A comparison of summer insectivory among four sympatric mesocarnivores on Izushima, a small island in northern Japan

Abstract Limited resources intensify interspecific competition and constrain the survival and distribution of species. A restricted, spatially isolated, small island can also pronounce this effect, as predicted in the island biogeography. This study compared the summer diets of sympatric carnivores, red foxes, raccoon dogs, Japanese martens, and Japanese weasels on the small island Izushima and evaluated niche partitioning, especially focusing on their insectivory. The results showed that insects were the main summer food source for all four species, with a significant overlap in their diets. However, in-depth investigation on insectivory down to the level of species and genera revealed that the species differences were more pronounced. Raccoon dogs frequently fed on ground-dwelling beetles, and Japanese martens fed on several arboreal beetles, while red foxes were less dependent on insects and more biased toward larger prey such as mammals and birds, and Japanese weasels foraged more on smaller prey, thereby reflecting niche partitioning between these species owing to differences in foraging ecology under the constrains of their body sizes.