Why the Alps are a paradise for rare butterflies

In lowland Europe, the big blue is “almost extinct”, mainly due to the spread of intensive agriculture, according to Johanna Propstmeier, an independent biologist. She was the one who spotted it near Auckenthaler Farm in 2021, while collecting data for a butterfly monitoring project run by the University of Innsbruck and other institutions.

In places like the Netherlands, Belgium and northern France, the big blue has been reduced to isolated and declining populations, she says. But “in the mountains, the situation is still a little better than in the plain”. Indeed, in the mountains, “intensive agriculture cannot yet spread in all regions. Many alpine meadows are used as pastures”.

These warm, nutrient-poor pastures provide ideal conditions not only for the big blue, but also for the other species involved in its complex and colorful life cycle: thyme and ants.

The big blue larva begins its life in wild thyme or oregano flowers. After about three weeks it falls to the ground and waits Myrmica sabuleti the ants are coming. It must be that specific ant; other species of Myrmica the ant would treat the larva as prey and eat it. When the good ants arrive, the butterfly larva mimics the sounds and smells of Myrmica sabuleti larvae, which trick the ants into returning them to their nest and raising them as one of their own. Still undetected, the larva feeds on the ants’ own larvae. After about 10 months, he turns into a butterfly and quickly escapes from the nest before the ants realize the trick and attack him.

Other types of big blues have their own specific host plants, and each can only fool specific types of Myrmica ant.

“Their way of life is so complex because the habitat must be suitable not only for the large blue butterflies, but also for their host ants, the plants that feed these caterpillars, and the flowers that provide the butterflies with nectar,” says Propstmeier. . .

Relatively small changes can break this complex network. When a meadow is heavily fertilized, for example, other plants take over and grow thyme, which prefers nutrient-poor soils, she says. Grazing animals avoid thyme, but keep the pasture short – which is another crucial factor, as grass length really matters to the big blue and its supporting species.

The UK, Phengaris arion disappeared after farmers stopped grazing their animals in its habitat, and the grass grew too long, which cooled the ground. The cooler ground was not suitable Myrmica sabuleti ants. The scientists recommended restoring the habitat by clearing the shrubs and letting the animals graze there. The butterfly was then successfully reintroduced.

A recent study, based on a larger project monitoring butterfly diversity in South Tyrol, northern Italy, highlights the enormous importance of these mountain pastures and meadows not only for the big blue, but for the diversity of butterflies in general.

Comparing areas such as pastures, grasslands, orchards and vineyards, the researchers found the highest butterfly diversity in traditional pastures and grasslands, and the lowest in intensive monocultures such as apple orchards. . They recorded 100 butterfly species in total, ranging from 32 found in traditional pastures to just one in apple orchards.

Semi-intensively managed grasslands fall between the two in terms of diversity, suggesting that the more traditional the management, with little or no fertilizer, the better for the butterflies.

This varied landscape is also in itself beneficial for the diversity of butterflies, explains Elia Guariento, ecologist and entomologist at Eurac Research in Bolzano (Bozen) in South Tyrol, co-author of the study.

“A characteristic aspect of a mountain environment is that the different habitats are not only close to each other, but also the management of the landscape is more fragmented,” he says. “So we tend not to have larger areas cultivated in the same way, but a mosaic combination of different land use types and different habitats, which increases butterfly diversity.”

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