Asking where earthworms live may seem like a silly question. Doesn’t their name say it all? Soil may appear to be a homogeneous place to you or me, but not to earthworms.
There are two ways to describe where an earthworm lives. The first is its habitat. Although most earthworms live at or under the surface of the soil, other factors influence their choice of habitat. Two key factors are climate and vegetation. Earthworms tend not to live in exceptionally dry or cold places. In New Zealand, native earthworms live in indigenous forests and tussock grassland, while introduced species are most commonly found in cultivated soils such as pasture, croplands and lawns.
A more specific way to describe where earthworms live is their niche – the position of the species within its habitat. A species’ niche includes both its physical location and the role it plays within the environment. By occupying a specific niche, earthworms make use of conditions that are best suited to their survival. Although all earthworms have common characteristics, features like size, pigmentation (skin colour) and quickness of movement reflect which niche different species occupy.
Soil-dwelling earthworms fall into three main niche groupings: compost and soil-surface dwellers (epigeic), topsoil dwellers (endogeic) and deep-burrowing subsoil dwellers (anecic).
Nature of science
Scientists often use curious terms. Sometimes, the words go back to their Latin or Greek origins. The three groups of earthworms are identified by their place within the soil. Epigeic is Greek for ‘upon the earth’, endogeic means ‘within the earth’ and anecic is Greek for ‘out of the earth’.
Epigeic: surface dwellers
Epigeic earthworms live in areas containing high amounts of organic matter. They live at or near the soil surface and feed on leaf litter, decaying plant roots or dung. These earthworms do not form permanent burrows. Epigeic species tend to have dark skin colour (pigmentation). The pigmentation acts as camouflage as they move through the leaf litter. It also helps to protect them from UV rays. Being close to the ground surface exposes the earthworms to predators so their muscles are strong and thick in proportion to their length, allowing for quick movement. Being so close to the surface also makes them vulnerable to stock treading in intensively grazed paddocks. Epigeic species tend to be small (1–18 cm in length). Introduced epigeic earthworms tend to live in compost (such as the introduced tiger worm Eisenia fetida, which cannot survive in soil) and under logs and dung. Native species usually live in forest litter.
Endogeic: topsoil dwellers
Endogeic earthworms are the most common earthworm species found in New Zealand. Their niche is the top 20 cm depth of soil. Endogeic earthworms eat large amounts of soil and the organic matter in it, although species sometimes come to the surface to search for food. They form shallow semi-permanent burrows. Endogeic earthworms have some pigmentation. Their muscle layers are not as thick nor do they move as quickly as epigeic earthworms. Endogeic species range in size from 2.5–30 cm. Introduced endogeic earthworms are often found in agricultural soils, while native endogeic earthworms are often found in tussock grasslands.
Anecic: subsoil dwellers
Anecic earthworms live in permanent burrows as deep as 3 m below the soil surface. They collect food from the soil surface and ingest organic matter from the soil. Anecic earthworms form extensive burrows that extend laterally and vertically through the subsoil. Their burrows can be up to 2 cm in diameter. Introduced anecic earthworms have some pigmentation. Indigenous anecic species tend to be sluggish and have weakly developed muscles. Because they live so deeply in the soil, native anecic species have little pigmentation, and being so pale, they are often referred to as milk worms. These deep-burrowing species are also the longest, ranging from 3 cm up to a very large 1.4 m.
Treetop dwellers and other unusual habitats
Earthworm habitats and niches are not all under the ground. Native earthworms are frequently found under the bark of dead trees, in the litter of epiphytes and sometimes in the crooks of tree branches! There are also a number of aquatic earthworms that live in semi-saturated habitats.