Australasian Science: Australia's authority on science since 1938

Tidal Flats Are Disappearing

By Nick Murray

The world is losing its tidal flats at an alarming rate, putting enormous pressure on threatened migratory birds.

Who speaks for the tidal flat? There are many voices for the mangrove forest, the coral reef and the seagrass meadow, but the chorus for tidal flats is often silent. Not only do hundreds of species of migratory bird depend on them for their existence, this coastal ecosystem also protects large chunks of humanity, providing ecosystem services to hundreds of millions of people.

The problem for all coastal ecosystems is the shifting character of the coastal zone. The past 50 years has seen the global human population migrating rapidly to coastal regions. As a result, coastlines around the world have become a focus of expansion of urban, agricultural and industrial areas.

This is having a major impact on coastal ecosystems, leading to the widespread loss and degradation of mangroves, seagrasses, coral reefs and tidal flats. This has major consequences for humans and nature. In terms of the human cost, coastal ecosystems are a frontline defence that protects billions of dollars of infrastructure from storms and sea level rise. Maintaining their integrity is among the most cost-effective options for coastal protection.

Tidal flats are a widespread coastal ecosystem that are frequently overlooked in the planning and management of coastal resources even though tidal flats provide important ecosystem services, such as food resources and storm protection. In addition, they support the majority of the world’s migratory shorebird species, enabling their yearly migration from their breeding regions in the Arctic to areas as far south as Patagonia.

In many countries tidal flats don’t receive the same protected status as mangroves, coral reefs or seagrasses, and consequently many tidal flats have been lost to development.

What’s the magnitude of the problem? Until now we have had no way of knowing just how much of this declining coastal ecosystem has been destroyed, or how much and where it remains. The principal reason for the lack of accurate maps of this ecosystem is due to the rapidly changing conditions they encounter: changing tides either expose or cover them, severely limiting the application of classical mapping methods.

To solve this problem, a small team of remote sensors and spatial ecologists from The University of Queensland have been developing methods to map tidal flats over very large areas. Using the heavily developed tidal flats on the mainland of East Asia as a case study, we developed a rapid mapping approach to identify the distribution of tidal flats while assessing their changing status at continental scales. Our methods use free data from the US Geological Survey’s Landsat Archive and freely available regional tide models, and allow fast implementation across thousands of kilometres. Implementing the methods, we determined the changing status of tidal flats across more than 14,000 km of coastline in East Asia.

The tidal flats in this region, which fringe the coastlines of Korea and China, are among the largest in the world and provide ecosystem services to an estimated 160 million people. Coastal development frequently occurs via land creation (often termed “reclamation”). Tidal flats have proven an ideal environment for cheap and rapid coastal development, involving the construction of seawalls, infilling and finishing for aquaculture, agriculture, suburbs and industry.

Our results demonstrate that tidal flats in East Asia are being destroyed at rates similar to other major threatened ecosystems, such as tropical forests and mangroves. The principal cause of these losses is related to rapid and widespread coastal development.

Loss of coastal wetlands to land reclamation is a global problem that is severely affecting the world’s coastlines. In China alone more than 1.2 million hectares of wetland reclamation took place in the past 50 years, perhaps accounting for more than 5% of the world’s tidal wetlands. This arc of growth will form one of the world’s largest urban areas by 2030: a continuous coastal urban corridor more than 1800 km long.

Effective conservation strategies for tidal flats must manage the complex economic and social trade-offs that drive coastal development. Decision-making that simultaneously plans for coastal development and coastal conservation along the world’s most rapidly developing shores is clearly needed. Not only might this avert catastrophic extinctions of coastal biodiversity, it will also help ensure we have a coastline capable of adapting to an increasingly uncertain future.

Nick Murray is currently based at The University of New South Wales Centre for Ecosystem Science. The work described here on mapping tidal flats was done with the Environmental Decisions Group at The University of Queensland.