小笠原諸島父島および母島におけるアフリカマイマイの分布ならびに個体数の変動（1995～2001年）

抄録

The distribution and population of the giant African snail, Achatina fulica, were investigated in 1995, 1998 and 2001 on Chichijima and Hahajima islands of the Ogasawara (Bonin) Islands, and the results were compared with those of a previous investigation conducted in 1985. On Chichijima Island, the distribution and population of the snail had decreased dramatically from the level in 1985. In contrast, on Hahajima Island, the snail's distribution and population decreased for a while from 1995 to 1998, but by 2001, its distribution had recovered to the same level, and its population had recovered to 40% of the level in 1985. Moreover, fecundity was higher on Hahajima Island than on Chichijima Island. The distribution and population of the predatory snail, Euglandina rosea, which was introduced to Chichijima Island from Guam in the 1960s, had decreased from the level of 1985. These findings indicate that the circumstances of these two islands, and the respective rates of snail decline, have been different since the 1980s. The cause of the snail's decline since the 1980s has yet to be verified, but it appears that the invasion of Platydemus manokwari, which occurred only on Chichijima Island in the 1990s, and the low fecundity on Chichijima Island, have been largely responsible for the snail's decline there since the 1990s.