Why Are Earthworms Important? Soil, Plants and Wildlife

Earthworms let air and water into the soil

As earthworms tunnel, they leave a network of channels behind them. Deep-burrowing species such as the common earthworm dig vertical burrows that commonly reach 1–2 m (3–6.5 ft) deep and are used for months or years. Shallow-living species leave countless smaller tunnels in the topsoil.

These channels let air reach plant roots and soil microbes, and they help rainwater soak in instead of running off the surface or pooling. Roots often grow down old worm burrows, where the soil is loose and lined with nutrient-rich mucus and droppings. Soils with plenty of earthworms are usually crumbly, well drained and easy for roots to push through.

They recycle dead leaves and plants

Soil scientists sort earthworms into three groups by lifestyle. Litter-dwellers live among fallen leaves and compost at the surface. Soil-dwellers burrow through the topsoil and eat soil with its organic matter. Deep burrowers, such as the nightcrawler, live in permanent burrows and come up at night to pull leaves down into the ground.

Together they break down huge quantities of dead plant material. Leaves are shredded, partly digested and mixed with mineral soil, where bacteria and fungi finish the job. Nutrients locked in dead leaves, such as nitrogen and phosphorus, are released in forms plants can take up again. Without this work, dead leaves would pile up and decay much more slowly.

Worm casts are natural fertilizer

Everything an earthworm eats passes through its gut, where a muscular gizzard grinds it with grains of sand. What comes out is a cast: a small heap of fine, crumbly soil glued together with mucus. Casts contain more plant-available nitrogen, phosphorus and other nutrients than the soil around them, and they form stable crumbs that resist being washed away.

Deep-burrowing worms leave many casts at the surface, which gradually buries leaves, stones and other objects. Over years, this turns over the top layer of soil and builds a fine, dark topsoil.

What Darwin discovered about earthworms

Charles Darwin was fascinated by earthworms for about 40 years. His last book, The Formation of Vegetable Mould through the Action of Worms, published in 1881, sold well and changed how people saw these “lowly” animals. He estimated that in parts of England worms brought up more than 10 tons of dry earth per acre each year, roughly 25 metric tons per hectare.

Darwin showed that worm casts slowly bury stones, fallen pillars and even Roman remains, and that a layer of chalk or cinders spread on a field ends up several inches below the surface after some years. He concluded that few other animals had played such an important part in the history of the world.

Earthworms feed a huge range of wildlife

Earthworms are a rich, easy-to-digest food, and a healthy pasture can hold hundreds of them in every square meter. That makes them a staple for many animals:

• Birds: American robins, European blackbirds, song thrushes, starlings, lapwings, gulls and woodcocks; see how robins find worms.
• Mammals: moles, shrews, hedgehogs, foxes and badgers, for which worms are often the main food; see what badgers eat.
• Others: toads, frogs, salamanders, slow worms, snakes, ground beetles and centipedes.

Worms are easiest to catch on damp nights and after rain. In dry summers, when they retreat deep underground, badgers, thrushes and other worm-eaters can struggle to find enough food and must switch to other prey.

Earthworms as a sign of healthy soil

Because they are sensitive to their surroundings, earthworms are used as indicators of soil health. Numbers are usually high in permanent pasture, woodland and gardens rich in organic matter, and lower in soils that are often plowed, compacted by heavy machinery, very acidic or treated with some pesticides.

Farmers and gardeners often count worms in a spadeful of soil as a quick check. In parts of north-west Europe, an invasive flatworm from New Zealand has wiped out earthworms in some fields, and soil structure there has suffered as the mixing and burrowing stopped, a clear example of how much the worms do.

When earthworms cause problems

Earthworms are not good everywhere. During the last ice age, glaciers wiped out earthworms across much of northern North America, and for more than 10,000 years forests there grew without them. These forests developed a thick, spongy layer of slowly rotting leaves on the ground.

European earthworms, including the nightcrawler, arrived with settlers, and Asian “jumping worms” spread more recently. In hardwood forests around the Great Lakes and elsewhere, invasive worms eat away the leaf layer. Studies have linked worm invasions to fewer native wildflowers and tree seedlings, changes in soil chemistry, and declines in salamanders and ground-nesting songbirds such as the ovenbird.

So earthworms are vital for farmland, gardens and grasslands where they belong, but in some North American forests they are unwelcome guests.