Ecological aspects of the invasive rats, Rattus rattus and R. norvegicus on Mikura Island, Japan

Abstract

In Mikura Island, Japan, feral cats (Felis silvestris catus) recently caused serious impacts by predation on streaked shearwaters (Calonectris leucomelas) that were breeding on this island. The population of the invasive rats, Black rat (Rattus rattus) and Norway rat (R. norvegicus) has increased on this island. It is important to understand the ecology of the invasive rats because they have become an alternative prey for feral cats and could possibly be the mesopredator in this ecosystem. However, there is no sufficient information on the ecology of these rats on this island. In this study, we established 8 trap lines across the island in 2017 and conducted a survey by capturing the rats in September, when shearwaters stayed on this island for reproduction, and in December, when shearwaters were absent. In September, Norway rats were captured mainly at low-altitude areas, whereas black rats were captured on a wide altitudinal range. In December, Norway rats occurred at both low- and high-altitude areas, whereas the number of black rats was lower at all trap line locations. Distribution and captured number per 100 trap-nights (CPUE) of these two rats changed spatially and temporally, and the pattern of changes was different between the two species. These results suggest that the two invasive rats have different ecological roles on Mikura Island.