Decomposers & Decomposition

As soon as a living creature dies it starts to decompose. Decomposition is nature's way of disassembling a dead animal or plant into its basic components, so they can be reused by other life forms. It also prevents the spread of disease. We explain the different stages of the decomposition process, and how decomposers and detritivores operate on land and in the ocean. We also look at scavengers, like vultures, who are equally important to the ecological health of the environment.
Vultures act as decomposers
Vultures, apex scavengers, can eat carrion which may be toxic to other animals. Image Credit: Mario Mata/CC BY-SA 3.0

In every ecosystem throughout the biosphere, there is a constant need for decomposers to deal with the remains of dead animals and plants, as well as waste from living creatures. These decomposers, known as saprotrophs, serve as Earth’s multi-functional clean-up crew, and include a variety of different organisms, all of whom share one important characteristic – they derive virtually all their energy and nutrition from dead organic matter.

What are Decomposers, Exactly?

A decomposer is an organism (typically a bacterium, or fungus) that feeds on and breaks down organic matter, such as the remains of dead animals and plants (saprotrophy), as well as the dung and waste of living creatures (coprophagy). These organisms assist in the process of decomposition, which happens to all living things after they die. Decomposition is an essential mechanism that facilitates the reabsorption of nutrients into the food web for the benefit of all. 1

Fungi decomposers recycle nutrients from rotten tree
Fungi like mushrooms are the predominant decomposers in forests, since only they possess the enzymes necessary to break down wood. Photo: © adege/pixabay

What’s the Difference Between Decomposers and Detritivores?

The word ‘decomposer’ is a loose term that is often used to describe two different types of organism: decomposers (saprophages), and detritivores (detritophages). The difference between decomposers and detritivores, concerns the way each group breaks down the dead flesh. Detritivores have to digest organic material inside their bodies in order to break it down. Decomposers, on the other hand – who are typically too small to ingest pieces of tissue – break down organic matter externally by chemical and biological means.

Types of Decomposers

Most decomposers comprise single-cell bacteria or fungi. Bacteria are microscopic organisms. A single teaspoon of fertile soil may contain anywhere between 100 million and one billion bacteria from as many as 10,000 separate species. 2

Bacteria involved in the decomposition of a dead animal include internal bacteria already resident inside the body, as well as bacteria joining them from outside. The soil contains thousands of species of bacteria that decompose cadavers. How do bacteria decompose a body? Usually by feeding on the exposed surfaces of organic matter.

By contrast, fungi are able to penetrate the surface of larger pieces of organic matter, by using their thin, thread-like appendages known as hyphae.

Fungi are the predominant decomposers in forests, since only they possess the enzymes necessary to break down lignin, the fibrous polymer found in wood. The method of decomposition employed by fungi involves the release of enzymes to break down the dead plant or animal cell tissue (a process called lysis), after which they absorb some of the nutrients in the resulting fluid (lysate). The rest seeps into the soil to be absorbed by other microbes or by plants. Some examples of fungi include yeasts, molds, mildew and mushrooms. 3

Mold growth on a decaying peach taken over a period of 6 days:

How mold decomposes a peach
Image Credit: © CC BY-SA 2.0

Types of Detritivores

Many detritivores live in forests and woodland, although the term also applies to certain bottom-feeders in wetlands and aquatic environments. Detritivores include invertebrate insects such as earthworms, woodlice, millipedes, mites, beetles, pillbugs, butterflies, dung flies, houseflies, blowflies; and mollusks such as slugs and snails.

Scavengers

Scavengers are another group of detritivores who consume dead organisms. They are not usually considered to be detritivores, as they are facultative scavengers who gain most of their energy and nutrients through predation.

Specialist or apex-scavengers include vultures, who are the only members of the animal kingdom that are obliged to scavenge in order to eat. Their digestive systems automatically neutralize many pathogens. Unfortunately, vulture populations are under threat from toxic drugs, with calamitous consequences for local ecosystems in India and elsewhere. 4 5 See: 10 Endangered Birds of Prey.

Other scavengers include carnivores, such as jackals and hyenas and jackals. Large predators, such as lions, cheetahs, wolves and black bears, will also scavenge given the opportunity, even though carrion is not their preferred food source. Birds are another common type of scavenger.

Scavengers of dead plant matter include termites that build earthen mounds in grasslands and then scavenge for dead plant material for consumption within the mound.

Hyena with zebra head in its mouth
A scavenger hyena carrying the head of a zebra, Tanzania. Image by BPBricklayer/Pixabay

How Do Decomposers Dispose of a Cadaver?

In the wild, the first arrivals at the site of a dead animal are usually scavengers, like vultures, followed by a variety of carnivores and opportunistic predators. Between them, they consume the majority of the carcass. Once they are done, decomposers and detritivores take over and consume the parts that the scavengers have left behind. That said, some decomposers like resident bacteria, and detritivores like flies, typically start feeding immediately: the former from inside the body, the latter from outside.

In developed areas, dead animals are typically scavenged first by birds, then small carnivores including rats, foxes, badgers, otters, weasels, ferrets, stoats and martens. Resident bacteria and flies are also early feeders. If the cadaver is lying on the soil, earthworms and other soil-based detritivores will also attack the corpse, once the scavengers have finished.

Decomposers in the Food Chain

Unlike autotrophs, such as plants, who use create their own energy using photosynthesis, decomposers and detritivores are heterotrophs who must find other organisms to eat, except in their case the plant or animal organisms are dead.

