Changes in the biodiversity of a deciduous forest ecosystem caused by an increase in the Sika deer population at Ashiu, Japan

Abstract

The deciduous forest of Ashiu, western Japan, the only unspoiled lowland forest ecosystem remaining in the Kinki district, harbors a diverse suite of characteristic flora and fauna. The recent increase in the population of Sika deer (Cervus nippon) has greatly impacted the unique forest-floor vegetation. From the 1980s to 2003, we monitored changes in (1) forest understory vegetation, (2) riparian vegetation, (3) roadside vegetation, (4) pollinator visits to flowers, and (5) abundance of leaf-miners. In an Aesculus-dominated site, the vegetation of the understory, which had been characterized by diverse perennials in 1987, was replaced by a monotonic stand of the deer-repellent fern Arachniodes standishii by 2003, and subendemic plants such as Cacalia peltifolia and Cirsium ashiuense are now locally extinct. Riparian vegetation, which had been dominated by Salix gracilistyla and Phragmites japonica, accompanied by diverse perennials in the 1990s, was replaced by sparse stands of deer-repellent Pterostyrax hispida and Clerodendrum trichotomum by 2003. The species-rich roadside vegetation has also been influenced, and most of the once common herbaceous plants have disappeared. In addition to the decline of herbaceous plants, drastic decreases in the numbers of foraging bumblebees and tephritid leaf-miners were observed in 2003. These results suggest that the population increase of Sika deer has caused reduced populations of pollinators and herbivores through a drastic reduction of their host herbaceous plants between 1990 and 2003, and that control of the Sika deer population by hunting or trapping is urgently required.