Japan’s ‘national butterfly’ sees alarming decline in population

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Japan’s magnificent great purple emperor butterfly is at risk of dying out, a nationwide research project showed.

Populations of nearly 40 percent of 87 familiar species of butterflies, including the great purple emperor, the “national butterfly,” are declining rapidly to the level of being designated as “threatened” by the Environment Ministry, the ministry and the Nature Conservation Society of Japan announced Nov. 12.

The 87 species were believed to have been abundant in undeveloped woodlands near residential areas known as “satochi and satoyama” in Japan, where sensitive management has helped maintain a rich ecosystem.

But now, populations of 34 species are at the level that meets one of the criteria for a threatened species.

Many of them had been categorized as common species, although two were listed as “vulnerable” in the category of threatened species on the Environment Ministry’s Red List.

The ministry has run a project, called Monitoring Sites 1000, since 2003, and has about 1,000 monitoring sites around Japan.

The project covered about 200 satochi and satoyama locations. A project team analyzed records from about 50 sites taken between 2008 and 2017 where the team obtained data on the kind and population size of butterflies.

An annual population decline of 15 percent or more is one of the criteria for “critically endangered” in the threatened category. The project found six species that met this criterion, including the Alpine black swallowtail butterfly, whose population shrank by 31.4 percent, the largest, and the great purple emperor butterfly, whose population declined 16.1 percent.

The habitats of both species stretch from Hokkaido to Kyushu. Their larvae grow by eating leaves of trees in undeveloped woodlands near populated areas.

Alpine black swallowtail butterflies consume leaves from the Ailanthus-like prickly ash tree, while great purple emperors prefer Chinese hackberry tree leaves.

“The populations of species that live in farmlands and grasslands are shrinking,” said Taku Fujita of the Nature Conservation Society of Japan. “Changes in the ecosystem, such as a lack of management of satochi and satoyama, might have contributed to the decline.”

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