The essential chemical nutrients that decomposers rescue from cadavers then become part of the normal food web, as the decomposers are either eaten by other heterotrophs, or die and are themselves eaten and recycled by other decomposers. Detritivores, in particular, tend to be eaten by consumers and therefore play an important role as recyclers of nutrients, thus supporting the biogeochemical cycles of essential chemicals.

Soil organisms contain a large number of active detritivores. Earthworms, for example, ingest rotting plant and animal matter as they swallow soil. After they’ve finished digesting, the waste that comes out of their bodies contains the valuable minerals, all ready for plants to take up again.

Not all the nutrients contained in the dead body are ingested by decomposers or plants. A small percentage is left to settle into the pedosphere as humus, before eventually being lithified into rock. Through their recycling activity, decomposers are the link that keeps the circle of life going.

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The Phosphorus Cycle

Decomposers in the Ocean

The principal decomposers in marine ecosystems are bacteria. Another important type are fungi. Marine detritivores (bottom feeders) include echinoderms, crustaceans, mollusks, and marine worms. In colder ocean waters, like the North Atlantic or Southern Oceans, only bacteria and fungi are active, as they are the only saprotrophic organisms who can survive the cold. Opportunistic decomposers include hagfish, fiddler crabs, sea urchins, sea stars and sea cucumbers. Freshwater detritivores also include mildew, trumpet snails and yeast.

Many decomposers in the ocean are microscopic organisms such as bacteria and other protozoa, whose importance within the marine food web has been generally underestimated. See also: Marine Microbes Drive the Aquatic Food Web.

Eutrophication is a growing problem in estuaries and coastal areas, as is ocean deoxygenation within dead zones on the high seas.

What Happens to a Animal Body During Decomposition?

Decomposition produces noxious odors and fluids but it’s a natural and critically important mechanism which recycles essential chemical nutrients back into the food chain, mainly for the benefit of plants (primary producers).

Decomposers break down complex organic materials into basic substances, such as water and carbon dioxide, as well as other simple inorganic substances containing nitrogen, phosphorus, and calcium. All of these substances help plants to grow and develop. Note that microorganisms involved in the decay process are not pathogenic.

What are the 5 Main Stages of Decomposition?

When an organism dies its remains undergo five stages of decay and decomposition: fresh, bloat, active decay, advanced decay, and dry/remains. 6 The process is driven by two main processes of chemical decomposition: autolysis and putrefaction. 7

Autolysis is the breakdown of the body by endogenous substances – in this case, digestive enzymes that flood the cells and tissues of the dead organism’s body. Post-mortem putrefaction involves the action of bacteria (such as bacteria in the digestive tract) that reproduce throughout the body after death, leading to the break-up of proteins, and the liquefaction of most organs.

Here is a brief summary of the five stages.

Fresh
This begins the moment the organism’s heart stops beating. Any oxygen remaining is rapidly used up by aerobic microbes who are naturally present in respiratory and gastrointestinal tracts, causing the proliferation of anaerobic microbes that consume the body’s carbohydrates, lipids, and proteins (autolysis).

Bloat
As putrefaction takes hold, gases produced by bacterial activity start to bloat the body. These include: methane, hydrogen sulfide, carbon dioxide, nitrogen and ammonia. As internal pressure rises, fluids are purged from the body through natural orifices, such as the nose, mouth, ears and anus.

Active Decay
This stage sees the greatest mass loss from the body, caused by the aggressive feeding of fly larvae (maggots) from houseflies and blowflies. The skin tissues eventually rupture, releasing the bacterial gas. Liquefaction of tissues leads to bodily disintegration. Active decay ends as maggots leave the body to pupate into fully developed flies.

Advanced Decay
By this stage, with most of the tissues consumed by now absent maggots or liquified and purged, there is little organic matter left to be decomposed. If the organism is lying on the ground, the surrounding soil – comprising the cadaver decomposition island (CDI) – typically displays a significant increase in nitrogen, as well as an increase in other nutrients, such as carbon, potassium, phosphorus, calcium, and magnesium – all valuable nutrients for plants.

Dry/Remains
At this stage, only dry skin, cartilage, and bones remain. Increased plant growth may be visible around the remains due to increased nutrient levels in the soil. When all soft tissue is gone, the cadaver is described as completely skeletonized, otherwise, it is classified as partially skeletonised.

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References

  1. “Recycling the dead.” Science News For Students. []
  2. “Decomposers – Bacteria Engines of Earth’s Nutrient Cycles.” []
  3. “Saprotroph.” []
  4. “Dropping dead: causes and consequences of vulture population declines worldwide”. Ogada, Darcy L.; Keesing, Felicia; Virani, Munir Z. (16 December 2011). Annals of the New York Academy of Sciences. 1249 (1): 57–71. Bibcode []
  5. “Scavenger community response to the removal of a dominant scavenger”. Olson, Z. H.; Beasley, J. C.; DeVault, T. L.; Rhodes, O. E. (31 May 2011). Oikos. 121 (1): 77–84. []
  6. “A summer carrion study of the baby pig sus scrofa Linnaeus”. Payne, J.A. (1965). Ecology. 46 (5): 592–602. []
  7. “Decomposition Chemistry in a Burial Environment”. Forbes, S.L. (2008). In ‘Soil Analysis in Forensic Taphonomy.’ M. Tibbett; D.O. Carter (eds.) CRC Press. pp. 203–223. ISBN 978-1-4200-6991-4. []
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