A deer-proof fence (900 ha) was constructed in 2001 in Nikko. The increasing deer population in Nikko was shown to probably increase the population of some insects and earthworms and decrease that of mice and voles outside the fence. The effects of the deer population on predators (foxes, raccoon dogs, martens, badgers, owls) are expected to differ, depending on the extent of the predators' dependency on invertebrates and small-sized mammals.

We examined the relationship between the ratio of sighting rates inside the fence to those outside the fence and the ratio of the extent of dependency on small-sized mammals to that on insects and earthworms. The ratio of sighting rates was computed from spotlight counts conducted during 2002-2011 and that of the extent of the predators' dependency on each prey was computed as the relative utilization frequency of each prey from the data of previous studies.

A positive correlation was observed between the ratio of sighting rates and that of the extent of dependency on prey: the higher the dependency on medium-sized mammals and invertebrates, the higher the singting rates inside and outside the fence, respectively. We conclude that the effects of the deer population on predators differ depending on the differences in the predators' food habits.