Examining the relationship between urban expansion and farmland protection under population decline: a multi-scale analytical framework

Rapid urban expansion has extensively transformed farmland into built-up areas, diminishing ecosystem services. In the twenty-first century, population decline in countries such as Japan has altered the dynamics of urban expansion and farmland preservation. Though development pressure has lessened, challenges such as under-cultivation and inefficient land management have emerged. The spatial heterogeneity of these changes complicates understanding their spatiotemporal patterns and mechanisms. This study proposes a multi-scale framework to investigate changing relationships between urban expansion and farmland protection during the transition from population growth to decline. It is applied to the Odawara region of Japan. Key findings include: (1) Since population decline began in 2010, conflicts between urban expansion and farmland protection have remained concentrated along the urban-agricultural fringe, causing continued farmland loss and fragmentation despite reduced pressure; (2) Population change indirectly causes paddy field loss via built-up land expansion, with a five-year lag; (3) The main driver of farmland loss has shifted from urban development to agriculture-related loss driven by labor shortages and low farming efficiency; and (4) Agricultural promotion areas moderately retain paddy fields but offer limited protection against fragmentation. These results confirm the framework's utility and highlight the need for adaptive land use policies in response to population decline. • Developed a multi-scale analytic framework on urban-farmland dynamics. • Explored how urban-farmland dynamics shift with population decline. • Reveals that population change drives farmland loss via built-up expansion, with a 5-year lag. • Showed that urban-agricultural interfacial zones are the most sensitive. • Agricultural promotion areas protect farmland but do little to curb fragmentation.