Decomposition and decay

In our modern-day human culture, decomposition and decay have often come to be viewed quite negatively.

The former is mainly associated with things that are rotten, have a bad smell and are generally symptomatic of death. The latter is similarly viewed as undesirable. Examples include urban decay, or, on a more personal level, tooth decay. However, decomposition and decay are vital processes in nature. They play an essential role in the breakdown of organic matter, recycling it and making it available again for new organisms to utilise.

Decomposition and decay are the yin to the yang of growth. Together they form two halves of the whole that is the closed-loop cycle of natural ecosystems. Everything dies, and without decomposition and decay the world would overflow with plant and animal remains. It would also experience a decline in new growth, due to a shortage of nutrients that would be locked up and unavailable in the dead forms.

What is decomposition?

Decomposition is the first stage in the recycling of nutrients that have been used by an organism (plant or animal) to build its body.

It is the process whereby the dead tissues break down and are converted into simpler organic forms. These are the food source for many of the species at the base of ecosystems. The species that carry out the process of decomposition are known as detritivores. Detritivore means literally ‘feeders on dead or decaying organic matter’. Many of these decomposer species function in tandem or parallel with one another. Each is responsible for a specific part of the decomposition process. Collectively they are known as the detritivore community.

Nature’s unsung heroes of recycling

A wide range of organisms takes part in the decomposition process. Most of them are inconspicuous and unglamorous. From a conventional human perspective, they are even undesirable. The detritivore community includes insects such as beetles and their larvae as well as flies and maggots (fly larvae). It also includes woodlice, fungi, slime moulds, bacteria, slugs and snails, millipedes, springtails and earthworms. Most of them work out of sight, and their handiwork isn’t immediately obvious, but they are the forest’s unsung heroes of recycling. Almost all of them are tiny, and their function happens gradually in most cases, over months or years. But together they convert dead plants and animals into forms that are useable either by themselves or other organisms.

Decomposition in plants

The primary decomposers of most dead plant material are fungi. Dead leaves fall from trees and herbaceous plants collapse to the ground after they have produced seeds. These form a layer of litter on the soil surface. The litter layer can be quite substantial in volume.  The litter fall in a Scots pine is around 1-1.5 tonnes per hectare per year, while that in temperate deciduous forests is over 3 tonnes per hectare per year. The litter is quickly invaded by the hyphae of fungi. Hyphae are the white thread-like filaments that are the main body of a fungus. (The mushrooms that appear on the forest floor, are merely the fruiting bodies of the fungus.) The hyphae draw nourishment from the litter. This enables the fungi to grow and spread, while breaking down the structure of the dead plant material. Bacteria also play a part in this process, as do various invertebrates, including slugs, snails and springtails. As the decay becomes more advanced, earthworms begin their work.

This decomposition process is usually odourless. It is aerobic, meaning that it takes place in the presence of air (oxygen in particular). On the forest floor it is spread out in both space and time. When people make compost heaps in their garden, they are utilising the same process. It is concentrated and accelerated by piling the dead material together in a heap, and the heat that is generated speeds up the process of decay.

Fungi that feed on dead plant material are called saprotrophic fungi. Common examples include the horsehair parachute fungus, which can be seen growing out of dead grass stems, leaves or pine needles. Another is the sulphur tuft fungus, which fruits on logs that are at an advanced state of decomposition.

In a forest, the rate of decomposition depends on what the dead plant material is. Leaves of deciduous trees and the stems and foliage of non-woody plants generally break down quickly. They are usually gone within a year of falling to the forest floor. Some plant material, such as the fibrous dead fronds of bracken, takes longer. But even these will still be decomposed within three years. The needles of conifers, such as Scots pine, are much tougher. It can take up to seven years for them to be completely broken down and recycled. The rate of decay is also determined by how wet the material is, and in general the wetter it is the faster it breaks down. In dry periods or dry climates, the organic matter becomes dessicated. Many detritivores, such as fungi and slugs, are inactive so the decomposition process becomes prolonged.

Decomposition of woody material – the rot sets in

In contrast to the softer tissues of herbaceous plants, the fibres of trees and other woody plants are much tougher and take a longer time to break down. Fungi are still, for the most part, the first agents of decay, and there are many species that grow in dead wood. The common names of species such as the wet rot fungus and the jelly rot fungus indicate their role in helping wood to decompose. The growth of the fungal hyphae within the wood helps other detritivores, such as bacteria and beetle larvae, to gain access. The fungi feed on the cellulose and lignin, converting those into their softer tissues. These in turn begin to decompose when the fungal fruiting bodies die. Many species of slime mould also grow inside dead logs and play a role in decomposition. Like fungi, they are generally only visible when they are ready to reproduce and their fruiting bodies appear.

