BEIJING—Along China’s coastline, rapid development has transformed marshes and mudflats into ports and urban sprawl; a line of concrete seawalls and sandbags now stretches longer than China’s Great Wall. The decline of wetlands is nearing a critical threshold below which the losses could inflict severe and lasting harm on ecosystems—driving numerous migratory bird species to the brink of extinction and jeopardizing nearly 20% of the world’s fisheries, warns a new report from Chinese and U.S scientists.

Recognizing the vital role that wetlands play in ecosystems management and flood control, on 25 April China’s central government drew a line in the mud, decreeing that no fewer than 53.33 million hectares of wetlands must be conserved. However, the new report forecasts that if current and planned coastal reclamation continue unabated, by 2020 the government’s red line “will be broken.”

Without “legislative power to stop reclamation, crossing the ‘red line’ is just a matter of time,” warns Jing Li, founder of the Shanghai-based nonprofit Saving the Spoon-Billed Sandpiper in China, named after a critically endangered species that depends upon China’s coastal mudflats.

Already half of China’s coastal wetlands have disappeared over the past 50 years, enclosed by seawalls or overrun by ports and other development, according to the analysis, released here Monday by China’s State Forestry Administration (SFA), the Chinese Academy of Science’s Institute of Geographic Sciences and Natural Resources Research, and the Paulson Institute, a nonprofit based in Chicago. Drawing on original research and published studies, the report also found that 70% of China’s mangrove forests and 80% of near-shore coral reefs have vanished in the past half-century.

The findings underscore a central tension in conservation in China: reconciling the directives of the central government, which increasingly strives to balance development and environmental protection, with the fervor of local officials for economic growth. “Huge economic returns from reclamation have prompted local governments to ‘bypass’ regulations issued by the central government,” the report explains. “Sea reclamation is deemed as the quickest and cheapest way to increase land supply in China’s eastern coastal areas.”

Competing policies can also undermine conservation. Coming into play is China’s need to safeguard its food supply by maintaining a minimum area of land—120 million hectares of arable land—for growing crops. “Ironically the crossing of the ‘red line’ on wetlands probably results from trying to achieve the ‘red line’ on agricultural land,” says Spike Millington, chief executive of the South Korea–based East Asian-Australasian Flyway Partnership (EAAFP), “since any agricultural land taken over for development has to be compensated by an equivalent area of newly created ‘agricultural’ land elsewhere.”

EAAFP is tracking the link between disappearing wetlands in China and the Koreas and the plummeting numbers of migratory birds in Asia, including more than 50 endangered or threatened species. The global population of the iconic spoon-billed sandpiper, for instance, has plummeted to only about 220 breeding pairs in 2010, down from an estimated 2000 to 2800 breeding pairs in the 1970s, mostly because of coastal habitat loss.

For China to achieve its national wetlands conservation target, local guidance for implementation must become much more specific—and meaningfully enforced, Li says. “The SFA has very limited political influence for reclamation, unfortunately,” she says. The agency “can only provide consultancy advice in terms of wildlife protection.”

If rumored development goes forward in one area of intact mudflats her group is closely monitoring, in Rudong County near Shanghai, it could “ring the death knell for that species,” Millington says. “We don’t have the luxury of time for these sites.”