The complexities of the natural world are easy to take for granted and we are often unaware of the extent to which different organisms are dependent upon one another. Take the large blue butterfly, for example. This, the grandest of our blues, was highly prized by 19th Century butterfly collectors and I have viewed cabinet drawers full of these illustrious insects, collected from sites scattered across the southern Britain. The large blue became extinct in Britain in 1979, not as a consequence of over-collecting but because of changes in the short chalk grassland habitats where it lived.
Like other species of blue, this butterfly has a surprising relationship with ants. After its final moult, a large blue caterpillar falls to the ground and waits to be discovered by an ant. Upon discovering a caterpillar, the ant, in something of a frenzy, will proceed to milk the caterpillar’s honey gland. This may go on for several hours but eventually the caterpillar arches its body and mimics the shape of an ant larva. This mimicry is convincing and the ant, now assuming that it has found one of its brood separated from the nest, takes the caterpillar below ground. Over the following months the caterpillar develops within the ant nest, feeding on ant grubs, before emerging the following summer as an adult. The success of this deception in dependent upon a number of factors. There needs to be an ant nest within about a metre of the thyme plant on which the butterfly egg was laid and this nest should be of one particular species – a red ant called Myrmica sabuleti; other red ants will adopt the caterpillar but large blues practically never survive in the nests of these species. Research has shown that it is the survival of caterpillars within ant nests that ultimately determines the fate of large blue populations. Only where conditions favour Myrmica sabuleti is the large blue able to survive. The ant itself is adapted to warmer climatic conditions than those normally found within Britain, so it is restricted to the shortest grassland swards that occur on steep south-facing slopes. If the sward height increases by just a few centimetres, then the drop in temperature results in the loss of the ant.
By developing a detailed understanding of the interactions that exist between the butterfly, the ants and the chalk grassland habitat, conservationists have been able to re-establish the species in Britain and, importantly, maintain sites at which the butterfly (and the ants) can flourish. It was one of these sites that I visited last weekend to see the successful results of the work – large blues on the wing again in southern England.