Some decomposers are highly-specialised. For example, the earpick fungus grows out of decaying Scots pine cones that are partially or wholly buried in the soil. Another fungus known as Cyclaneusma minus grows on the fallen needles of Scots pine.

As the wood becomes more penetrated and open, through, for example, the galleries produced by beetle larvae, it becomes wetter. Being wet facilitates the next phase of decomposition. Invertebrates such as woodlice and millipedes feed on the decaying wood.  Predators and parasites, such as robber flies and ichneumon wasps, will also arrive, to feed on beetles and other invertebrates. For trees such as birch the wood becomes very wet and rotten, and falls apart quite easily after a few years. Earthworms and springtails are often seen at this stage, when the decomposing wood will soon become assimilated into the soil. They can reach high densities – there can be 1 tonne or earthworms in a single hectare of broadleaved European forest! The wood of Scots pine, however, has a high resin content. This makes it much more resistant to decay, and it can take several decades for a pine log to decompose fully.

It’s a fungus eat fungus world

Most fungi are soft-bodied and having a high water content. This means they often disintegrate and disappear within a few days or weeks of fruiting. The tougher, more woody fungi, such as the tinder fungus, can persist for several years. Even so, they often have specialist decomposers at work on them. The tinder fungus, for example, is the host for the larvae of the black tinder fungus beetle and the forked fungus beetle. These feed on the fungal fruiting body, helping to break down its woody structure

Another bracket fungus that grows on dead birch trees, is the birch polypore. This fungus is itself colonised by the ochre cushion fungus, which feeds on and breaks down the polypore’s brackets. The bolete mould fungus is another species that grows on fungi, in this case members of the bolete group. (Boletes have pores on the underside of their caps and include edible species such as the cep.) The silky piggyback fungus and the powdery piggyback fungus fruit on the caps of brittlegill fungi. They speed up the process of breakdown and decay in them. Slime moulds, although not fungi, are somewhat fungus-like when they are in the fruiting stage of their life cycle. The fruiting bodies of a species called Trichia decipiens are susceptible to fungal mould growing on them. This in turn accelerates their decomposition.

Decomposition in the animal kingdom

Fungi play a key role in breaking down plants, but this isn’t the case then it comes to dead animal matter. The vast majority of the decomposers in this case are other animals and bacteria. Animal decomposers include scavengers and carrion feeders. These consume parts of an animal carcass, using it as an energy source. They also convert it into the tissues of their own bodies and the dung they excrete. These animals range from foxes and badgers to birds such as the hooded crow. They also include invertebrates such as carrion flies, blow-flies and various beetles. Their dung in turn is eaten by other organisms, particularly dung beetles and burying beetles. Some fungi, including the dung roundhead grow out of dung, helping to break it down.

Not all animal carcasses are immediately consumed by large scavengers. In these cases there are five main stages in the decomposition process. The first of these is when the corpse is still fresh. At this stage carrion flies and blow-flies arrive and lay their eggs around the openings, such as the nose, mouth and ears. In the second stage, the action of bacteria inside the corpse causes putrefaction. These bacteria produce gasses which make the carcass to swell. This is anaerobic decomposition, or decay in the absence of air. It is characterised by its bad smell, in contrast to the odourless nature of aerobic decomposition.

The next stage commences when the skin of the corpse is ruptured. The gases escape and the carcass deflates again. In this decay stage, the larvae or maggots of flies proliferate and consume much of the soft tissue. Predators such as wasps, ants and beetles also arrive, to feed on the fly larvae. In the following stage, only cartilage, skin and bones remain. At this point different groups of flies and beetles, along with their parasites, take over the decomposition process. Finally, only bones and hair remain, and they can persist for several years or more. Eventually even these are consumed – for example, mice and voles will gnaw on old bones, to obtain the calcium they contain. Clothes moths help break down hair or feathers. The progression through these stages depends to some extent on the time of year when death occurs. But typically it takes several months from beginning to end.

One example of a fungus that helps break down animal matter is the scarlet caterpillar club fungus. This species grows out of the living pupa or larva of a moth or butterfly. It converts the body of its host into a fruiting body, which is club-shaped and orange, with a pimply surface.

Decomposition feeds new growth

Decomposition and decay may appear to be unpleasant processes from our human perspective. However they are vital for the functioning of ecosystems. Just like compost in a garden, they provide essential nutrients for the growth of new organisms. They are a key aspect of the cyclical processes that maintain all life on Earth. A renewed appreciation of their importance will help humans to protect and sustain ecosystems. This appreciation may even provide inspiration for alternatives to the unsustainable unlimited growth model that drives human culture today.

Sources and further reading

In This Section