Multiscale dietary analysis of brown bears in Hokkaido, Japan

Abstract

Understanding regional dietary differences in wildlife has traditionally focused on regional-scale comparisons. However, integrating individual- and intra-individual-scale analyses offers a more nuanced approach. In this study, we conducted a multiscale dietary analysis using stable isotope analysis to investigate spatial dietary patterns in brown bears (Ursus arctos) across mainland Hokkaido, Japan, with a particular focus on the Shibecha region in eastern Hokkaido, where significant livestock depredation has recently occurred. We measured isotope ratios in bone collagen from bears harvested in mainland Hokkaido, compared regional-scale diets through a mixing model, and assessed individual-scale isotopic variations using geostatistical models. Additionally, we conducted a retrospective isotope analysis of a single bear (OSO18) responsible for large-scale livestock damage in the Shibecha region, using samples of its humerus and body hair. Our results revealed that brown bears in the Shibecha region rely more heavily on sika deer (Cervus nippon) than do bears elsewhere in mainland Hokkaido. A subsequent individual-scale analysis indicated that elevated δ¹⁵N values in the bears harvested from the Shibecha region were not strongly correlated with local deer density. Intra-individual analysis of OSO18 showed reliance on sika deer year-round, supplemented seasonally by anthropogenic dietary components such as dairy cattle and dent corn. Although the data were insufficient to provide a direct link between a highly carnivorous diet and the large-scale livestock predation by OSO18, it is possible that the increase in carnivorous bears in the Shibecha region represented a dietary shift caused by anthropogenic factors, such as mismanagement of sika deer carcasses left after harvest. Thus, investigations into how carnivory among brown bears may be affecting human–bear conflicts and what factors cause the high dependence of brown bears on sika deer are needed. This study demonstrates the utility of multiscale isotope analysis in elucidating regional dietary patterns and informing solutions to issues related to local wildlife management strategies.