The Book of the Earthworm Page 2

  Earthworms, rather like bees, are one of the groups of animals known as ‘keystone species’. They have such a profound effect on our ecology that, without them, humans would struggle to survive.

  Modern agriculture relies on healthy, productive soil. From growing cereals, fruit and vegetables to industrial crops such as biofuels, fibre for clothing and animal feeds – good soil supports it all. Even some of the things we might not first think of – from paper to pharmaceuticals – started life as a plant growing in ground made fertile by the actions of earthworms.

  Without earthworms, the soil would die. They play a crucial role in breaking down organic matter (such as dead leaves and flowers) and putting that goodness back into the soil. Thanks to their constant eating and pooing, earthworms release the nutrients in organic matter by breaking it down into tiny pieces, making it available for use by living plants, fungi and bacteria. An estimated 3.5 × 1010 (35 billion) tonnes of terrestrial leaf litter are turned over worldwide each year in soils.4

  As they wriggle and squirm underground, earthworms also aerate the soil, leaving it light and fluffy. Their constant burrowing creates air pockets, through which water can drain. Without this sponge-like texture, soil soon becomes compacted and prone to flooding. This never-ending churning and ploughing also helps bring nutrients that are deep in the soil back up to the surface, where plants can easily access them. Scientific experiments have shown, for example, that earthworms increase plant growth by, on average, 20 per cent5 in comparison to plants growing in soils with no earthworms.

  In fact, earthworms do such an incredible job at improving the soil in our gardens that they actually help plants resist attack from other pests. Studies have shown that earthworms boost the nutritional value plants get from the soil, enabling them to better defend themselves against the insects and herbivores who eat them. Plants that are constantly damaged by insects or herbivores try to protect themselves by changing the chemical composition of their leaves. When the nutrition of the plant is improved by earthworm activity, the plant can produce more of these defensive compounds. In one experiment, for example, where invasive Spanish slugs (Arion vulgaris) were introduced to micro-ecosystems to monitor how much damage they would do, the sites where earthworms were present reduced the number of leaves damaged by 60 per cent.6

  EARTHWORMS AND SEEDS

  Earthworms also have an interesting role to play in seed germination. Soil is full of seeds; when plants drop their seeds, not all of them will germinate straight away. Depending on where they land, seeds will either grow, rot or go into dormancy, waiting for the right conditions to thrive. The seeds in this last category inevitably get covered over and worked into the soil, where they can persist for years. It’s a clever strategy and means that plants can survive, in the form of an underground ‘seed bank’, even if conditions above ground won’t support them.

  Studies have shown that some species of earthworms eat seeds as they chomp through the soil and leaf litter. By ingesting seeds, and then depositing them on the soil surface in their casts, earthworms are one of the main ways seeds get a chance to come back to the soil surface and grow. Scientists have also shown that seeds that have passed through the guts of an earthworm have a better chance of germinating than seeds that haven’t – the chemical composition of worm casts seems to give seeds a better chance of germinating.7 A reverse effect is also beneficial – by burying seeds through their day-to-day activity, earthworms may be key to the formation of large seed banks in many different ecosystems.

  ARE EARTHWORMS IN TROUBLE?

  To forget how to dig the earth and tend the soil is to forget ourselves.

  Gandhi

  Some earthworms are having a rough time. Unlike nature’s more dazzling, attention-grabbing species, the earthworm and its story have gone largely unnoticed. Some of the more exotic species – such as Australia’s Giant Gippsland earthworm (Megascolides australis) – are endangered, thanks in most part to habitat loss. And the picture isn’t too rosy elsewhere.

  Studies have shown that, overall, numbers of earthworms are declining in certain areas, especially those regions that are intensively farmed. No one quite knows what’s happening, but researchers suspect that excessive tillage (the mechanised digging and turning of soil), certain types of crops, microplastics in the soil and the overuse of pesticides could play a huge role in the decrease.

