2017 年 8 巻 1 号 p. 117-123
Water pollution due to agriculture has become a serious global problem. Also, agricultural development has had major impacts on biodiversity. Impacts on the ecology of a watershed should be considered when proposing changes to land use and management for water quality conservation. Habitat diversity is one of the most important factors for conserving biodiversity in an agricultural landscape. In recent years, several fields of research have shown a positive correlation between a mosaic of land types and species richness. Herein, we evaluated the land use diversity of the Tokachi River basin and examined an appropriate spatial unit to calculate a land cover heterogeneity index for river water quality. The Tokachi River basin is located in the northern part of Japan and plays an important role in a food production area. The main land use types are forest, cropland, pasture, and dairy farming. Here we applied the Satoyama Index (SI) as a land cover heterogeneity index to examine land use diversity. SI evaluates a diverse mosaic of agricultural and non-agricultural land as an index from 0.0 to 1.0 based on Simpson’s diversity index. High SI values are an indicator of high habitat diversity. The mean SI value of the Tokachi River basin (No.17) was 0.6, and SI tended to be high in the central basin and low in the headwater of mainstream and tributaries. SI values had negative correlations with total nitrogen, nitrate, and electrical conductivity (EC) of river water. Unified land uses for extensive agricultural land results in increasing nitrogen and EC of the river water and leads to monotonic habitat conditions in the Tokachi River basin.
