Sustainable urban development depends on natural resources, especially ecosystem services. However, as global urbanization accelerates, the conflict between urban expansion and natural resource conservation has become more apparent. With population trends shifting from growth to decline, this conflict may evolve in new ways. Balancing urban expansion and natural resource protection in this context is now a key challenge for sustainable development. Farmland, as one of the most impacted resources by urban expansion, deserves particular attention. This chapter focuses on how the nexus principles of investing on nature contributes to improve conservation of farmland ecosystems during the transition from population increase to decline. It highlights the importance of farmland in periphery of metropolitan regions, examines how population changes affect urban expansion and farmland use, and explores how urban expansion leads to farmland loss and reduced ecosystem services. Strategies for balancing urban development and farmland protection are also discussed. By investing in natural resources, promoting compact urban planning, and optimizing resource distribution, it is possible to use land more efficiently, protect ecosystems, and foster urban–rural coordination. These approaches can ease the pressure of urban expansion on farmland while achieving a balance between ecological, social, and economic benefits. This study provides practical insights and guidance for future urban planning and farmland conservation.