  In a recent scientific study of English agricultural land, for example, nearly half of the land surveyed was seriously deficient in earthworms. Roughly 1,300 hectares of farmed land was studied in total. Over 40 per cent of fields had ‘poor earthworm biodiversity’ – meaning very few or no surface-dwelling and deep-burrowing worms were found. And while most fields had healthy numbers of shallow-burrowing earthworms, a fifth of all fields had no surface-dwelling worms and about one in six fields had no deep burrowers at all.

  The farmers involved in the study, who did most of the surveying, were so alarmed by the results that over half of them pledged to change their soil management practices as a result.8 Similar findings have been recorded across Europe – in Slovakia, for example, one study found the density of earthworms was nearly twice as high in the soil under permanent grassland compared to agricultural land.9

  But the situation is not all bad. Habitats such as pasture and gardens seem to be supporting high numbers of earthworms. A citizen science count by Earthworm Watch, for example, recently found densities of 200 earthworms per metre which, if extrapolated out to the average UK home, could mean we each have over 32,000 earthworms in the everyday back garden.

  HOW DOES A DECLINE IN EARTHWORMS AFFECT US?

  While it’s brilliant that suburban gardens and grassland are havens for earthworms, the situation for farmland is a serious one. Fewer earthworms means lower soil fertility, which in turn equals lower crop yields – a real problem in terms of food security for the world’s growing population.

  Poor soil fertility also produces less nutritious plants – a 2004 study published in Scientific American compared nutrients in crops and vegetables grown in the 1950s with those grown at the turn of the millennium and revealed significant drops in levels of protein, calcium, phosphorus, iron, vitamin B2 and vitamin C.10

  A decline in the number of earthworms also affects the wildlife that relies on them for food; the song thrush population, for example, has plummeted in recent years, a crash thought to be due, at least in part, to an absence of earthworms to feed their young in springtime. Fewer worms means that baby chicks either starve in the nest or fail to thrive after they leave. Without earthworms, other wildlife such as hedgehogs, foxes, robins, shrews, moles, wood mice and badgers also struggle to find enough food. Earthworms, for instance, make up about 60 per cent of a badger’s diet, while the American robin can catch 4 metres’ worth in a single day.

  A recent study also found that long-term pesticide use seems to be having a direct effect on earthworms. A Danish/French research collaboration discovered that agricultural pesticides, used on crops, are stunting the growth of earthworms and causing a drop in their ability to reproduce. The scientists involved in the study found that, although earthworms can tolerate a certain level of pesticides and fungicides in the soil and have developed strategies to detoxify themselves, this behaviour comes at a cost. It takes a huge amount of energy for the earthworm’s system to be constantly ridding itself of toxic chemicals – the result is that earthworms in heavily sprayed areas suffer from stunted growth and are less successful at breeding.11

  Fewer earthworms also means that soil can get very compacted. If agricultural land can’t absorb rainwater, the run-off ends up in local watercourses. This flow is often contaminated with pesticides, nitrates and other chemicals, which, if it trickles into streams and rivers, causes a great deal of harm to aquatic wildlife. For soil to absorb water effectively, earthworm burrows need to connect to one another like a huge network of drains – research has shown that tillage can significantly damage these water conduits, st
opping them from working. Other agricultural activities, such as the overuse of pesticides, have also been shown to affect how quickly soil can absorb water, because of increased earthworm mortality.

  It might not seem much of a problem if earthworm burrows are damaged and don’t drain as effectively, but flooding can cause untold misery to communities and the people whose homes, public spaces and services are ruined when rainwater has nowhere to go. Soil that is full of earthworm tunnels is less likely to flash flood; when you consider that more people are killed in the US by flooding each year than by tornadoes, hurricanes or lightning,12 the ecological significance of the lowly earthworm is thrown into stark relief.

