Ogasawara-shotō (Bonin Islands)
The Ogasawara-shotō (often still referred to as the Bonin Islands in English) consists of three separate groups
of islands stretching for 160 km north to south across the northwestern Pacific: the Chichijima Islands in the centre
of the chain, the Mukojima Islands in the north and the Hahajima Islands in the south. The islands form part of the
Japanese prefecture of Tokyo. They are located south of the Izu-shotō and 148 km northeast from the
Kazan-rettō (Volcano Islands) — forming part of the volcanically active
island arc system that includes the Mariana Islands farther south — and around 1,100
km south of Tokyo. In all, there are 18 larger islands and dozens of smaller islets and offshore emergent rocks.
The Chichijima-rettō (Chichijima Islands) have a total land area of 38.89 km², consisting of Otohto-jima (5.30 km²), Ani-jima (7.85 km²), Chichi-jima Ohmura and Okumura (23.99 km²), Higashi-jima (28 ha), Nishi-jima (49 ha) and Minami-jima (34 ha). The Mukojima-rettō (Mukojima Islands) are the smallest group (with a total land area of 6.57 km²) consisting of Kita-no-shima (19 ha), Muko-jima (3.07 km²), Nakohdo-jima (1.58 km²) and Yome-jima (85 ha). The Haha-jima-rettō (27.54 km²) consist of: Haha-jima (20.80 km²), Mukoh-jima (1.45 km²), Hira-shima (60 ha), Ane-jima (1.67 km²), Imohto-jima (1.36 km²) and Mei-jima (1.13 km²).
The islands of the Ogasawara-shotō generally have convoluted coastlines with numerous bays, headlands and offshore islets, and are often lined by steep slopes or by cliffs of between 50 and 100 m in height. The interiors contain a rugged and highly eroded topography of low hills, sharp limestone peaks, valleys and deep ravines, with the highest point above sea level (462 m) occurring on Haha-jima. Geologically they are composed mainly of volcanic rocks (such as lavas and tuff breccias), with smaller amounts of sedimentary rocks (such as sandstone, mudstone and limestone). There is also some limited coral reef development around the islands.
Although much of the native vegetation of the islands has been lost to over-grazing or clearance for cultivation, what remains consists of a mix of grasslands and broad-leaved evergreen forest — with moist lowland and dry upland forest subtypes. Scrub-type forest, found on exposed areas such as cliff-tops and mountain ridges, have the highest species diversity. There is a fairly high degree of endemism amongst the Ogasawara flora: the endemism rate is around 43 percent, with 73 of the 97 species of tree found on the islands thought to be endemic.
The climate is classified as being subtropical with an annual mean temperature of 23 °C — with maximum and minimum monthly means of 27.6 °C in August and 17.6 °C in February. An annual rainfall of around 1,210 mm falls in two pronounced wet seasons (May to June, and October to November), although it rains throughout the year. Conversely, there are two marked dry seasons from January to March and from July to August. Severe typhoons can sometimes develop in the area.
