Deer often act as an ecosystem engineer and affect ecosystems through compositional and functional changes by their feeding. Exclosure experiments comparing two points of deer density (“no deer” and “high density of deer”) have become a major approach for studying deer effects. In exclosure experiments, however, it is difficult to establish multiple density levels of deer, which have previously masked the exact relationships between deer density and forest vegetation (especially plant species composition and richness). In addition, it is also important to consider the effects of forest managements by human on their relationships, because forest managements largely change the plant species diversity and vegetation composition. However, there is no clear understanding about the effects of forest management on the relationships between deer density and forest vegetation. In Yakushima Island, Japan, deer density is different widely among areas, and there are several forests with different management histories (primary, secondary and plantation forests). To test the relationships between deer density and forest vegetation and compare these relationships among different types of forest managements, we investigated plant species richness, coverage and species composition in multiple levels of deer densities and different forest types (primary, secondary and plantation forests).
In primary forest, plant species richness responded to deer density nonlinearly and was maximized around 20 deer / km2 in primary forest. The relationships between the relative abundance of each species and deer density were different depending on deer food preferences, which may result in the nonlinear response of plant species richness to deer density. On the other hand, in secondary and plantation forests, plant species richness linearly decreased with deer density increase, and was not be maximized at intermediate deer density. In secondary forest, coverage in forest floor vegetation is very low even in deer-scarce area. In plantation forest, the most part of forest floor was covered by some fern species. Thus, forest management largely affects the plant species composition and abundance, which consequently alter the relationships between deer density and forest vegetation and may obscure the peak of plant species richness in intermediate deer density.