Is deer herbivory directly proportional to deer population density? Comparison of deer feeding frequencies among six forests with different deer density

The intensity of deer herbivory, rather than simply the deer population density, directly affects the forest ecosystem, but a linear relationship between these two factors has generally been assumed. To assess their relationship, we investigated deer population density and tree sapling vegetation in six forests with different deer density on Yakushima Island, Japan. The feeding frequency was used as an index of deer herbivory. Palatable saplings showed high feeding frequency and became rare in deer-abundant areas, while unpalatable saplings showed low feeding frequency and increased with increasing deer density. In addition, feeding frequency on sapling vegetation did not continue to increase with increasing deer population and was limited to only 0.24–0.32 in deer-abundant areas (more than 20 deer/km2). These data suggest that deer shift their main food items from living palatable saplings to other alternatives such as litter fall rather than living unpalatable saplings. Clearly, the nonlinear relationship between deer density and deer herbivory on forest vegetation could result from a change in the food eaten by deer, and it is therefore necessary to assess the impacts of deer based on not only the size of the deer population but also the intensity of direct herbivory.

► The relationship between deer density and feeding frequency on saplings was tested.
► Sapling composition and main foods of deer changed with increasing deer density.
► Feeding frequency did not continue to increase and was limited to only 0.24–0.32.
► In deer-abundant area, deer may shift their diet to alternatives such as litter fall.
► It is important to consider that herbivory can respond non-linearly to deer density.