Photo/Illutration The rapid decline in number of the butterfly species Papilio maackii could lead to its inclusion on the Environment Ministry’s Red List. (Provided by the Nature Conservation Society of Japan)

Butterfly and bird populations plummeted in and around human habitats throughout Japan between fiscal 2005 and 2022, and higher temperatures were a likely cause of the declines, a study showed.

The Environment Ministry and the Nature Conservation Society of Japan on Oct. 1 released findings from the ministry’s Monitoring Site 1,000 program, which has been conducting biodiversity surveys at 1,000 locations nationwide since fiscal 2003.

The latest report was based on data from 325 study points in and around human settlements, including on mountains.

Thirty-four, or more than 30 percent, of the 103 butterfly species that were once commonly found in those areas had annual population declines of 3.5 percent or higher over the 18-year period.

Such a fast pace is among the criteria for listing a species on the Endangered Class II division of the Environment Ministry’s Red List. However, only six of those butterfly species were on the Red List.

The annual decrease rate reached 20 percent for both the “restricted demon” and Celastrina sugitanii. That percentage is high enough for listing in the Endangered Class IA category, which comprises animals with an extremely high likelihood of going extinct in the wild in the very near future.

Another butterfly variant, called Parnara guttata, underwent an annual population decline of 6.9 percent. It used to be spotted readily across Japan.

Minoru Ishii, an insect ecologist who specializes in butterflies and serves as chairman of Osaka Prefecture’s Research Institute of Environment, Agriculture and Fisheries, gave a warning about the results.

“Insects are declining due to a range of factors, such as invasive alien species and agricultural chemicals,” Ishii said. “Japan’s natural environment will be ruined unless steps are taken to improve the conditions of human settlements and peripheral mountain zones to provide diverse habitats for wildlife.”

Bird numbers similarly plummeted. The populations of 16 bird varieties, including the azure-winged magpie and the Asian house martin, fell by 3.5 percent or more annually.

The populations of sparrows and other species that prefer to live in agricultural fields, grasslands and open spaces have decreased considerably since fiscal 2015.

Climate change was cited as the most influential element.

The annual average temperature nationwide had generally risen until fiscal 2022. In areas that had sharp increases, fewer species were found.

On the other hand, more butterfly species from the tropics and the subtropics have been discovered in Japan each year.

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