Ogasawaras: Invasive species threaten precious ecosystem

The Yomiuri Shimbun

Kazuo Horikoshi shows drawings of wild cats on June 18 that his NPO has caught on Chichijima island.

The Yomiuri ShimbunThis is the second installment of a series.

A rock mass juts out of the ocean floor, reaching a height of 916 meters above sea level. Minami-Iwoto in the Ogasawara Islands is known as “the devil’s island” due to its inhospitable appearance, but a team of about 30 researchers set out for the island in June last year.

Their objective was to survey the ecosystem on the Ogasawara Islands, registered on the list of World Natural Heritage sites (see below). Minami-Iwoto has no land where a boat can safely dock, so after the 19-hour journey to Minami-Iwoto island from Chichijima island, the researchers had to swim the last about 30 meters to shore, carrying packs full of food and drinking water with them.

“I’ve never seen this before,” said Satoshi Chiba, a professor of ecology at Tohoku University, upon discovering a snail just two millimeters long. Chiba also discovered a new species of snail during an expedition to the island in 2007.

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“The more we discover, the better we’ll be able to tell the story of Ogasawara’s natural diversity to the world,” he said.

Ogasawara is sometimes described as the “Galapagos of the Orient” because of the unique evolution of its flora and fauna. Minami-Iwoto is a symbolic place, an uninhabited island where unspoiled nature thrives.

Conversely, on Chichijima, which is connected with the mainland by a regular ferry service, the delicate ecological balance is being threatened due to the spread of flora and fauna brought to the island by human beings.

Solving wild cat problem

“I want to restore, through human efforts, the nature that humans destroyed” — this is the desire of Kazuo Horikoshi, chairman of the nonprofit organization Institute of Boninology.

The organization is working to remove wild cats, which are sent to the mainland to be kept as pets. The cats prey on red-headed wood pigeons, an endemic species of birds that are endangered.

The cats on the island are believed to descend from domestic cats taken to Ogasawara in the late Edo period (1603-1867) that went feral and multiplied. Originally, the NPO had considered culling the cats that it had caught in cooperation with the islanders, but they then received a proposal from the Tokyo Veterinary Medical Association that would allow them to protect both the birds and cats.

In the mid-2000s, therefore, the NPO and the veterinary association started a project in which the NPO transports the cats in baskets by ship to the mainland and the veterinary association receives and tames them at animal hospitals.

At the facility on Chichijima where the cats are housed before being transported, pictures of about 720 cats that have been sent so far are on display.

The number of red-headed wood pigeons has begun to recover, increasing from a mere 40 in 2006 to around 500 today.

Eradication methods sought

The Ogasawara Islands are home to many rare living things, including 100 confirmed endemic species of snail, and have been registered as a World Natural Heritage site.

However, according to the Environment Ministry, planarian flatworms that prey on snails were found on Chichijima in the 1990s. Flatworms measure 1 to 10 centimeters long and are thought to have been brought from mainland Japan hidden in goods such as plants and soil.

The invasion of these flatworms has seen four out of the five large snail species on Chichijima disappear. An effective method to eradicate them has not been established, so in order to prevent them spreading to neighboring Hahajima, the ministry is requesting ferry passengers to thoroughly clean the soil from the soles of their shoes before boarding.

“If things continue as they are, the area may be registered on the list of ‘World Heritage in Danger,’” warned Naoki Kachi, a professor of ecology at Tokyo Metropolitan University who has been conducting research on the Ogasawara Islands for over 20 years.

Indeed, if it is judged that the measures against invasive species are insufficient, the World Heritage Committee’s advisory body may recommend that it be listed as a World Heritage in Danger, which carries the possibility of it losing its status as a World Natural Heritage site.

“We really must use our brains to remove invasive species. There’s not much time left,” Kachi said.

■ World Natural Heritage site

A natural site that has been recognized by the World Heritage Committee of the United Nations Educational, Scientific and Cultural Organization (UNESCO) as an important common asset for humanity that should be protected.

The Ogasawara Islands were registered as a World Natural Heritage site in 2011, in recognition of the fact that they have a unique ecosystem inhabited by rare species of snail and other living things. Other sites in Japan — Shiretoko (Hokkaido), Shirakami-Sanchi (Aomori and Akita prefectures) and Yakushima (Kagoshima Prefecture) — are also registered.


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