This chapter contributes to a comprehensive understanding of the island socio-ecological system by a synthesis of the state and trend of, and the drivers of changes in ecosystems and their services, focusing on mangroves in Ishigaki Island. The current extent of mangroves on the Island is limited to small patches along a few river estuaries, which nevertheless provide critical habitat for several threatened species as well as important ecosystem services that mediate the Island’s terrestrial and coastal systems. It however is subject to gradual change due to changing hydrological and sedimentation patterns affected by land uses within the upstream watershed. The chapter went further to identify plausible future island socio-ecological scenarios with concrete narratives and key metrics, building on downscaling existing scenarios on one hand, and on the other hand regional policies and periodic statistics over the past two decades. The scenarios are fourfold: (A) nature-sensitive integrated tourism and agriculture; (B) nature-centered tourism; (C) integrated tourism and agriculture; and (D) resort island scenarios. We also identified ten key quantitative metrics to gauge the four scenarios: (1) tourist number, (2) nature tour destination area, (3) residential area, (4) land development area, (5) coastal development, (6) dry and paddy field area, (7) grassland area, (8) conservation agriculture area, (9) protected area, and (10) restored area. The synthesis of the Island’s socio-ecological system and the plausible future scenarios articulated in this chapter provide a robust basis for modeling future island ecosystems and their services centering on mangroves in Ishigaki Island. Furthermore, the chapter demonstrates methodological advancement in downscaling existing high-level scenarios with the aid of regional periodic statistics and a comprehensive review of regional policy documents.