Thursday, 31 January 2013

100 Species, Number 5: Earthworm


David North, Head of People and Wildlife       

He thought of all the infinitesimal motions of the world, the obstinate, heartbreaking progress of an earthworm, eating its own route forward.
(Carrie Brown, Rose’s Garden)

The power of earthworms comes not from their individual strength but from their collective strength – something we in the conservation movement could perhaps learn from!


Earthworm, photo by Richard Burkmarr
Out of sight and out of mind. In your garden, in the fields and woods of Norfolk, and across Britain, from our Cley Marshes reserve on the Norfolk coast to the Cors Dyfi nature reserve on the Welsh coast (run by Montgomeryshire Wildlife Trust), uncountable numbers of worms are doing what worms do best, eating and tunnelling, and in the process building our Living Landscapes.

How often do we wonder about what lives under our feet – the life in the soil. Millions – no billions - of living creatures inhabit every garden, living their lives unnoticed in the soil. None are more important than earthworms. There are worlds within worlds under our feet – soil is a habitat that supports a myriad life forms, from the microscopic to the massive, including the earthworms most feared enemy, the mole. (A species for a later post perhaps!)

Worms are blind, deaf, have no spine, no bones, no teeth, a length of just a few inches and their bodies are 80% water. So why include them in my list of 100 species that make Norfolk? 

Well Charles Darwin said of the earthworm, ‘It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organised creatures.’ The power of earthworms of course comes not from their individual strength but from their collective impact – something it took the genius of Darwin to recognise. 

Charles Darwin, one of my naturalist heroes, was the first person to take earthworms seriously. Though perhaps seriously is the wrong term, as his research included getting his wife to spend many hours singing and playing music to worms to discover if they could hear! However from his observations of worms Darwin drew remarkable and profound conclusions. This interest in earthworms was encouraged by an observation made by his uncle Josiah Wedgewood (of Wedgewood pottery fame) who observed that pieces of brick he had spread in a field had slowly become buried. Darwin immediately thought from his studies that earthworms were a likely cause.

His great insight was to realise that the small changes made by the individual earthworms he observed , when multiplied by billions for all the worms in England, and across thousands of years, could bring ‘geological’ scale change.

As he modestly records, 'The subject may appear an insignificant one but we shall see it possesses some interest’. The results of his 35 years of observations on earthworms were published in his final book, ‘The Formation of Vegetable Mould, Through the Action of Worms, With Observations on Their Habits’ published in 1881. A book which surprisingly sold more copies in its first year of publication than ‘Origin of the Species’ achieved!

So how are earthworms landscape builders? Darwin estimated that an acre of grassland might support 50,000 worms and that the weight of their casts (posh term for earthworm poo) would amount to 18 tons a year. We now know that Darwin’s estimates were on the low side – indeed if you count worms of all species then an acre of good land could support perhaps a million worms!

To understand the importance of earthworm poo, sorry casts, it’s necessary to know a little about their lifestyle. Earthworms are a bit like archaeological JCB diggers. They tunnel through the soil, eating as they travel, passing the earth through their muscular guts and rising to the soil surface to leave their casts. In this way Darwin estimated they increase the depth of the soil by 0.2 of an inch each year. That might not sound much but try multiplying it up through time! In ten years an object on the soil surface will be buried 2 inches deep and after 1,000 years by 200 inches. So Darwin had cracked Josiah Wedgewood’s puzzle of his vanishing bricks. Earthworms are great survivors. Worms (though not our lumbricus terristris) have been around for at least 500 million years and survived at least five great extinction events. 

But worms of course provide far greater services to us than gently burying former cities and civilizations for today’s archaeologists to unearth. Earthworms have been described as ecosystem engineers. They change the structure of their environments. Their burrows allow both air and water to penetrate soils but also help prevent water-logging. It’s estimated the tunnelling or worms each year on an acre of grassland creates a drainage system equivalent to installing 2,000 feet of 6 inch pipe!

Darwin described earthworms as nature’s ploughs, 'The plough is one of the most ancient and most valuable of man’s inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earth-worms.’ Darwin. 