  Alongside intensive agriculture, there’s a rather more exotic threat to earthworms. Certain species of flatworms – particularly New Zealand’s Arthurdendyus triangulatus – pose a serious threat to European populations of native earthworms. These tiny ribbon-like creatures originally landed on European shores by accident, hidden in the soil of gardeners’ pot plants. Slim and very stretchy, this particular flatworm seems to love eating earthworms, which it devours by first wrapping itself around the worm and then liquidising it by secreting digestive enzymes. Researchers have found that one flatworm can wolf its way through fourteen earthworms a week and, even after it has decimated an earthworm population in a particular place, can survive for up to a year waiting for the earthworm numbers to improve, so it can once again resume its assault.

  A recent study also found a new and unexpected challenge to earthworm health – microplastics in the soil.13 These are pieces of plastic less than 5 millimetres in size; they are absolutely everywhere, widely distributed in our seas and in our soils. They come from two sources – one is ‘primary microplastics’, such as microfibres, microbeads and pellets that enter the environment as tiny fragments. The other source is larger pieces of plastic – such as plastic bottles, carrier bags and packaging – that, over time, degrade and break down into minuscule pieces. No one had studied the effect of these microplastics on earthworms until this research project looked into the problem. Surface-dwelling earthworms (Eisenia fetida) were exposed to soil that contained minute amounts of fluorescent polystyrene microplastics over a two-week period. Even at very low concentrations, the results showed that the earthworms suffered both cell and DNA damage from exposure to microplastics.

  How to help earthworms #1

  FEED THE WORMS

  ● The more organic matter you can add to your soil, the more earthworms you’ll have. Under natural, ‘wild’ conditions, the condition of the soil tends to stay fairly even – the year-round cycle of growth and decay ensures that enough organic matter is reincorporated into the soil, along with animal droppings and decomposing creatures.

  ● In gardens and other growing spaces such as allotments, we tend to take more goodness out of the soil than we put in. We tidy up fallen leaves, pull out dead plants and clear away the natural debris left behind at the end of the season – all the things that would ordinarily contribute to the richness of the soil.

  ● There are a number of ways to add organic matter to soil – home-made compost (see How to help earthworms #3 page 52), shop-bought compost, municipal compost (from council-run recycling centres), mushroom compost from commercial growers, spent hops from breweries, well-rotted manure, spent coffee grounds, leaf mould and composted bark.

  ● Apply organic material in spring, before the growing season starts – gardening bodies usually recommend applying organic matter at least 5 centimetres deep but even just a generous sprinkling will help improve the nutrition in the soil for earthworms. Keep adding extra compost every year.

  WHAT’S WORM-FRIENDLY FARMING?

  Who are the farmer’s servants? … Geology and Chemistry, the quarry of the air, the water of the brook, the lightning of the cloud, the castings of the worm, the plough of the frost.

  Ralph Waldo Emerson, from ‘Farming’ in Society and Solitude (1870)

  The same principles for keeping garden earthworms happy apply to farmed land; they need plenty of decaying plant matter to feed on and minimum disturbance.

  If a farming system constantly removes all crop residues before they are recycled back into the soil, earthworms will struggle to find anything to eat. If we can find a way to increase the amount of organic material that gets left on agricultural land, either through crop stubble, using green manures such as clover or vetch that act as a mulch, crop rotations that add organic matter to the soil, or even switching some arable land to permanent pasture, all these measures would increase the amount of plant material available to earthworms.

  Digging and turning soil, especially on a large mechanical scale, also creates a number of problems for earthworms – it destroys their burrows, displaces their cocoons, and often brings them to the surface, where they either die from UV exposure or desiccation, or get gobbled up. The constant turning of soil also tends to dry it out – a disaster for creatures that need moist soil. Practices such as no-till agriculture, leaving behind crop residues, planting cover crops and using no-drill equipment are all being tested as ways to improve soil health and bring back earthworm numbers.

