The Satogawa Index: A landscape-based indicator for freshwater biodiversity in Japan

Nationwide biodiversity indicators based on landscape heterogeneity have been developed primarily for terrestrial ecosystems, but not well for freshwater ecosystems worldwide. A plausible reason for this limitation is the difficulty in the fine discrimination of freshwater environments by remote sensing. However, in recent years, Geographic Information System (GIS) vector data of freshwater land cover are classified in detail (i.e. rice paddy fields, other marshy wetlands, rivers, ponds and lakes) in Japan. Using these GIS data, we developed an index for freshwater landscape heterogeneity with 1 km square as the spatial unit, which was named the 'Satogawa' Index (SGI). The SGI formula consisted of two-component terms, one reflecting the diversity of freshwater land covers, and the other representing their spatial shapes. We calculated the SGI at a central region in Japan and found significant relationships with the species richness of Odonata (dragonflies and damselflies) and freshwater fish. The amounts and spatial patterns of freshwater landscape elements by which the SGI was estimated were different among regions in Japan; however, the SGI may be adapted to other regions as a surrogate of aquatic species richness, a typical measure of freshwater biodiversity. Although several modifications are underway that may have negative consequences for freshwater biodiversity, land use has been changed to prevent flood damage and improve the freshwater environment. The SGI may reveal the influences of the changes in land use on freshwater biodiversity and may identify the type and amount of lotic and lentic environment that should be supplemented to improve freshwater biodiversity at a certain location.

Automated summary

Although biodiversity indicators based on landscape heterogeneity have been developed for terrestrial ecosystems, they are not well suited for freshwater ecosystems worldwide due to the difficulty in discriminating freshwater environments using remote sensing. However, in Japan, detailed classification of freshwater land cover using Geographic Information System (GIS) data has allowed the development of an index for freshwater landscape heterogeneity called the 'Satogawa' Index (SGI). The SGI, calculated at a central region in Japan, showed significant relationships with Odonata species richness and freshwater fish. The SGI can be adapted to other regions as a surrogate of aquatic species richness, providing valuable insights into the impacts of land use changes on freshwater biodiversity.