Earthworms can be divided into three ecological categories: epigeic, endogeic and anecic.
1. epigeic (gr. epigeios – on the soil surface; epi – on + gê – soil/ground). They live above the mineral layer, under plant debris or other organic matter. They are small, pigmented, fast and have short life-cycles. They are under a great predation pressure. Earthworms of this category survive unfavorable conditions usually in cocoon-stage. They feed on organic matter and/or associated microflora, digesting small amounts or nothing at all. Some species like Lumbricus rubellus and Lumbricus castaneus prefer droppings, while most species of genus Dendrobaena prefer rotten wood. Several species, most famous among them being Eisenia fetida and Eisenia andrei, are specialists of places with great amounts of organic matter like compost, manure etc.
2. anecic (gr. anekas – reaching up). These species live in permanent vertical passages in the mineral layer (depths up to 3 m). Species belonging to this group are large, with dark dorsal pigmentation (at least anteriorly), quickly enter their passages, but are mostly quite slow. Generally, long lived species with long life cycles, exposed to predation while being on the surface, but are mostly protected inside their tunnels. They survive dry periods mainly in a resting state. They feed on the surface, pulling fallen leaves and other plant debris in their passages, at he same time also digesting some soil. These species prefer leaves rich in nitrogen and without tannins, like lime (Tilia sp.), ash (Fraxinus sp.) and maple (Acer sp.). The most famous representatives are Aporrectodea longa and Lumbricus terrestris.
3. endogeic (gr. endogeios – in the ground/soil; endo – in + gê – soil/ground). These species dig horizontal passages in the soil that are not permanent, usually on depth of 10-15 cm from soil surface. They are usually not pigmented (pink, whiteish or seeming darker because of contents of the intestine), move slowly, vary in size and are relatively under low pressure of predation even from species digging in the surface. Considering their lifespan and the length of life cycle they are between epigeic and anecic earthworms. In dry periods they enter diapause. These species feed on soil more or less rich in organic matter i.e. are geophagous species. In species Aporrectodea rosea active mineral and organic soil particle recognition is noticed. The most important element in their nutrition is the microflora attached to soil particles.
This classification suites species present in the temperate continental climate, while it needs to be extended for tropical regions. Ecological categories represent functional adaptations to the environment which enable co-existence of different species allowing them to harness different food sources and occupy different areas of the habitat.
Identification of earthworm species can be done based on external morphological features of adult individuals, on which the ErIK2 key is based on, but in some cases a section is required to identify the species based on some internal characteristics.