In our modern-day human culture, decomposition and decay have often come to be viewed quite negatively, with the former mainly associated with things that are rotten, have a bad smell and are generally symptomatic of death, while the latter is similarly viewed as very undesirable, whether it be in terms of urban decay, or, on a much more personal level, tooth decay. However, they are vital processes in nature, playing 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, and together they form two halves of the whole that is the closed-loop cycle of natural ecosystems. Everything dies, and without the processes of decomposition and decay the world would quickly become not only overflowing with the remains of dead plants and animals, but also would experience a decline in new growth, due to a shortage of nutrients, that would be locked up and unavailable in the dead forms.
Decomposition is the first stage in the recycling of nutrients that have been used by an organism (plant or animal) to build its body, and are surrendered back to the ecosystem upon its death. It is the process whereby the dead tissues break down and are converted into simpler organic forms that are the food source for many of the species at the base of ecosystems. The species that carry out the process of decomposition, and feed on the 'waste' products produced by it, are known as detritivores, which means literally 'feeders on dead or decaying organic matter'. Many of these decomposer species function in tandem or parallel with one another, with each being responsible for a specific stage or aspect of the decomposition process, and collectively they are known as the detritivore community.
A wide range of organisms takes part in the decomposition process, with most of them being relatively inconspicuous, unglamorous and, from a conventional human perspective, even undesirable. The detritivore community includes beetles and their larvae, flies and maggots (the larvae of flies), woodlice, fungi, slime moulds, bacteria, slugs and snails, millipedes, springtails and earthworms. Most of them work out of sight, with their handiwork not immediately apparent, but they are the forest's unsung heroes of recycling. Almost all of them are small in size, and their function happens gradually in most cases, over time periods measured in months or years, but cumulatively they convert all dead plant and animal material into forms that are useable for growth either by themselves or other organisms.
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, forming a layer of litter on the soil surface. The litter layer can be quite substantial in volume, with the litter fall in a Scots pine (Pinus sylvestris) forest estimated to be between 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 - the white thread-like filaments that are the main body of a fungus (the mushrooms that appear on the forest floor, mostly in late summer and autumn, are merely the fruiting bodies of the fungus). The hyphae draw nourishment from the litter, enabling 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 and snails, springtails and, as the decay becomes more advanced, earthworms.
This decomposition process is usually odourless, and is aerobic, meaning that it takes place in the presence of air (oxygen in particular). On the forest floor it is spread out both spatially and in time. When people make compost heaps in their garden, they are utilising the same process, which is concentrated and accelerated by piling the dead material together in a compost heap, where the heat that is generated speeds up the process of decay.
Fungi that feed on dead plant material are called saprotrophic fungi and common examples include the horsehair parachute fungus (Marasmius androsaceus), which can be seen growing out of dead grass stems, leaves or pine needles, and the sulphur tuft fungus (Hypholoma fasciculare), 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, and are usually gone within a year of falling to the forest floor. Some plant material, such as the fibrous dead fronds of bracken (Pteridium aquilinum), takes longer, but will still be fully decomposed within three years. The needles of conifers, such as Scots pine, are much tougher and 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 - in general the wetter it is the faster it breaks down, while in dry periods or dry climates, the organic matter becomes dessicated and many detritivores, such as fungi and slugs and snails, are inactive so the decomposition process becomes prolonged.
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 mostly 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 (Coniophora puteana) and the jelly rot fungus (Phlebia tremellosa) 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, which 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, or sporocarps, appear.
Some decomposers are highly-specialised. For example, the earpick fungus (Auriscalpium vulgare) grows out of decaying Scots pine cones that are partially or wholly buried in the soil, while another fungus (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 and this facilitates the next phase of decomposition. Invertebrates such as woodlice and millipedes feed on the decaying wood, and 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 (Betula spp.), 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, and they can reach high densities - the biomass of earthworms in broadleaved forests in Europe has been estimated at up to one tonne per hectare. The wood of Scots pine, however, has a high resin content, which makes it much more resistant to decay, and it can take several decades for a pine log to decompose fully.
