Why Is That Mud Worth Protecting? It Feeds Millions of Birds

Fine particles carried by rivers and stirred from the seabed stay suspended while water moves and settle out when it slows, so they accumulate wherever the flow stops for a period, which is precisely what happens at every high and low tide. Sheltered coasts, estuaries and the landward side of barrier islands therefore build up deposits of silt and clay over a gentle slope, exposed as the tide falls and covered as it rises. The particles are small enough to pack tightly and hold water, which is why the surface is soft and why oxygen penetrates only a centimetre or two before the sediment turns black.

The apparent emptiness conceals very high densities of life:

• •Worms, shellfish and crustaceans buried in the sediment
• •Densities that can exceed tens of thousands of animals a square metre
• •Microscopic algae on the surface, visible as a brown film
• •Those algae fixing carbon at rates comparable to a rainforest
• •Almost no plants above the surface, which is why it looks bare
• •Fish moving in with each tide to feed over the flats

Waders crossing continents fly in stages and must refuel completely at a small number of stopover sites, which makes those sites disproportionately important. A bird arriving after a flight of several thousand kilometres has consumed much of its own body, including part of its digestive system, and must rebuild before continuing, which requires food available in enormous quantity in a short window. Flats supply exactly that, and individual sites support hundreds of thousands of birds within a few weeks. Bill length and shape determine which depth each species probes, so many species feed on the same ground without competing. Losing one site in a chain can break the whole migration.

The flats are not uniform and the banding across them is legible once known. The upper part, exposed for most of the tidal cycle, is drier, saltier and colonised by salt-tolerant plants that trap more sediment and eventually build into marsh. The middle zone is exposed and covered roughly equally and holds the greatest abundance of burrowing animals. The lower part, exposed only briefly at the largest low tides, is softest and holds species that cannot tolerate long exposure. Creeks draining the surface concentrate the flow and are where fish and their predators move. Each zone supports a different community, which is why a wide flat is more valuable than a narrow one of the same area.

Reclamation has been severe and the figures are stark. Estimates using satellite imagery published in 2019 found that roughly sixteen per cent of the world's tidal flats disappeared between 1984 and 2016, with the greatest losses in Asia. The Yellow Sea, which is the critical staging area for waders on the East Asian flyway, has lost most of its flats to land reclamation for industry and agriculture, and populations of several species using that route have declined steeply and measurably as a direct consequence. Sea level rise adds a further pressure, since flats can migrate landward as the sea rises only where there is undeveloped land behind them, and a sea wall prevents that entirely.

Fine particles settle wherever tidal flow pauses, building soft deposits that hold tens of thousands of buried animals a square metre and surface algae fixing carbon at rainforest rates. Migrating waders arriving after flights of thousands of kilometres must refuel completely at a few such sites, so losing one breaks a whole route. Roughly a sixth of the world's flats vanished between 1984 and 2016.