Will an aging and shrinking global population reshape nature conservation? How do we construct new sustainability goals to address changing demographics and yet slow progress in biodiversity protection? These questions need to be answered when planning long-term nature conservation as well as a future society with a demographically skewed population to secure the human population’s future well-being. This research is to identify the direction of nature conservation and its role in sustainable development in the coming decades. The study is composed of four case studies: (1) evacuation-designed zones in Fukushima as a more futuristically shrunk population with rebounding secondary forest with its biodiversity; (2) secondary forests in Okinawa prefecture as a place of both shrinking and aging; (3) Bialowieza Forest, Europe’s primeval forest, as a case study of nature conservation conflict in an aging society in a diffusely populated area; and (4) a mix of old-growth and secondary forest in Kachin State, Myanmar. The research will apply a set of interdisciplinary methods to produce a novel recommendation for policy and practices.