Location and General Description
The archipelago extends from the island of Mukojima in the North to the Iwo Islands in the South. The Bonins can be divided into three main clusters of islands: the Mukojima, Chichijima, and Hahajima-rettos groups. In general, the islands are quite small. The largest, Chichijima, has an area of 24 km2 and its highest point is 317 m elevation. Hahajima, the second largest, is about 11 km long and 2 km wide (Mueller-Dombois & Fosberg 1998).
The climate of islands is subtropical, with a marked seasonal temperature variation, ranging from a sea level mean of 18ºC in February to 25ºC in July and August. A regular dry season occurs from January through March; sometimes there is also a secondary dry season in July and August (Mueller-Dombois & Fosberg 1998).
The basal rocks of the Bonins were formed during the Tertiary 65 to 1.8 million years ago by submarine volcanic activity. Boninite, an andesitic volcanic rock rich in magnesium oxide, chromium, and silicon dioxide, is widespread, and is overlain in some areas by volcanic breccia. Most of the islands drop sharply to the ocean, with sea cliffs ranging from 50 to 100 meters in height.
The rugged topography of the islands allows for a patchwork of vegetation communities. The small northern islands are mostly covered with grasses, a secondary vegetation resulting from grazing by introduced goats. On these islands only fragments of forest remain, mostly in ravines. On the larger islands, cleared areas have been colonized by almost pure stands of Leucaena leucocephala. Leucaena is a small tree or shrub that forms dense thickets, giving intense shade, and limiting seedling growth of other species. Broadleaf evergreen forests still cover extensive areas on Hahajima, parts of Chichijima, and some of the smaller islands, although much of this is secondary forest. The primary native forests are restricted in their range to more remote and inaccessible parts of the islands (Mueller-Dombois & Fosberg 1998).
The primary subtropical broad-leaved evergreen forest that remains on the islands has been classified into 3 major types by Shimizu (1992):
Type I - Elaeocarpus-Ardisia mesic forest is a lowland forest that grows in moist habitats that have deep soils. The closed canopy layer (with a height of about 15 m) is dominated by Ardisia sieboldii. Other important species in the canopy are Elaeocarpus photiniaefolius, Pisonia umbellifera, and Pouteria obovata. Type I forests were almost completely destroyed before 1945 by clearing for cultivation.
Type II – Distylium-Raphiolepis-Shima dry forest is another closed-canopy forest type. It is lower in stature than Type I, averaging from 4 to 8 m in height, and is found in drier lowland and upland sites that have shallow soils. It is dominated by Distylium lepidotum, Rhaphiolepis integerrima, Schima mertensiana, Pouteria obovata, and Syzgium buxifolium. Type II forests can be further subdivided into:
Type IIa – Distylium-Schima dry forest occurs principally in upland areas with fine-textured soils that are often covered with clouds. Pandanus boninensis and Syzgium buxifolium are common in this forest type. There is a very high endemism with many rare species.
Type IIb – Raphiolepsis-Livistona dry forest is found on dry, rocky slopes where there are few clouds. The fan palm Livistona chinensis var. bonensis is a second dominant and is associated with Pandanus boninensis and Ochrosia nakaiana.
Type III – Distylium-Pouteria scrub forest (.5 – 1.5 m tall) grows on mountain ridges and on the exposed, eroded tops of sea cliffs. The soils are thin, and the climate is windy and dry. Other common species in this habitat are Myrsine okabeana, Symplocos kawakamii, and Pittosporum parvifolium. This forest type appears to have the highest species diversity on the islands.
The flora of the Bonin Islands is noted for a high degree of endemism. Out of approximately 500 species, the endemism rate is around 43 percent. Just over 75 percent of the native tree species (73 of 97 species) are believed to be endemic (Kitayama 1991). There are two endemic genera, Dendrocacalia (Asteraceae) and Boninia (Rutaceae) that occur in both the Bonin and the Volcano Islands. Another endemic, Dendrocacalia crepidifolia (Asteraceae) dominates an association that is widespread on Hahajima, and is classified as Vulnerable.
Shizimu (1992) postulates that many of the endemic species found on the islands are relict endemics that evolved in moister conditions when the islands were higher, and orographic precipitation and fog were more common. Gradual subsidence of the islands after volcanic activity ceased has led to drier conditions. This, combined with anthropogenic disturbances including goat grazing, is currently stressing much of the remaining native vegetation.
According to Shimizu (1995), 58 rare species of vegetation have been identified in the Bonin Islands; 20 of these are officially listed as endangered in the Japanese Red Data book. These include Pittosporum parvifolium, Metrosideros boninsis, Melastoma tetramerum, and Rhododendron boninense.
M. boninsis is of particular interest. The genus Metrosideros, part of the Myrtaceae family of Gondwanan origin, has a nearly Pacific-wide distribution with its apparent center of diversity in New Zealand. It is notable that this genus appears in the Bonin Islands, but is completely absent from the rest of Micronesia. To date, it is not understood what dispersal mechanisms were involved in the transport of Metrosideros to the Bonins (Wilson 1996).
