A question of height

The Large Blue butterfly (Maculinea arion) is rather rare throughout many European countries. It is found in meadows and pastures where thyme grows because the caterpillars eat only this plant or its close relative, wild marjoram (Origanum vulgare). The butterfly's survival depends on a large number of factors - as it belongs to the genus Maculinea. This means that it is dependent not only on a specific food plant for the caterpillars, but also on a particular species of ant. The caterpillars trick the ants into carrying them into their nest where they feed on the ants' brood through the winter. However, this trick works only on one very specific type of ant – only Myrmica sabuleti is tricked by the scent of the caterpillars' of the Large Blue. Other ants see through the disguise and remove any caterpillars that have been carried into their nest. During the decline of the Large Blue in Britain, M. sabuleti ants were crowded out by a competing species of ant, M. scabrinoides, which copes better with lower soil temperatures. "A change in the height of the grass by one or two centimetres can result in a two or three degree temperature change in the ants' brood chambers just below the surface," explains Prof. Jeremy A. Thomas of Oxford University. The biologist spent decades studying the complicated interplay and his findings have now been published in the same issue of Science - in a publication to which the Helmholtz scientists refer. The soil temperature dropped because the meadows on which the butterflies and ants had lived together for so long were grazed less, and because an epidemic among the wild rabbits, which used to keep the grass short, caused their numbers to plummet. A change in land use had thrown the sensitive interplay between the species off balance. By the time this was realised, it was already too late for the Large Blue in Britain. Once the precise reasons for the disappearance of the butterfly populations became known, scientists kept a lookout for suitable donor populations in other European countries so that the butterfly could be reintroduced. Eventually, butterflies were brought in from Sweden and the meadows were kept short in line with the scientific findings. It was this that turned the reintroduction of the Large Blue into a success story. There are now more butterflies of this highly endangered species living in the UK than there were when records began in the 1950s. "The fact that it was possible to stop and reverse the decline could make this a model for many other insect conservation projects," hopes Jeremy Thomas.

His fellow scientists from the UFZ go a step further in a second article published in Science. The problem with the Large Blue butterfly was that the longer grass made the soil cooler. However, climate change will pose the opposite problem, and many species could simply find it too hot. "We could counter this by simply letting the grass grow longer. Then the microclimate at soil level would remain constant," suggests Dr Josef Settele of the UFZ, one of the two authors of the article. "This may sound trivial, but it is a very simple example of how modified countryside management can cushion against climate changes to give many species a breathing space to adapt or to migrate." Settele coordinates a number of large research projects investigating risks to biodiversity (including ALARM; see www.alarmproject.net).