Saving the Large Blue Butterfly - a conservation success story

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25 years ago, the Large Blue Butterfly was brought back from extinction in the UK. For the first time, the research that informed the successful re-introduction has been published, providing a model for similar insect conservation projects.

In 1979, the Large Blue Butterfly (Maculinea arion) became extinct in the UK. In-depth research uncovered the reasons behind its extinction; these were rectified and the butterfly was then successfully re-introduced.

The research examined the population dynamics on the last UK site of the Large Blue Butterfly from 1972-1978, the period before its extinction. The study reduced 18 influential factors down to seven and discovered that the main driver for extinction was the loss of a particular species of ant - Myrimica sabuleti.

The caterpillars of the Large Blue Butterfly are parasites of this ant. They use a specific scent to trick the ant into carrying them into their nest where they feed on the ants' brood during the winter.

The EU-funded study1 demonstrated that greater numbers of M. sabuleti were found where the grass was shorter. A change in the height of grass by just one or two centimetres can cause a temperature change of two or three degrees in the ants' nests just below the surface. M. sabuleti cannot cope with lower soil temperatures and are crowded out by other ant species that survive better under these conditions.

Rabbits normally grazed the site, but an epidemic of the disease myxomatosis caused the rabbit population to shrink significantly. As a result, the site became overgrown. This change unbalanced the sensitive interdependence of the different species. The longer grass meant cooler soil and a decline in M. sabuleti, which reduced the food supply of the caterpillars of the Large Blue Butterfly.

Once the causes were identified, the grasslands were restored and the grass was maintained at a short height. Although habitat fragmentation was not seen as influential in the decline of the Large Blue Butterfly, possible corridors between sites were also established.

From 1983 to 1992, butterflies were introduced from Sweden to three UK sites and more introductions followed. By 2008, the Large Blue Butterfly had naturally colonised 25 other conservation sites. The largest populations contained about 1000 adult butterflies per hectare, which is greater than any previous population recorded worldwide.

Across Europe, similar declines in Large Blue Butterfly numbers have been observed. Based on the UK results, successful but smaller-scale management is occurring in other countries and the Large Blue Butterfly conservation status will shortly be downgraded from Endangered to Vulnerable in Europe. The study proves that good ecological data can inform the preservation of ecosystems and communities to aid insect conservation.

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