There are two extant restricted-range species in the Ogasawara and Iwo Islands, the Japanese wood-pigeon (Columba janthina) and the Vulnerable Bonin honeyeater (Apalopteron familiare)(not actually a honeyeater but a white-eye). The wood-pigeon was extirpated from the Iwos in the 1980s, while three other species became permanently extinct in the 1800s. These species comprised another wood pigeon, a Zoothera thrush, and Chaunoproctus grosbeak endemic to Bonin (Stattersfield et al. 1998, Hilton-Taylor 2000).
There were at least two native mammals in the ecoregion, the Critically Endangered Bonin fruit bat (Pteropus pselaphon), and now Extinct vespertilionid bat Pipistrellus sturdeei. The Bonin fruit bat population was estimated at ‘a hundred to several hundreds’ (Ishii 1983) in 1983, but was declining due to deforestation and hunting (Mickleburgh et al. 1992). P. sturdeei was known from the Bonin Islands in the North West Pacific, south of Japan. It has not been recorded since its original description in 1915 and is thought to be extinct (Hilton-Taylor 2000).
The Bonin Islands have been subjected to human disturbance for the past 170 years. They were first inhabited in 1830, when an American colony was established there. This colony persisted until World War II, although Japan had ruled the islands since 1875. The larger islands are still mostly forested, probably because of their extremely rugged terrain. Forests of Pinus luchuensis are extensive, especially on Chichijima; however, they appear to be of recent origin (introduced from the Ryukus)(Mueller-Dombois & Fosberg 1998).
Types and Severity of Threats
These pine forests have recently been deteriorating because of an introduced pine wood nematode, Bungaphalenchus zylophelus, and its vector, the beetle Monochamus alternatus. Many endemic species, especially orchids and one woody plant, Claoxylon fulica, in the Type IIb forests are threatened with extinction due to human disturbance, goat grazing, and the introduction of alien plant species. In response to this threat, large areas have been set aside as national parks by the Japanese government (Mueller-Dombois & Fosberg 1998).
Justification of Ecoregion Delineation
This ecoregion comprises the Ogaswara and Iwo Island groups of Japan. Van Balgooy et al. (1996), based on floristic affinities, isolates the Ogasawaras (also known as the Bonins) as reasonably distinct from the Marianas and the rest of Micronesia. Birdlife International (Stattersfield et al. 1998) identifies the Ogasawara Islands as an Endemic Bird Area based on the presence of two endemic bird species. The Iwo Islands are similarly delineated by Birdlife as a Secondary Endemic Bird Area because the island once contained (until the 1980’s) a restricted-range Columba pigeon. Due to geographic proximity and avifaunal affinity, the Ogasawaras and Iwo Islands groups were lumped into the one Ogasawara ecoregion.
Ishii, N. 1983. Mammals of Minami-Iwojima Island, in Nature of Minami-Iwojima Island (eds. Nature Conservation Branch, Environmental Agency) p.225-242. (Japanese with English Summary)
Kitayama, K. 1991. Threatened endemic species of the Bonin (Ogasawara) Islands. Pac. Sci. Assoc. Inform. Bull. 43 (3/4): 9-10.
Mickleburgh, S.P., Hutson, A.M. and Racey, P. (compilers) 1992. Old World Fruit Bats. An action plan for their conservation. IUCN/SSC Chiroptera Specialist Group. IUCN, Gland Switzerland.
Mueller-Dombois, D. and F.R. Fosberg. 1998. Vegetation of the tropical Pacific islands. Springer-Verlag, New York.
Shimizu, Y. 1992. Origin of Dystilium dry forest and occurrence of endangered species in the Bonin Islands. Pac. Sci. 46: 179-196.
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Shimizu, Y., and H. Tabata. 1991. Forest structures, composition and distribution on a Pacific island with reference to ecological release and speciation. Pac. Sci. 45: 28-49.
Stattersfield, A.J., M.J. Crosby, A.J. Long, and D.C. Wege. 1998. Endemic bird areas of the world: priorities for biodiversity conservation. BirdLife Conservation Series No. 7. BirdLife International. Cambridge, UK.
Van Balgooy, P.H. Hovenkamp, and P.C. Van Welzen. 1996. Phytogeography of the Pacific – floristic and historical distribution patterns in plants. Pages 191-213 in Keast, A. and S.E. Miller, editors. The origin and evolution of Pacific island biotas, New Guinea to Eastern Polynesia: Patterns and processes. SPB Academic Publishing, Amsterdam.
Wilson, P.G. 1996. Myrtaceae in the Pacific, with special reference to Metrosideros. Pages 233-245 in A. Keast and S.E. Miller, editors. The Origin and Evolution of Pacific Island Biotas, New Guinea to Eastern Polynesia: Patterns and Processes. SPB Academic Publishing bv. Amsterdam, The Netherlands.
Prepared by: Sandra Zicus
Reviewed by: In process