Rapid decline of endemic snails in the Ogasawara Islands, Western Pacific Ocean

Abstract

The distribution of Mandarina spp., endemic land snails (ground-dowelling ecotype species) of the Ogasawara (Bonin) Islands (Japan), was examined from the 1980's to the present in Chichijima and Hahajima islands. In Chichijima Island, M.mandarina has been rapidly declining since the 1990's in the northeastern area of the island. On the other hand, M. chichijimana has only slightly declined since the 1990's in the southern area of the island. In Hahajima Island since the 1990's, M. polita has slightly declined in the central area while M. ponderosa has been rapidly declining, and M. aureola has shown almost no decline in the southern area of the island. These circumstances offer evidence of the expansion of land snail predators (flatworms). Moreover, Chichijima and Hahajima islands differ in the pace of their respective decline, perhaps because of a predatory flatworm, Platydemus manokwari, used previously as a biological control agent abroad for the giant African snail, Achatina fulica, which only invaded Chichijima island in the 1990's.