the specific ant species the slight cooling of the ground temperature caused that species of ant to abandon the hillsides. They still colonized the heavily grazed fields in the valleys, but Wild Thyme did not. So the chain was broken and the LB declined and went extinct in Britain, subsequently re introduced and surviving on specifically managed sites.

So the Large Blue had a breeding cycle that depended on two other species living close by, but the overlap of their habitat requirements was at best marginal, neither species needed the other and as the LB predated both of them it`s absence could be seen as a bonus. So the low numbers of Common Blue my well be a product of it`s specialist life cycle – it can only successfully breed if Birds Foot Trefoil and a specific species of ant are both present in the same location.