報告書番号

U03070

タイトル（和文）

イヌワシを頂点とする生態系の解明 -－食物連鎖の定量的解析－-

タイトル（英文）

Study on the ecosystem sustaining a pair of Golden Eagles-Quantitative analysis of food chains-

概要 （図表や脚注は「報告書全文」に掲載しております）

上位性の視点による生態系アセスメントの基本的な手法を提示するため、イヌワシ－ノウサギ－植生という食物連鎖に焦点を絞り、イヌワシ行動圏内の現存量をGISデータベース化した。更に、狩り・探索を行う際に制限要因となる植生の展葉状況を衛星データなどにより定量化し、イヌワシの利用可能餌量という指標を導入した。各栄養段階の現存量を比較した結果、下位に対する上位の相対割合は0.05～0.43％で、下位が非常に大きいものとなった。季節間では、植生とノウサギの現存量は植生繁茂期の方が積雪期よりもそれぞれ10.3倍、6.8倍多かったが、利用可能餌現存量は1.2倍と近似していた。更にイヌワシの重要な餌場である伐採跡地が消失した場合の影響をＧＩＳ解析した結果、下位に比べ上位における現存量の減少率が大きく、とくに積雪期においてノウサギの現存量やイヌワシ利用可能餌現存量に与える影響が大きいことが示唆された。

概要 （英文）

While an ecosystem approach has been highly demanded for the environmental impact assessment, methods and procedures have not been thoroughly explored. Here, as a practical approach for an ecosystem assessment, we demonstrate the GIS-based quantitative analysis of food chains sustaining a pair of Golden Eagles on Mt. Akita-Komagatake, Japan. Our objections in this study were (1) to estimate the influence of food availability on the Golden Eagles during the nestling period and (2) to examine the quantities of the food availability and the three trophic levels in spatial and temporal changes. (1) In order to estimate the influence of food availability on the Golden Eagles during the nestling period, we compared 1) the total number of Japanese hares in the territory of the Golden Eagles, 2) food availability in consideration of 党shade effect媒 by seasonally-changing vegetation cover during hunting activity from the sky estimated partly by satellite MODIS-LAI data, and 3) dietary width obtained by the results of direct video recordings in the nest. The results showed that the total number of the hare was between about 500 and 1300, and no clear tendency was found in the population trend during the period. On the other hand, the food availability rapidly decreased to about 100 hares at the end of the nestling period. The dietary width also showed a rapid increase in the period when the food availability decreased. (2) To examine the quantities of the trophic levels (vegetation, Japanese hares, and Golden Eagles) of the food chains, we developed GIS databases of two periods (vegetation growth and snowing) as follows; 1) vegetation biomass accessible to the hares; 2) biomass of the hares predicted by Generalized Linear Models based on the relationships with the vegetation biomass and environmental factors; 3) food availability for Golden Eagles. While net biomass of vegetation and the hares in the vegetation growth period were substantially higher than those in the snowing period, the biomass of the food availability was similar between the two periods. The results of our studies suggest that the food availability and the structure of the food chains differ dramatically temporally and specially, and the Golden Eagles correspond to the differences by changing their food use, hunting area, and territoriality. Our GIS-based quantitative approach in consideration with spatial and temporal changes would be effective for ecosystem evaluation in the environmental impact assessment.

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