Seasonal fluctuation of invasive flatworm predation pressure on land snails: implications for the range expansion and impacts of invasive species.
Introduction of the snail-eating flatworm Platydemus manokwari has caused extinction and decline of native land snails on tropical and subtropical islands. As the factors influencing flatworm predation pressure on land snails remain unclear, I examined the effects of seasonal variation in flatworm predation pressure on land snail survival in the wild on a subtropical island. I also examined the feeding activities of P. manokwari under laboratory conditions. Survival rates of land snails experimentally placed on the forest floor for 7 days ranged from 0% to 100% among seasons on the oceanic island Chichijima [Ogasawara (Bonin) Islands]. More than 90% of snails were killed by P. manokwari within 7 days in the period from July to November, while less than 40% of snails were killed in other months. Snail mortality rate (0-100%) attributable to P. manokwari was positively correlated with mean temperature (17.1-27.3°C) in the study area. Laboratory experiments showed that low temperature influenced snail survival and regulated feeding activity of P. manokwari. Laboratory experiments also suggested that high densities of P. manokwari may cause a rapid decline in snail survival. Therefore, ambient temperature and density of P. manokwari may regulate seasonal variations in predation pressure on land snails. Recent global warming may increase the probability of invasion and population establishment, and elevate the impacts of P. manokwari in temperate regions.