Abstract
Stenomelania boninensis is a thiarid snail endemic to the Ogasawara (Bonin) Islands. Although an alien thiarid snail, Melanoides tuberculata, which presumably has a potential to replace native snails, was recorded from Chichijima in 2005, nothing is known concerning the relationships between these two species. Field surveys on the geographical distribution of the both species were conducted in 39 stream systems on eight islands of the Ogasawara Islands, including Mukojima, Chichijima and Hahajima. As a result, whereas S. boninensis was collected from 33 streams of five islands, M. tuberculata was collected from only five streams in Chichijima. In addition, whereas while S. boninensis was found at both natural and concrete bank sites in freshwater zones, M. tuberculata was found observed at concrete bank sites in both freshwater and tidal zones. The effects of a stream bank construction on both M. tuberculata and S. boninensis were also assessed. Quantitative samplings of the snails were conducted at six stations along the Yatsuse River in Chichijima during a period from immediately before to nine months after the construction was completed. Before the construction, S. boninensis and M. tuberculata coexisted in the reaches of the construction site. During the construction period, both snail species were eliminated by the diversion of all water from the stream. Whereas M. tuberculata recovered at the construction site three months after the construction, S. boninensis failed to recover even after nine months. Moreover, M. tuberculata expanded its distribution range to the upper reaches after the construction. In conclusion, a disturbance like a stream bank construction possibly induces the replacement of native species by aliens.
