Assessment of habitat and flood control functions by the nested landscape structure of watersheds in Imba-Numa Lake basin, Japan

Flood risk is increasing in monsoon Asia, where socio-ecological production landscapes/seascapes (SEPLS), such as Japanese Satoyama ecosystems, are being degraded by urbanization and land abandonment. SEPLS are heterogeneous landscapes that potentially enhance ecosystem-based disaster risk reduction (Eco-DRR), but their functions have rarely been assessed with consideration of the nested structure of watersheds. This study evaluated the relationship between watershed nesting and Eco-DRR potential in the Imba-Numa Lake basin, Japan. We evaluated habitat suitability for Rana japonica (Japanese Brown Frog) as an indicator of habitat function using land-use data and weighted-HAND as an indicator of rainwater storage function using elevation and land-use data. These functions were integrated to represent Eco-DRR potential, and their spatial patterns were analyzed with respect to both the top and lower watershed units. Results showed that habitat and rainwater storage functions exhibited strong synergistic effects within approximately 100 m of water bodies. Eco-DRR potential score calculated using the nested watershed approach were 0%–16% higher than those obtained with conventional single-scale aggregation. Watersheds with high landscape heterogeneity were particularly prone to information loss under the traditional method, highlighting the importance of accounting for nested structures to avoid the modifiable areal unit problem (MAUP). Our findings suggest that Eco-DRR assessments in SEPLS should define conservation indicator species, evaluate functions relevant to their habitats and integrate multi-scale watershed structures. This approach provides a framework for prioritizing conservation and flood control in heterogeneous landscapes, with applicability not only in Japan but also in other SEPLS worldwide.