The Bonin Islands are about 1000 km south of Honshu, Japan's largest island, and north of the Tropic of Cancer. The Bonin islands form three clusters, Mukojima, Chichijima, and Hahajima, composed of one larger island and several smaller ones. Chichishima, the largest island, and Hahajima, the second-largest, are the only inhabited Bonin islands. The highest point on Hahajima is 326 m, and Chichishima's highest point is 462 m. The Bonin Islands have sea cliffs along the shore. The center of Chichishima and other islands in the group is rolling plateau, while Hahajima has steep ridges and Mukojima is mostly flat. The Volcano or Kazan islands lie south of the Bonin Islands. Iwo Jima is the largest of the Volcano Islands. The others are Kita Iwo Jima, Minami Iwo Jima, and Nishinoshima. Iwo Jima and Kita Iwo Jima are inhabited. Nishinoshima is volcanically active. The Bonin and Volcano islands are the central portion of the Izu–Bonin–Mariana Arc, an island arc created by the subduction of the Pacific Plate under the Philippine Sea Plate that stretches 2800 km.
Flora
Flora has evolved differently on each of the islands. The Ogasawara Islands are sometimes referred to as the Galápagos of the Orient. The endemic palm Clinostigma savoryanum is the northernmost species of the palm genus Clinostigma. Metrosideros boninensis is an endemic tree of genus Metrosideros, a common tree in the southern tropical Pacific but generally absent from Micronesia. The islands are home to about 500 plant species, of which 43% are endemic. There are three main types of forest on the islands:
Type I: Elaeocarpus-Ardisia is a mesic forest found moist lowland areas with deep soils. The forests have a closed canopy with a height of about, dominated by Ardisiasieboldii. Elaeocarpusphotiniaefolius, Pisonia umbellifera, and Pouteriaobovata are other important canopy species. These forests were almost completely destroyed by clearing for agriculture before 1945.
Type II: Distylium-Raphiolepis-Schima dry forest is found in drier lowland and upland sites with shallower soils. It is also a closed-canopy forest, with a canopy composed mostly of Distylium lepidotum, Rhaphiolepis integerrima, Schima mertensiana, Pouteria obovata, and Syzygiumbuxifolium. The Type II forests are of two sub-types:
*Type IIa: Distylium-Schima dry forest occurs in cloudy upland areas with fine-textured soils. Pandanus boninensis and Syzygium buxifolium as the predominant trees, and these forests contain many rare and endemic species.
*Type IIb: Raphiolepsis-Livistona dry forest is found in drier upland areas with rocky soils. Rhaphiolepis integerrima is the dominant tree species, along with the fan palmLivistona chinensis var. bonensis, Pandanus boninensis and Ochrosia nakaiana.
Type III: Distylium-Pouteria scrub forest is found on windy and dry mountain ridges and exposed sea cliffs. These forests have the highest species diversity on the islands. Distylium lepidotum and Pouteria obovata are the dominant species, growing from tall. Other common shrubs are Myrsineokabeana, Symplocos kawakamii, and Pittosporum parvifolium''.
The islands are home to 40 species of freshwater fish, including an endemic goby, Rhinogobius ogasawaraensis. There are two endemic species of caddisflies. Goera ogasawaraensis inhabits headwater streams on Chichijima, and Hydroptila ogasawaraensis is found in streams on Chichijima and Hahajima. The islands have nine species of freshwater and brackish-water shrimps – three of the family Palaemonidae, five of the family Atyidae, and one of genus Metabetaeus. The brackish-water shrimp Palaemon ogasawaraensis from Chichijima is endemic, as is Paratya boninensis, a freshwater shrimp found in headwater streams on Chichijima and Hahajima. There are endemic species of mitten crab, sesarmid crab, and fiddler crab, and the freshwater snailStenomelania boninensis.