  It’s unfair to blame farmers. We all have a role to play. Downward pressure on prices, encouraged by consumers wanting cheaper food, has forced farmers to increase their output while reducing inputs. This approach is exacerbated by issues such as short-term tenancies, which don’t allow farmers to think long-term about their land, and the demand for energy crops such as maize, which can exacerbate soil degradation. There’s also been little in terms of education, government incentives or national policy to help farmers to get the right advice and access to new methods and equipment.

  DO EARTHWORMS AFFECT CLIMATE CHANGE?

  The health of soil, plant, animal and man is one and indivisible.

  Sir Albert Howard (1873–1947)

  In recent years it was thought that earthworms might be actually contributing to greenhouse gas emissions. As earthworms go about their business, turning over soil, scientists wondered whether they were releasing carbon dioxide that was trapped in the soil into the atmosphere.

  Soil acts as a huge storage system for carbon, which is pulled out of the air by plants and transported by their roots into the soil. When soil is dug up, turned over and tilled, it speeds up the release of carbon back into the atmosphere. It was thought that, because earthworms churn up soil, they might also be speeding up this carbon-release process.

  However, new studies have shown that earthworms actually help stabilise carbon into a form that is more likely to stay in the soil. As soil passes through an earthworm’s digestive system, the soil carbon is converted into a form that is more resistant to decay. Once it has come out of the other end of the worm, the soil also has a crumblier structure, which not only helps to hold on to the carbon, but also makes more plants grow, which in turn pull more carbon from the atmosphere.

  Other research has shown that deep-burrowing earthworms actually change the distribution of carbon in the soil, dragging it further down underground. When soil carbon is near the surface, it’s very vulnerable to being released when the soil is tilled. Deep-burrowing earthworms move the carbon from the surface soil and take it to the deep subsoil, where it is much less likely to be disturbed.

  EARTHWORMS AND SPACE TRAVEL

  While we’ve yet to find life on other planets, that doesn’t mean earthworms won’t play their part in space exploration. One of the most interesting recent experiments involving earthworms involved trying to see if they could survive in soil similar to that found on Mars. If humans want to live on the ‘Red Planet’, working out how to grow crops on Mars will be one of our biggest challenges. Scientists believe the only way to do this will be to modify the planet’s soil, which is sterile and full of toxic compounds.

  One fascinating piece of research not only managed to keep earthworms alive in the simulated Mars soil, but the worms actually started to reproduce and produce new baby worms. The key
to any self-sustaining agricultural system in outer space would have to revolve around the recycling of nutrients – from plant and human waste – back into the soil. Earthworms, NASA believes, will be critical to this process. The next stage in the study is going to investigate what kinds of bacteria and fungi will be able to cope in Mars’ soil and, crucially, how crops will be pollinated. The bumble bee is next on the research list.

  Space travel also uncovered another scientific puzzle back down on Earth. First described by scientists in the 1940s but well known by locals, the wetlands around the Orinoco river in South America are studded with strange mounds of earth. Known as ‘surales’ by the native population, no one could explain how this vast mosaic of regularly spaced, bizarrely uniform mounds came into being – they cover 29,000 square miles and can reach up to 2 metres high and 5 metres wide. Only from space could the true extent of the surales be seen and yet no one, until recently, could explain their presence. Giant earthworms turn out to be the culprits, creating huge piles of worm casts as they dig through the waterlogged soil.

  WHY ARE WORMS CALLED ‘WORMS’?

  Then the worm woke; cause of strife was renewed: for then he moved over the stones, hard-hearted beheld his foe’s footprints…

  Beowulf (trans. T. Donaldson)

  ‘Worm’ is an ancient word but throughout time its meaning has shifted. In Anglo-Saxon, or Old English, worm or ‘wyrm’ was a general word for any creature that crawled, wriggled or slithered its way around. Such a broad category meant that frogs, snails, snakes, scorpions, earthworms and many other unrelated animals came into the same category. What unified them all, however, was their unpopularity. ‘Wyrms’ were creatures viewed as noxious, repulsive and dangerous – ‘vermin’ comes from the same root and we still use it to describe troublesome pests.