Optimum Density of Sika Deer for Tree Seedling Survival

Sika deer have both direct negative and indirect positive effects on tree seedlings because deer browse on seedlings themselves as well as the floor vegetation that suppresses their growth. As such, a non-zero optimum deer density may exist for tree seedling survival. We conducted a field experiment combining exclusion of deer and removal of floor vegetation to investigate possible conditions under which an optimum density of deer with respect to seedling survival could exist. We constructed a hierarchical Bayesian model and used it to predict the survival rate of tree seedlings under different deer densities and floor leaf biomass conditions. Results indicated that a non-zero optimum deer density existed when the following conditions were present: a negative relationship between deer density and floor leaf biomass, negative effects on seedling survival caused both by deer and by floor vegetation, positive interaction of deer and floor leaf biomass on seedling survival, and considerably large potential leaf biomass. We estimated optimum deer density was 9.5 deer/km(2) when floor leaf biomass of the dwarf bamboo was 0.15 kg/m(2). This optimum deer density may have been excessive because we examined only the initial stages of seedling development, in which seedlings were less vulnerable to deer browsing. These results suggested that in this system, managing deer density at moderate levels might be more effective from a forest regeneration perspective than the complete exclusion of deer. (C) 2014 The Wildlife Society.