Effects of sika deer browsing on the arthropod communities on understory vegetation in a thinned Japanese cypress plantation

摘要

We assessed how arthropod communities on understory vegetation are affected by sika deer browsing in a Japanese cypress plantation in central Japan, about 5 years after thinning, by comparing understory plants and arthropod communities between fenced and unfenced plots at two different scales. Deer browsing reduced the volume of plants per plot, which is a quantitative index of understory habitat for arthropods. As a result, the per-plot abundances of the herbivore guild and the tourist guild, including epiphyte fauna, scavengers, ants, and unknown taxa, which mainly use plants as temporary rests or habitats, decreased. Because the abundance of prey (herbivores and tourists) declined, the per-plot abundance of the predator guild, including parasitoids, also decreased. Thus, the negative effect of deer browsing cascaded from understory vegetation to higher trophic levels on a per-plot scale. In the plants preferred by deer, however, browsing increased the number of twigs and leaves per unit plant volume, which represents the structural complexity of plants and thus is an index of habitat quality for arthropods. At the per-unit-plant-volume scale, the abundances of the herbivore and tourist guilds increased, followed by a subsequent increment in the abundance of the predator guild. In the present thinned plantation, deer browsing improved the habitat quality (per unit plant volume), although it reduced the quantity of habitat (per plot). Thus, deer browsing had a positive cascading effect on the abundance of arthropod communities at the unit-plant-volume scale, but the negative effect at the plot scale outweighed this positive effect.