Threats to Indigenous Biota from Introduced Species on the Bonin Islands, Southern Japan

High endemism has evolved on the Bonin Islands, a group of oceanic islands in the northwestern Pacific, due to a disharmonic, unique biota. However, human settlements have caused extensive deforestation and the introduction of invasive species over the last 200 years. Insular populations are vulnerable to the devastating effects of introduced predators, competitors, and diseases, because they have evolved in the absence of such factors. Introduced species (e.g., goats, cats, black rats, green anole lizards, invasive plants, and predatory flatworms) have caused population declines and the extinction of indigenous species through direct and indirect effects. Unpredictable events often occur within these food webs. Clearly, efforts must be made to control these introduced populations, and to that end, attempts to eliminate certain species (e.g., goats and rats) have begun. However, some invasive species cannot be completely eradicated from their ranges because of their wide distributions and high population densities. In such cases, these species must be confined to their current ranges, which, for the primary invasive species, is mainly limited to two inhabited islands. Problematically, some introduced species have become essential components of the current ecosystem due to long-term naturalization, and their elimination may cause decreases in native species. Thus, the construction of sustainable ecosystems that include both native and introduced species is a realistic goal of current conservation efforts.