Does the Introduced Japanese Weasel Outnumber the Native Population?

A research team led by Professor Eiji Inoue from the Department of Biology, Faculty of Science, Toho University conducted a study on the Japanese weasel (Mustela itatsi). The Japanese weasel is an endemic species to Japan, but it was intentionally introduced to a number of non-native islands and its naturalization on some islands has resulted in severe negative consequences to the local ecosystems. This study examined habitat use and its abundance on two islands of Izu Islands: Miyakejima Island, where the species is introduced and Izu-Oshima island, where it is native.
Using fecal surveys, researchers estimated that the relative abundance of Japanese weasel feces was higher on Miyakejima Island than on Izu-Oshima island, suggesting that the introduced population may be expanding its habitat and increasing its density. The study also revealed differences in habitat use between the two islands, highlighting that the vegetation types supporting higher weasel populations differed between them.
Additionally, density estimation using fecal DNA analysis in a specific area of Miyakejima Island indicated that approximately 20 individuals inhabited each square kilometer. These findings provide valuable baseline data for future measures for the management and control of introduced species.
This research was published in PLOS One on May 30, 2025.