Chapter 2. Description of Study Site in The Ogasawara Islands
1. Outline of Ogasawara Islands
The Ogasawara Islands, Ogasawara, Tokyo Metropolis, Japan, are located in the northern Pacific Ocean about 1000Km south of Tokyo. These society islands include some twenty small islands consisting of three archipelagoes, from the north to the south, Muko-jima retto, Chichi-jima retto and Haha-jima retto (Fig. 1). These islands have been formed by submarine volcanic activities in Lower Tertiary and upheaved on the surface of the sea in Lower Pleistocene. The land was mainly composed of Andesite, Boninite, Pillow lava, and Limestone (Kuroda et.al., 1981). The climate is subtropical with annual mean temperature of 23.0. No frost and snow has been recorded, and temperature does not fall down below 5 throughout the year. The annual precipitation was less than 1300mm through 10 years from 1971 to 1980 (Ono et al., 1986). These Islands has one of the most unique flora and fauna in Japan. A great number of plant and animal species are endemic. For example, 90 species of land snails, approximately 90% are unique to this islands (Kurozumi, 1988). They include the following five endemic genera: Ogasawarana, Boninena, Hirasea, Boninosuccinea and Mandarina.
Both flora and fauna are, accordingly, extremely precious from the systematical view point. The species endemism to five hundred species of vascular plants recorded reaches to 43% (Yamasaki, 1970 ; Ono et al., 1986). The subtropical flora of the Ogasawara Islands shows a close relationship to those of the tropics and subtropics in Asia, such as the Ryukyus, Taiwan and the Philippines. However, it comprises very disharmonic flora, and furthermore contains some other floristic elements such as the Japanese or East Asiatic, Hawaiian and Polynesian elements (Tuyama, 1970, 1972 ; Ono & Kobayashi, 1980). These endemic plant species form very characteristic communities which show peculiar stand structure in evergreen broad-leaved forests and sclerophyllous scrubs (Okutomi et al., 1983).
The evergreen broad-leaved forests, which are 15m high, occur in moist habitats with thick soils. It is phytosociologically named as "Pisonio-Elaeocarpetum photiniaefoliae". The canopy of the forest is composed of large trees of Elaeocarpus photiniaefolius, Pouteria obovata, Pisonia umbellifera and Hibiscus glaber. As old stumps of Morus boninensis which are larger than 2m in diameter, have been found in the forest, this endemic Morus tree must have been a member of the canopy as well (Ono et al., 1986 ; Okutomi et al., 1983 ; Shimizu, 1988).
The sclerophyllous scrubs, which are 0.5-8m in height, are found in dry habitats on thin layers of soils. It is named "Distylio-Pouterietum dubiae". The canopy consists of Pouteria obovata var.dubiae, Distylium leidotum, Rhaphiolepis indica var.integerrima, and Juniperus taxifolia. These scrubs are mainly distributed in Chichi-jima and Ani-jima. The typical ones that are 0.5-3m in height cover the upper parts of Ani-jima most widely in the Ogasawara Islands (Okutomi et al., 1986; Tsuyama, 1972). There is no doubt that these communities of endemic species have survived in the ecosystem that is extremely local in Ogasawara.
2. Distribution of Achatina fulica in Ogasawara Islands
A. fulica introduced as a primitive medicine from Taiwan through Tokyo in 1936 by Mr. Nishimura who was a storekeeper in Chichi-jima. Several years later the snails were already quite common in the lowland near the Hutami harbor. According to Mead (1960) and my hearing investigation in Ogasawara village, there were records that this species was once distributed in a wide area of Ogasawara Island, such as Muko-jima, Ototo-jima, Ani-jima, Nishi-jima and Higashi-jima, in 1940's .
Mead (1960) reported the situation of A. fulica in this Islands. Mead, M. Kondo and D.B.Langford made the first survey on A. fulica which had been established on Chichi-jima, Haha-jima, and Ani-jima.
At the present time, A. fulica is distributed only in Chichi-jima and Haha-jima according to my survey (Fig. 2). A. fulica has been extinguished in Muko-jima, Ototo-jima, Ani-jima and Higashi-jima, although weathering shells were found in Ototo-jima and Ani-jima (Fig. 3). In Haha-jima, A. fulica was limited to a very small area in the Okimura in 1949, but expanded its distribution along the roads stretching to the north and south across the island, since road construction was begun in the late 1960's.
3.@
4.@
5. Description of study sites
This study was conducted in Chichi-jima, the Ogasawara Islands. The following four study sites were set in Chichi-jima (Fig. 3); A: Mikaeri- yama, B: Miyanohama-michi, C: Yoake-yama, and D: D-site.
In order to observe seasonal changes in weight of protein gland, weight of bursa copulatrix, and percentage of snails with eggs in a popula tion, an appropriate number of snails was periodically sampled from the site of A, B, and C. Observations on activity pattern and mating behaviour by means of radio-tracking and mark-recapture were conducted in mainly D-site.
The flora of the four sites were consisted of various broad-leaved evergreen tree species, such as Leucaena leucocephala, Schima mertensiana, Pinus luchuensis, Fogara boninsimae, Hibiscus glaber, Planchonella obovata, and Osmanthus insularis . The forests are secondary and dominated by Leucaena leucocephala. The litter layer is thin and covered with poor vegetation and fallen timbers.
Copyright© 1999, Snailz Home Page
Revised--06/21/99
url:http://www.geocities.com/snailz_2000/