Invasive mammalian predators are the most damaging alien animal species in terms of biodiversity, particularly in island fauna. Currently, four mammalian carnivores (raccoon dogs, feral cats, American mink, and masked palm civets) have been introduced to Okushiri Island, Hokkaido, northern Japan; however, basic information for invasive species management such as their distribution and activity patterns on the Island are still unclear. This study aimed to reveal the distribution, habitat selection, and diel activity patterns of alien carnivores on Okushiri Island. Camera trapping was conducted at 31 sites from June to November 2022. We evaluated the diel activity patterns using the kernel density method and habitat selection using a generalized linear mixed model. During the 2094 camera days, we obtained the occurrence frequencies of 4105 raccoon dogs, 208 masked palm civets, 71 feral cats, and 69 American mink. The raccoon dog was active at night and day and avoided streams. The masked palm civet was only active at night, preferred grasslands, and avoided residential areas. The American mink was active at night and day and preferred streams. The feral cat exhibited crepuscular patterns and preferred residential areas. Our study revealed that raccoon dogs have a high population density on Okushiri Island.
Biological invasions are major drivers of biodiversity loss, and introduced carnivorans are the most negatively affecting group of animal species, introducing population decline and extinction of indigenous species (Bellard et al. 2016; Doherty et al. 2016). In particular, island fauna is vulnerable to invasive mammalian predators, because island species often evolve in the absence of strong natural predators and lack defensive traits (Tershy et al. 2015; Doherty et al. 2016). Therefore, assessing the impact of introduced alien carnivores on island ecosystems and their management is important worldwide.
Okushiri Island, located in southwestern Hokkaido, in northern Japan, has endemic and threatened species. The Ezo brown frog (Rana pirica) and Far Eastern skink (Plestiodon finitimus) are listed as “Threatened Local Population” in Hokkaido Red List (Hokkaido Government 2015). Two threatened terrestrial snails are also known from the Island (Minato 2000); Euhadra brandtii sapporo is ranked as “Near Threatened” and the endemic species Megalophaedusa kanjiookuboi is listed as “Vulnerable” in the national Red List (Ministry of the Environment 2020). There are no native carnivoran species on the Island; however, four alien medium-sized carnivores–raccoon dogs (Nyctereutes procyonoides albus), feral cats (Felis catus), American mink (Neovison vison), and masked palm civets (Paguma larvata)–have been introduced by humans (Yoshiyuki 1988; Hokkaido Government 2010). Raccoon dogs had been farmed for the fur industry in many places in Japan during the 1930s (Nakamura et al. 1989) and introduced to Okushiri Island around the same time (Yoshiyuki 1988). Feral cats were estimated to have been introduced to many places in Japan for rodent pest control and as pets approximately 900–1200 years ago (National Institute for Environmental Studies 2023). American mink was introduced to Japan in the 1950s for the fur industry and has been spreading to the wild in Hokkaido since the 1960s (Uraguchi et al. 1987). Masked palm civets were introduced to Japan from Taiwan during the 1930–40s for the fur industry and released into the wild in1940s (Nakamura et al. 1989). At least, raccoon dogs and masked palm civets were recorded on the island in the 1980s (Yoshiyuki 1988; Hokkaido Government 2010), while the others were not. Since previous studies reported that some medium-sized alien carnivores could spread at a rate of several kilometers per year (Koike 2006; Ćirović et al. 2011), the four alien carnivores must have spread throughout the Island with approximately 11 km distance from east to west and 27 km from north to south (Fig. 1).