Most fungi, being soft-bodied and having a high water content, decompose quickly, often disintegrating and disappearing within a few days or weeks of fruiting. The tougher, more woody fungi, such as the tinder fungus (Fomes fomentarius) and other bracket fungi, can persist for several years. However, in many cases they have specialist decomposers at work on them. The tinder fungus, for example, is the host for the larvae of the black tinder fungus beetle (Bolitophagus reticulatus) and the forked fungus beetle (Bolitotherus cornutus), which feed on the fungal fruiting body, helping to break down its woody structure.
Another bracket fungus that, like the tinder fungus, grows on dead birch trees, is the birch polypore (Piptoporus betulinus), and it in turn is colonised by the ochre cushion fungus (Hypocrea pulvinata), which feeds on and breaks down the polypore's brackets. The bolete mould fungus (Hypomyces chrysospermus) is another species that grows on fungi, in this case members of the bolete group, which have pores on the underside of their caps instead of gills and includes edible species such as the cep (Boletus edulis). The silky piggyback fungus (Asterophora parasitica) and its close relative the powdery piggyback fungus (Asterophora lycoperdoides) fruit on the caps of various brittlegill fungi (Russula spp.), accelerating the process of breakdown and decay in them. Slime moulds, although not actually fungi themselves, are somewhat fungus-like when they are in the fruiting stage of their life cycle, and the sporocarps of one species (Trichia decipiens) are highly susceptible to fungal mould growing on them, accelerating their decomposition process.
In sharp contrast to decomposition in plants, fungi play a very limited role in the breakdown of dead animal matter, where the vast majority of the decomposers are other animals and bacteria. Animal decomposers include scavengers and carrion feeders, which consume parts of an animal carcass, using it as an energy source and converting it into the tissues of their own bodies and the dung they excrete. These range from foxes and badgers to birds such as the hooded crow (Corvus corone cornix), and also include invertebrates such as carrion flies, blow-flies and various beetles. The dung they produce in turn forms the food source for other organisms, particularly dung beetles and burying beetles, while some fungi, including the dung roundhead (Stropharia semiglobata) grow out of dung, helping to break it down.
For animal carcasses that are not immediately consumed by large scavengers, ecologists identify five stages in the decomposition process. The first of these is when the corpse is still fresh, and is typified by the arrival of carrion flies and blow-flies, which lay their eggs around the openings, such as the nose, mouth and ears, that allow easy access to the inside of the carcass. In the second stage, the action of bacteria inside the corpse causes putrefaction and swelling of the carcass due to the production of gases. This is anaerobic decomposition, or decay in the absence of air, and 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, which allows the gases to escape and the carcass to deflate again. In this decay stage, the larvae or maggots of flies proliferate in large numbers and consume much of the soft tissues. 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, and different groups of flies and beetles, plus their respective parasites, take over the decomposition process. Finally, only bones and hair remain, and they can persist for several years or more, although even they are consumed - for example, mice and voles will gnaw on old bones, to obtain the calcium they contain. The progression through these stages depends to some extent on the time of year when death occurs, but typically it will take several months from beginning to end.
One example of a fungus that plays a role in the decomposition of animal matter is the scarlet caterpillar club fungus (Cordyceps militaris). This species grows out of the living pupa or larva of a moth or butterfly, converting the body of its insect host as it dies into the hyphal structure of its fruiting body, which is club-shaped and orange in colour, with a pimply surface.
While decomposition and decay may appear to be unpleasant processes from our human perspective, they are vital in terms of the functioning of ecosystems. Just like compost in a garden, they provide essential nutrients for the growth of new organisms, and 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, and may even provide inspiration for the establishment of an alternative to the unsustainable unlimited growth model that drives human culture today.
Alan Watson Featherstone
Sources & further reading
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