Invasive rats alter assemblage characteristics of land snails in the Ogasawara Islands

Although a large number of studies have shown that rats have a serious effect on island ecosystems, questions remain regarding how their invasion alters native species diversity and assemblage patterns on islands. In particular, a long history of invasions makes it difficult to understand the impact of rat invasion. In the present study, I investigate how an increase in predation by black rats (Rattus rattus) alters the assemblage characteristics of land snails on the Ogasawara Islands in the West Pacific. Radical declines in many land snail species have occurred since 2006 due to increasing predation by black rats. The frequency of shells with predation scars was greater on larger species and on those living on the trees and in a shallower position in the litter, suggesting greater predation pressure on these species. As a result of this selective predation, large species living on trees and in shallower parts of the litter declined. Accordingly, when black rats increased on the island, small species and species with a burrowing microhabitat became dominant in the land snail assemblage. The present findings can be used to aid in predicting the effects of rat eradication and in estimating the assemblage patterns and diversity prior to rat invasion.