Most gardeners are very well aware that worms are crucially important garden friends, and that their presence (or absence) in soil is a good indicator of how healthy that soil is. But earthworms do virtually all of their work undetected and invisible to us surface dwellers, so the exact mechanisms by which they operate are somewhat more mysterious.

Earthworms have been dubbed “ecosystem engineers”, and, like their human counterparts, they are able to alter the physical structure of their environments. Worms move through the soil by means of burrowing, with different species specialising in either horizontal or vertical runways some of which can extend deep into the soil. These burrows create a network of virtual pores through which beneficial oxygen and water can enter and carbon dioxide can exit the soil. At the same time worms digest their way through their surroundings, literally breaking larger particles into smaller, and their resulting faeces (worm casts) are responsible for much of the open, crumb structure of the the best soils, which in turn allows access to plant roots and myriad other subterranean organisms.

An earthworm cast.

Improvement in soil structure is only the start of the story, however. Earthworms are detrivores – meaning that they feed exclusively on dead material – and they play an equally vital role in the decomposition of that organic material. The earthworms resident in your compost are busily munching their way through, and thus decomposing your garden and kitchen waste, and soil-dwelling earthworms are engaged in exactly the same process. Together with a galaxy of fungi and bacteria this process of decomposition releases the nutrients tied-up in dead plants and animals and allows them to be accessed by living plants. The volume of material processed in this way is pretty staggering – just consider, for starters, all of the autumn leaves shed by deciduous trees and shrubs each season that are efficiently and rapidly recycled back into the soil, a job done largely by worms.

The industrious work of earthworms also acts to mix together the different strata of soil. Nutrition is incorporated deeper into the soil (without which it would be rapidly leached of it’s goodness by rain and the feeding of living plants) and the fertile top-soil region is slowly expanded. These actions lead Charles Darwin to refer to earthworms as “nature’s ploughs” due to their ability to mix and fertilise soil with organic matter, but worms are hugely more efficient, and entire ecosystems are dependent opon their actions.

An earthworm - this one is Lumbricus terrestris - in its burrow.

Earthworms, as mentioned above, aren’t the only operators in the area of decomposition. In fact they are the  first level – the macro-scale when it comes to matters of micro-organisms – and fungi and bacteria take over where they leave off, breaking particles into even smaller pieces and fully releasing the nutrients within. For the most part, however, these fungi and bacteria are unable to survive without the presence of worms, who are, effectively, responsible for providing their food, so whilst worms can operate independently of their decompostion partners the same can’t be said of the bacteria and fungi. What’s more the higher the concentration of worms in a given soil the larger the presence of beneficial bacteria and fungi will be.

It comes as a surprise to many gardeners to learn that there are actually many species of earthworm present in a typical garden, and even more so to hear that they all specialise in different terrains. From a gardeners point of view the species can be broken down into four broad groups.

First up are the so-called compost earthworms, so-named for their love of compost bins. They require a warm, continually moist environment with a regular supply of new material on which to feed and are highly active, digesting rapidly and reproducing quickly too. The most commonly encountered species of the compost group are Eisenia fetida and E. veneta, both characterised by their relatively small size and bright red colour with distinctive paler stripes, which lends them the common name of Tiger Worms.

Eisenia fetida - one of the familiar Tiger Worm compost species.

The second group are known as the Epigeic earthworms. They live on the soil surface amongst the leaf litter – Epigeic literally means to crawl across the surface of the soil – and can be regarded as the first line of attack in the decomposition of material and the recycling of nutrients. These species do not burrow at all, but feed exclusively above ground, breaking-down organic material into pieces small enough for the subterranean species to access easily. Epigeic earthworms are generally also bright red or red-ish-brown, but they lack the stripes of the compost species. Common Epigeic earthworm species include: Dendrobaena species, Eiseniella tetraedra, Helodrilus oculatus, Lumbricus species and Satchellius mammalis.

Next up come the Endogeic earthworms. As might be imagined, these species both live in and feed directly upon the soil. They create networks of horizontal burrows through which they move and digest/excrete as they go and are able to burrow deep into the soil. Although they do re-use their burrows to some extent they characteristically create new burrows as they go. Endogeic earthworms are large in size and generally pale in colour, with different species appearing  pink or grey as well as green-ish or blue-ish. Common endogeic species include: Allolobophora chlorotica, Apporectodea species, Murchieona muldali and Octolas species.

Allolobophora chlorotica - commonly known as the Green Earthworm.

The final group are the anecic earthworms. These species make permanent, vertical burrows in the soil and feed upon surface leaves that they capture on the surface and drag into their burrows. The anecic species are the ones responsible for the familiar surface piles of worm-casts so often seen in lawns and other grasslands where they create middens that surround the entrance to their burrows. Anecic species are the largest of our native earthworms and are dark red or brown in colour at the head end with paler tails. The two most common species are: Lumbricus terrestris and Apporrectodea longa.

Apporrectodea longa - the Black-Headed Worm.

The anatomy of an earthworm consists of a simple digestive tube housed within a thick cylindrical muscular sheath that forms the body. That body is segmented with furrows on the surface of the body marking the division between each segment. The first segment incorporates the mouth of the animal, and has a fleshy, muscular lobe on the top. This lobe can be pulled in to seal the mouth off, or pushed outwards to investigate its surroundings. Aside from the mouth, each of the segments has a series of eight retractable bristles which together allow the earthworm to propel itself along.

Earthworms are hermaphrodites, with each individual possessing both male and female reproductive organs. When two earthworms are ready to mate they adopt a head-to-tail position, cover themselves in a layer of mucus, and exchange sperm. Each then forms a tube of mucous that detaches and moves forward along the body, collecting both the individuals own eggs as well as the sperm received from its partner en route.

Mating earthworms.

Fertilization subsequently occurs within this mucous tube which detaches from the front of the body and is deposited in the soil where it dries to form an egg capsule or cocoon, from which one or more young earthworms will eventually hatch. Many species are able to breed several times a year.

Earthworm egg cocoons - these are from L. terrestris.