Earthworms are saprophages (detritovores) - feeding on decaying organic matter. They eat decaying leaves, stems,
roots, seeds, algae, fungi and protozoa. As such they have important jobs (niches) in the ecosystem in helping to recycle
Many common British or northern temperate species (Lumbricus terrestris, Allolobophora caliginosa, Allolobophora longa,
Allolobophora rosea, Dendrobaena octaedra) prefer nitrogen-rich and sugar-rich foods and avoid high concentrations of
polyphenols and so prefer, for example, nettle, dog's mercury, ash, elm, alder, birch and sycamore leaves more than the
leaves of oak, beech and conifers, which have high carbon/nitrogen ratios and large amounts of polyphenols.
Earthworms can be divided into several categories, depending on their lifestyle:
Epiges (surface dwellers)
These are litter-dwellers and are small earthworms, 10-30 mm long, and uniform in colour, e.g. Lumbricus rubellus,
Lumbricus castaneus, Dendrobaeria octaedra, Dendrobaeria mamalis, Dendrobaeria rubida and Eiseria foetida (the 'red
wriggler'). They are mesophagous, consuming medium-sized food particles, such as leaves and leaf-fragments.
Endoges (topsoil dwellers)
These live in horizontal, branching burrows in the organo-mineral soil layer (the organic-rich layer beneath the leaf litter).
These worms are larger, weakly pigmented, small to large in size and microphagous (feeding on minute food particles by
ingesting soil and extracting nutrients from the tiny organic particles in the soil). E.g. Allolobophora caliginosa,
Aporrectodea caliginosa (the 'grey worm'), Allolobophora rosea, Allolobophora muldali, and Octalasion cyaneum.
Anecigues (deep-soil dwellers)
These are deep-burrowing worms that live in vertical burrows and leave surface casts. They emerge at night to draw down
organic material into their burrows, which they nibble on during the daytime. They are large (200 to 1100 mm or more in
length) and the anterior end and dorsal surfaces tend to be dark brown. They are macrophagous (consuming large
particles such as grass and whole leaves) and eat 10-30% of their own body weight every day. E.g., Lumbricus terrestris
(the 'night crawler').
Effects on Soil
Earthworms are important agents in affecting soil formation and soil quality. Their effects can be summarised as follows:
- Increase soil pH
- Increase decomposition rates
- (Probably) Increase humus formation rate
- Improve soil texture
- Enrich the soil
Earthworms excrete surplus nitrogen in the form of ammonia, which is excreted into the intestine and then removed with
the faeces during defecation. The earthworm also ingests soil containing nitrogen bound to clay and organic particles in
forms which plants can not easily absorb through their roots, but on passage through the gut, this nitrogen is converted /
replaced by ammonia, nitrites and nitrates and excreted in these forms which plants can easily absorb. When the pH of
soil becomes neutral (pH 7) or alkali (greater than 7) the growth of bacteria is favoured and these bacteria increase the
rate of organic decomposition, producing a mull-type humus. In acidic soils, fungi are more dominant as decomposers,
producing a mor-type humus.
Earthworms also increase mixing between soil layers. Worm casts are richer in minerals that are in forms more readily
available to plants, such as salts of calcium, sodium, magnesium, potassium, phosphorus and molybdenum. In English
pasture, lumbricid earthworms have been shown to produce 7 500 to 16 000 kg of casts per acre per year. In South
African grassland, microchaete earthworms produce 20 000 to 100 000 kg per acre per year!