Earthworms, including Lumbricus terrestris, by the way they feed, are very effective at increasing the organic matter in the soil. They pull leaves and decaying vegetation from the soil surface down into the soil. This method of feeding releases the nutrients locked up in dead plants, making it available to living plants. As they tunnel and feed they break dead organic matter into tiny crumbs with a large surface area that bacteria and fungi can then break down releasing plant nutrients.

Darwin’s observations on the importance of earthworms have more than been confirmed by scientists today. Indeed we are still discovering new benefits that earth worms provide. Recent studies have shown that worm casts contain strange structures of calcium carbonate but in the form of amorphous calcium carbonate which is a very strange and special structure. A single earthworm may produce between 0.2 and 4.3 milligrams of calcite a day. That doesn’t sound much but earth worm produced calcite could lock up 564kg of carbon per hectare per year. That’s as much carbon as would be locked up by tree growth if a hectare was planted up. Pretty amazing. Maybe it’s the earthworm that holds the answer to climate change. Its already busy working to protect us from sea level rise of course by raising the land surface.

It has even been suggested that the origins of human civilization and farming can be linked to earthworms. For the last 11,000 years of human history, ever since we first became farmers, people have benefited from earthworms tilling the soil and providing nutrients to our crops. It may be no coincidence that the first great civilizations, first cities, and perhaps even the invention of agriculture, occurred in areas rich in earth worms. The Nile, Indus and Eurphrates valleys, where our first great civilizations prospered, were exceptionally rich in earthworms. Earthworms in the Nile Valley have been shown to deposit up to a thousand tons of castings per acre per year, explaining the astonishing fertility of the valley. The rise and fall of civilizations may be as strongly linked to soil fertility as it is to emperors and kings, or human wars and battling philosophies. As Andre Voisin suggested, ‘One often reads of the thousands of slaves that built the Pyramids of the Pharaoh. In actual fact, these enormous edifices owe their existence in the main to the thousands of slaves inhabiting the sub-soil of Egypt.’ (Andre Voisin Better Grassland Sward 1960)

Darwin may not have used the term ecosystem service but he certainly discovered, and understood, the principle that people benefit from nature’s services. His pioneering studies on earthworms are still to my mind the best on an ecosystem service. He recognised the great truth about soil, which he refers to as vegetable mould, ‘‘All the vegetable mould over the whole country has passed many times through, and will again pass many times through, the intestinal canals of worms.’ (Darwin). 

Of course its not just people who benefit from earthworms. They are vital in the food chains of many other species. And it’s not just early birds that catch the worms, a whole host of wildlife, from shrews and voles to moles and badgers, rely on earthworms in their diets. This key role in food chains was understood by another great natural observer, Rachel Carson. She also recognised that worms ingesting soil would mop up and concentrate in their bodies chemical poisons such as DDT being used by farmers. She found that worms were able to take up huge quantities of persistent DDT from the soil without it killing them, but a small bird would be killed, or rendered infertile, by eating just 11 worms. Her studies led to the publication of Silent Spring in 1962, A book which did change the world and eventually led to the banning of these poisons. Even more recently it’s been shown that other poisons, such as PCBs, are broken down much faster in soils with healthy worm populations. Today worms are used both for pollution control and waste disposal.

So next time you stand in your Norfolk garden, or walk through a Norfolk wood or meadow, think about the worms under your feet. There is a whole world down there in the soil – a world as full of mysteries, dramas and magic; of predators and prey, food-chains and habitats, parasites and symbiants, as in our world. The hidden depths under our feet are as strange and biodiverse as any of the ecosystems in the superficial world of the surface. I wonder, when we own a nature reserve how deep does our ownership go? Do we own the world up to a metre down? 10 metres? 100 metres? Life penetrates deep – is that life part of our nature reserves? 

I don’t have an answer to that question, but I do know, if I had to choose one species that puts the life in our Living Landscapes, it would be the earthworm. They were once known as ‘angels of the soil’. That seems a pretty good name to me too.

 ‘I would not enter on my list of friends the man who needlessly sets foot upon a worm.’ (William Cowper)

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