Syndicate content

Triumphant Return of the Large Blue Butterfly

An upcoming report in Science celebrates the 25-year effort to restore the large blue butterfly (Maculinea arion) in the UK, where it reached extinction in 1979. Meticulous research showed that the extinction was caused by a subtle change in habitat that disrupted the unusual life cycle of this spectacular butterfly. Previously, the extinction had been attributed to the work of overzealous collectors. Adult M. arion females lay their eggs on thyme flowers in the summer. After hatching, the caterpillars stay very small and many eventually fall to the ground. They secrete chemicals that attract red ants and fool them into thinking the caterpillars are ant grubs. The ants then carry the tiny caterpillars into their underground nests. In most cases, only caterpillars that have landed in the nest of one particular ant species, Myrmica sabuleti, will survive to adulthood. The caterpillars' secretions are a sufficiently close match to those of M. sabuleti grubs that the ants never discover that they have been duped, and instead continue to protect the caterpillars for 10 months even though they are feeding on the ants' own brood. In early June, the caterpillars form a chrysalis near the colony entrance and then emerge to crawl aboveground two weeks later as butterflies. Using laboriously collected field data, lead author Dr. Jeremy Thomas and his coauthors explored the possible factors that could be causing the butterflies’ decline. They realized that the grass in the butterflies' habitat had grown too long, as farmers had gradually stopped grazing their livestock on these hillsides and a viral infection had killed many of the wild rabbits in the 1950s. The soil on these overgrown grasslands was therefore too cool to support adequate numbers of M. sabuleti ants. And, without enough ants to raise their young, the populations of large blue butterflies dwindled. The researchers combined these ecological relationships into a numerical model, which is also being published for the first time in the Science study. Starting in 1983, Dr. Thomas and his colleagues began introducing large blue butterflies imported from Sweden into restored habitat sites. As of 2008, the butterflies occupied 30 percent more colonies than they had in the 1950s, before the major decline began. The large blue is now one of just three UK butterflies on course to meet the Convention of Biological Diversity's target to reverse species' declines by 2010. This rebound has closely followed the predictions generated by the Thomas et al. model. "Human beings are so much larger than insects, it's very hard for us to appreciate that what to us is an imperceptible change in habitat can have devastating consequences for a species like the bizarre and beautiful large blue butterfly. A difference of a centimeter in grass length can change the soil temperature by 2 or 3 degrees C. If you're the size of an ant or butterfly that difference is massive," said Dr. Thomas. The report will be published online in the June 18 edition of Science Express. [Press release]