We examined the distribution patterns of native animals on Amami-Oshima Island, southern Japan, along a historical gradient of mongoose establishment and estimated the effects of mongoose on the native fauna. To assess the relative abundance of various ground-dwelling animals, we used the following four methods; sensor cameras for exotic mammals, nighttime driving census for nocturnal native vertebrates, line census for ground-dwelling lizards, and adhesive traps for arthropods. The results indicated that seven species with larger body size, including mammals, birds, reptiles, and amphibians, were rarely observed in mongoose-infested area. By contrast, medium-sized animals showed neutral relationships with mongoose establishment. Interestingly, the densities of smaller-sized animals were higher in mongoose-infested area. It could be interpreted that smaller species have increased in abundance through top-down cascades, i.e., decreases in native predators such as frogs and lizards caused by the mongoose have resulted in increases in the abundance of smaller animals. Predation pressures by mongoose and native predators may be canceled out for medium-sized animals, causing neutral responses to mongoose by these animals. This study appears to be the first example that shows the influence of mongoose on a wide variety of native animals. In addition, our findings indicate the importance of considering the food web structure of a recipient ecosystem and contribute to the prediction and assessment of ecological risks caused not only by mongoose, but also by other invasive top predators.
