To many people, a compost heap is nothing more than a pile of useless rubbish of kitchen waste and garden weeds. However, inside, a compost heap teems with life, from bacteria, worms, proliferating fungi, and insects, to hibernating reptiles and amphibians.
A well-made compost heap can rapidly turn waste vegetable matter into valuable plant food for spreading on the garden and feeding the next generation of flowers and vegetables.
The process depends on microscopic plants and animals, and some larger organisms as well, which feed upon the refuse and break it down gradually into simple substances which plant roots can absorb.
This is the same as what happens to fallen leaves and other dead vegetation in its natural habitats.
What Lives In A Compost Heap?
If you dig into the center of a compost heap, you will find it to be warm, with temperatures reaching 60c (140f), thanks to the activities of millions of unseen bacteria. Their chemical attack on the softer plant materials releases energy in the form of heat, which cooks the material in the center of the heap.
Most living things are killed by the heat. The temperature is lower towards the outside of the heap, and weeds are not always killed here.
A powerful microscope is needed to see the bacteria and the other microorganisms, such as protozoans and actinomycetes, that live there, but some fungi are easier to see. Many of the fungi are simple molds, and you might see the spore capsules of Mucor on rotting fruit or other food.
The capsules are like minute black pinheads on slender stalks, giving the fungus its common name of pin mold. You may find delicate toadstools, such as Psathyrella gracilis, sprouting from the edge of the heap.
The fungal threads help break down dead leaves and twigs and soften them up for a further assault by bacteria and an assortment of small animals.
Huge numbers of animals live in compost heaps. Many are microscopic, but others can be seen with a magnifying glass. These animals include vegetarian and predatory species, all bound up in elaborate food webs.
Those that eat the decaying vegetation do not always digest it fully, but they break the material up into small particles before passing it out in their droppings.
It is then easier for the bacteria to work on the material and finally reduce it to the structureless organic matter that we call humus, a material that maintains soil fertility. The dead bodies of the animals also contribute to the richness of the compost.
Worms
Several species of earthworms invade the outer parts of the compost heap and contribute to the decomposition processes by dragging plant remains into where the bacteria more readily attack them.
The worms’ droppings also contain finely divided organic matter and valuable minerals. One of the most conspicuous species in the compost heap, especially where animal dung is incorporated, is the brandling Eisenia foetida, purplish brown with bold orange bands. E. rosea is also very common and easily recognized when adult by its bright pink body and very swollen orange clitellum.
The pot worms or enchytraeids are related to the earthworms, although much smaller. These are small white worms that you will often see in clusters among the decaying leaves outside the compost heap.
While no more than a centimeter long, they feed on decaying matter. They could be mistaken for fly maggots, but they have no obvious head, as found in most fly larvae.
These are also roundworms or nematodes that can be seen under a microscope. There will be millions in your compost heap. Some are predators of the protozoans, but most are scavengers feeding on the decaying material, contributing to its eventual conversion to humus.
Slugs and Snails
The decaying matter of the compost heap attracts large numbers of slugs and snails belonging to several different species. The great grey slug is the largest of the slug’s Limax maximus. Up to 18cm (7in) long, this grey creature can form large colonies in the compost heap.
It feeds only on fungi and decaying matter and does not harm the garden. The pearly white eggs of these slugs are commonly found in the compost heap.
Among snails is the cellar glass snail, a pale grey animal with a pale brown glassy shell. Its smaller cousin, the garlic glass snail, can often be found in compost heaps. The shell is much darker and shinier, and the animal smells of garlic when handled.
Flies
It should be no surprise that flies breed in compost heaps, and their larvae can be found in the damper parts of the compost. The pale-bodied, black-headed larvae of fungus gnats are especially common. Even the adults crawl about in the compost.
They are very delicate flies and are often accompanied by numerous moth flies or owl-midges. These are very small flies with pointed wings densely clothed with hair. Their tiny larvae are white and pointed at both ends.
Many-legged Animals
Mites are usually the most conspicuous arthropods when the compost is examined closely. They are very numerous, and they move about freely. Many are spiky creatures with relatively long legs, while others are encased in smooth shells with tiny legs protruding just enough to slowly trundle the animal. The mites imbibe fluids from living and dead organisms, with fungal threads playing a major role in their nourishment.
Springtails are also very common, wandering slowly and chewing the decaying matter. Both groups fall victim to the fascinating false scorpions, similar in look to scorpions, except that they lack a tail and sting.
Although the animals are only 2-3mm long, their pink claws look distinctly menacing as they creep slowly through the compost. False scorpions are not the most abundant insect in a compost heap, but they can be found. If touched, they pull in their claws and scuttle backward quickly.
Centipedes find a wide range of suitable prey in the compost heap and are numerous. Most individuals are of the species Lithobius forficatus. They are brown and move very quickly using their 15 pairs of legs.
They are about 3cm (1in) long when mature. The flat-backed millipede Polydesmus angustus is often mistaken for a centipede. It is largely the same color as Lithobius, but closer inspection will reveal two pairs of legs on each segment, unlike the one in centipedes.
Like other millipedes, Polydesmus is a vegetarian and loves the decaying vegetation of the compost heap. You may be able to see its nest, a small chamber built from its excrement and saliva.
Woodlice also revel in the moist conditions of the compost heap. Large species such as Oniscus asellus are usually very obvious, especially at night, but the most common species— Trichoniscuspusillus and T. pygmaeus need to be searched for because they are under 5mm long.
Predatory Animals
The abundance of small animals in the compost heap attracts many predatory creatures, which may take up temporary residence. These include numerous beetles, such as the devil’s coach horse and the violet ground beetle, and larger animals, including the hedgehog.
The warmth of the compost heap is an attraction for some of these larger animals, and it is not uncommon for hedgehogs, grass snakes, and slow worms to nest there. They may also use the heap for hibernation, together with frogs, toads, and newts.
Less welcome lodgers are rats and mice, which frequently excavate snug nests for themselves and their young, emerging at night to sniff out our freshly sown peas and beans.
The wasp is another generally unwelcome lodger, although it does control garden pests. It is likely to nest in your compost heap if there is a fair amount of uncompacted woody material from which it can hang its nest.
Bumble bees can be seen quite frequently taking over abandoned mouse nests in the heap. It is possible to see how much pollen they bring in on their hind legs. Keep your eyes open for the remains of any nests and the bees’ waxy cocoons when spreading the mature compost in the garden.