シカの利用頻度が草本群落に及ぼす影響

Abstract

シカの利用頻度が草本群落のバイオマスと種構成に与える影響を明らかにすることを目的とした.京都大学フィールド科学研究センター芦生研究林上谷のイグサが優占する群落に3つの調査サイトを設置し, 各サイトに6つの処理区を設けた.各処理区は周囲を柵で囲み, 毎月, 処理区ごとに柵を開閉してシカの利用頻度を変化させた.開放しているときにプロットに付加されたシカの糞塊数を利用頻度の指標とした.調査は2007年6月から9月におこない, 9月末に各処理区の草本植物の乾燥重量を測定した.シカの利用頻度が減少しても, 植物群落全体のバイオマスや種数に影響はなかったが, 多様度指数は増加する傾向にあった.優占度を種ごとにみた場合, シカの利用頻度が減少すると, イグサは減少し, ヤノネグサなどで増加した.シカによる利用頻度が特に高いプロットでは, 利用頻度が減少してもすぐには優占度に変化は見られず, 対照区の3分の1程度にまで利用頻度が減少してから変化が検出できた.シカの利用頻度と草本群落のバイオマスや種構成は非線形的な関係にあり、この関係を考慮して生態管理をすることが重要であることが示唆された。

To examine relationships between the intensity of deer habitat use and the biomass and species composition of herbaceous communities, three experimental sites were set on Juncus effusub dominated stands at the Ashiu Experimental Forest of Kyoto University on May 22th, 2007. In each site, six plots of different treatments, including control plot, were set. Plastic net fence surrounding each plot was opened in zero to 30-31 days per month to yield deer's habitat use gradient. Fecal pellet groups newly added to the plots were counted when any plot was opened or closed. Accumulated numbers of fecal pellet groups were regarded as the indices of the intensity of habitat use. At the end of September of the same year, we measured plant biomass of each treatment plot. Increasing habitat use of deer didn't affect total herbaceous biomass and species richness, but pushed up diversity index of herbaceous communities. In each plant species, decreasing habitat use intensity causes a decrease in dominance of Juncus effususb and an increase in dominance of Persicaria nipponensis and other minor species. In the site where deer heavily used, there was no clear change in herbaceous stand until habitat use intensity decreased to one-third of that in control. It suggested that relationships between the intensity of deer habitat use and the biomass and species composition of herbaceous communities were nonlinear. Therefore, it would be important to consider them when we planed ecological management.