The Efficiency of Line-thinning in Making Foraging Habitat for the Japanese Golden Eagle

27 Mar The Efficiency of Line-thinning in Making Foraging Habitat for the Japanese Golden Eagle

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Abstract

現在,ニホンイヌワシAquila chrysaetos japonicaは天然記念物および絶滅危惧IB類に指定されており,その繁殖成功率は最近30年間で急速に低下している。繁殖の失敗をもたらすと考えられる要員の中で,近年,鬱閉した針葉樹人工林の増加による採餌環境の悪化が注目されつつある。この対応策として,2002年,林野庁は岩手県北上高地に生息するイヌワシの繁殖成績を改善するため,列状間伐による森林ギャップの創出を試みた。イヌワシの採餌環境としての列状間伐の有効性を評価するため,林野庁が試験的に実施した列状間伐区,間伐区と環境が類似している非処理対照区および事前調査によりイヌワシの採餌行動が度々確認された採餌区の3調査区を設け,イヌワシの採餌頻度および北上高地に生息するイヌワシの主要な餌であるノウサギとヘビ類の個体数を比較した。ノウサギ生息密度の指標となる糞粒数は,間伐区において伐採翌年に著しく増加したが,伐採2年後には減少し,3年後には伐採前とほぼ同じ水準まで減少した。ヘビ類の発見個体数は,調査期間を通していずれの調査区においても少なかった。イヌワシの採餌頻度は,調査期間を通して間伐区よりも採餌区の方が高かった。このように,本研究で実施された列状間伐は,イヌワシの餌動物を一時的に増やすことに成功したものの,イヌワシの採餌行動を増加させることはできなかった。今後,イヌワシとの共存を可能にする森林管理方法を提唱するため,イヌワシの採餌環境を創出するための技術的な問題が早急に解決される必要がある。

English Abstract

The breeding success of the Japanese Golden eagle, Aquila chrysaetos japonica, desig ted a precious tural product and a class IB endangered species in Japan, has declined rapidly during the last three decades. Of the likely factors causing this breeding failure, a rapid decrease in the foraging habitat, caused by an increase in coniferous plantations with closed canopies, was recently noted. In 2002, the forest agency attempted to make forest gaps, using line-thinning, to form a foraging habitat for Golden eagles in order to improve their breeding success in the Kitakami Plateau, in Iwate prefecture. In order to evaluate the efficacy of line-thinning in creating a foraging site for Golden eagles, we determined numbers of hares and s kes, which are the main prey of the Golden eagle in this region, and the foraging activities of Golden eagles in three plots, i.e., line-thinning, control, and normal foraging habitat plots. In the line-thinning plot, the number of hare pellets, which was used as an indicator of the number of individual hares, increased remarkably the year after line-thinning, although the number decreased in the second year after line-thinning, and fi lly ended up being similar to that before line-thinning. S kes were rarely observed in any plot. The foraging activity of Golden eagles in the line-thinning plot was lower than in the normal foraging habitat plot throughout the study period. Therefore, unfortu tely, the line-thinning practiced in this study was not effective for increasing the foraging activity of Golden eagles, although this trial was successful in causing a temporary increase in the prey animals. Therefore, it is necessary to devise a method for making foraging habitat as quickly as possible, while promoting practical forest ma gement, in order that Golden eagles may continue to exist in this area.