In February, a landmark study in the journal Science delivered rare good news for conservation: a five-year fishing ban in the Yangtze River had halted seven decades of biodiversity near-collapse. Fish biomass has surged, and the critically endangered finless porpoise has begun to rebound. Researchers showed that sustained, collective action can revive an ecosystem. The lesson for Hong Kong is both inspiring and urgent. Hong Kong is at a critical juncture in its environmental history. Last...