Not Rainforest, Not Reef: on the beauty and significance of mud

Every year, millions of shorebirds migrate from Australia to Siberia via the Yellow Sea – but populations are declining catastrophically. As Harry Saddler discovers, the reason may have a lot to do with our lack of respect for mud.

If I turn my head in an arc from left to right, I see a train station, a long line of traffic, endless city blocks, a wide brown river, a bridge and North Korea.

I'm in Dandong, in north-east China. Forty kilometres down the highway is the booming Dandong Port. The main reason tourists come to Dandong is because there's no better place from which to launch a tour of North Korea – but I'm here because of the river.

The river is called the Yalu, and it forms a natural border between China and North Korea. Starting in the Changbai Mountains, it flows for nearly 800 kilometres to the Yellow Sea. As it flows, it drains an area of 30,000 square kilometres. When I walk along the river on my first day in Dandong, declining the offers of touts trying to sell me tours of North Korea, I can see sediment from all that land suspended in the water like milk poured into tea. Even this far from the sea, the Yalu is tidal, and at low tide slick banks of dark brown mud line the river.

The Yellow Sea could, in an ideal world, support the migration of two million shorebirds twice a year. But mud is mud. It's not rainforest, it's not coral reef. It doesn't look pretty – so we don't value it.

It's this mud that makes the Yalu River special. Since the end of the last ice age 10,000 years ago, the Yalu River, along with half a dozen other rivers in China and the Korean peninsula, has disgorged all the sediment it has collected from its catchment straight into the Yellow Sea. When the tide in the Yellow Sea goes out, mudflats extend for kilometres from the shoreline.

If I told you that mudflats were one of the most ecologically rich environments in the world, you might not believe me. After all, mud is mud – it's not as obviously biodiverse as rainforest or reef. But beneath the surface of the Yellow Sea's mudflats are billions of animals: shellfish, crabs, worms, snails. If you stand very still and look closely at the mud, you can see some of them rushing to their burrows – and if you visit the Yellow Sea at the right time of year, in autumn or in spring, you'll also see one of the most spectacular natural phenomena in the world.

Mudflats on the shore of the East China Sea. Image: johnlsl (CC BY-NC-ND 2.0)

Every spring, and every autumn, millions of birds from three dozen species – known, collectively, as shorebirds – migrate between Australia and the Arctic. They breed in the far north, but they're there for only a few weeks: the Arctic summer is fertile but brief. When their eggs hatch, the birds fly back to Australia, where they spend the southern summer resting and preparing for the next flight north.

When they migrate, they fly night and day, shutting down half their brain at a time while their wings keep flapping. Most of their migration is over open ocean, but they can't swim – if they get exhausted and fall, they drown. They fly at speeds of up to 60 kilometres an hour, always flapping, never gliding, and they fly astonishing distances: 6000, 7000, even up to 11,000 kilometres without stopping.

Before they fly, they feed constantly. They nearly double their body weight in a few weeks, piling on fat to fuel their journey, many of their internal organs shrivelling to lighten the load. By the time they leave Australia, many shorebirds will have started moulting into colourful breeding plumage, turning rich orange, black, and white. Then, when they do finally arrive at their breeding grounds, they'll switch their diet completely – to insects, berries and leaves. They repeat this process twice for every year of their life, beginning their first migration only weeks after hatching. No other group of animals on earth undergoes such extensive bodily changes on such a regular basis.

To fuel their extreme migration, these birds need one thing: the Yalu River’s mud. If they couldn't gorge on all those tiny animals living under the surface of the mudflats, shorebirds wouldn't be able to fly the distances they do. Nearly all of the species that migrate to and from Australia pass through the Yellow Sea.

Mudflats on the shore of the East China Sea. Image: johnlsl (CC BY-NC-ND 2.0)

The Yellow Sea could, in an ideal world, support the migration of two million shorebirds twice a year. But mud is mud. It's not rainforest, it's not coral reef. It doesn't look pretty – so we don't value it.

Already, nearly half of the mudflat that once existed in the Yellow Sea has been destroyed. Six hundred million people live in the Yellow Sea's catchment area – nearly 30 times the population of Australia, clustered around a sea that's only 1000 kilometres long and 700 kilometres wide. As the Chinese and South Korean economies boom, factories are built on land that used to be mudflat. As products from those factories are shipped through the Yellow Sea to the world, the sea is being pushed to breaking point. Its mudflat ecologies are collapsing.

And as the mudflats go, so do the shorebirds. Population declines of some species are as high as 8% a year. The population of the largest migratory shorebird in the world, the eastern curlew, is estimated at only 38,000 birds, and dropping. You could sit them all in the MCG and it would only be a third full. Other species are faring worse.

A bar-tailed godwit. Almost the entire population of this species pass through the mudflats of the Yalu River in May each year. Image: Sergey Yeliseev (CC BY-NC-ND 2.0)

In Victoria, one of the best places to see shorebirds is the Western Treatment Plant. The sewage works of Australia's second-largest city is a shorebird paradise. Another good place to see them is Cheetham Wetlands, near Altona – the site of a defunct saltworks, once owned by Cheetham Salt. But most shorebird habitat in Australia is still unprotected, and much of it is subject to development. Each species of shorebird requires a specific type of mud; the consistency of the mud, the depth of the mud, and the types of animal the mud is home to all affect whether a given species of shorebird can utilise it or not. And shorebirds know this: they return to the same feeding grounds, in every country along their migration route, year after year. If a particular habitat is lost, whether it's in China or Australia or anywhere else, shorebirds can't go elsewhere.

Animals have no choice now but to navigate the human world. If you go down to the coast next summer, keep an eye on the tideline: you might see a bunch of small brown birds, busily feeding. You could hold the smallest of them in your cupped hands. They'll be gone again in a few months, their migratory instincts drawing them north.  But how long they'll keep coming back depends on us.

Portrait of Harry Saddler

Harry Saddler is a Melbourne-based writer. He was joint-winner of the 2014 Melbourne Writers Festival/Blurb Inc 'Blog-to-Book Challenge' for his blog Noticing Animals. He is currently travelling through China and South Korea researching a book about migratory shorebirds, to be published by Affirm Press in 2017.

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