A Multi-Scenario Assessment of Ecosystem Service Trade-offs under Population Decline and Farmland Reallocation

: Population decline and aging are reshaping farmland management in agricultural regions, yet the ecological consequences of farmland withdrawal and reallocation remain insufficiently examined. This study develops a rule-based multi-scenario framework to examine how population-decline trajectories and farmland reallocation strategies are associated with ecosystem service (ES) trade-off patterns. Using the Kushida River Basin in Japan as a case study, we constructed three demographic scenarios—business-as-usual, macro-decline, and compact settlement—and combined them with three farmland reallocation strategies: production-oriented, energy-oriented, and ecosystem-oriented. These combinations generated nine comparable land-use configurations. Four regulating-service indicators were assessed using InVEST models, whereas the remaining provisioning, cultural, and proxy-based indicators were derived from GIS-based spatial analyses; cross-scenario ES association patterns were then compared using correlation-based analysis. The results show relatively consistent positive associations among regulating services across scenarios, while negative associations between provisioning and regulating services are more evident under production-oriented strategies. Population decline alone does not necessarily lead to ecological improvement; ES outcomes vary according to farmland withdrawal and reallocation rules. Across demographic trajectories, ecosystem-oriented strategies showed more coherent positive association patterns among regulating services, while compact settlement tends to show weaker trade-offs than more spatially dispersed decline scenarios, although these tensions remain. By linking demographic contraction with rule-based farmland withdrawal and reallocation, this study shows that ES outcomes in shrinking agricultural regions depend less on population decline per se than on how released farmland is spatially reorganized. The framework provides a reproducible basis for comparative scenario assessment and supports planning discussions in aging and depopulating agricultural regions.