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What are decomposers? (What do they eat?)




I admit it. I like decomposers like fungi, bacteria, and insects, and I think they are vastly undervalued creatures on this planet.  

In fact, decomposers like bacteria, fungi, earthworms, crabs, and termites play a vital role in every ecosystem and are perhaps the most important organism on earth! They basically turn dead plants and animals into nutrients that they channel up the food chain so the circle of life can keep going!

Sometimes I hear this misconception that decomposers are ugly bugs, crabs, shrimp, bacteria, and stinky worms and beetles that belong in garbage dumps. This is not true at all!

These are amazingly beautiful creatures found everywhere in nature that help break down dead leaves, small animals, etc. Decomposers are actually nature’s cleaners, making sure everything looks good and is not floating with rotting animals and plants!

It is amazing to see dead branches turn into fertile soil in a matter of months thanks to the amazing small creatures that live on logs and decompose them from the inside out.

This blog article, and more to come, will be dedicated to these creatures and to my fellow nature lovers who feel the same way and want to learn everything there is to know about decomposers!

What do decomposers do?

Decomposers break down organic waste into simpler inorganic material, making them accessible to plants and algae.

True decomposers release enzymes that break down organic waste into simple molecular nutrients such as water, carbon dioxide, nitrogen, phosphorus, calcium, and magnesium. These nutrients are turned into humus, a nutrient-rich substance, and they are released into the soil and absorbed by plants and algae.

Organic waste includes dead plant matter, feces, and animal carcasses. The decomposition process consists of multiple stages and the various organisms involved in these stages are called decomposers.

The term “decomposer” is often used to refer to both true decomposers and detritivores. Scavengers are not considered decomposers, but they also form part of the decomposition process.

The first stage of the process is the fragmentation of large pieces of organic waste into smaller pieces.

Scavengers and detritivores ingest organic waste, digest it internally and excrete smaller pieces of organic waste for other decomposers to act upon.

Water-soluble nutrients in fragmented organic waste leach into the soil. The fragments that remain after leaching, are then decomposed by microbes such as bacteria, yeast, or protozoa.

The exact process, and species of decomposers involved, depend on the type of organic waste, the habitat where the waste has been deposited, and the environmental conditions at the time.

What animals are decomposers?

Some detritivores have specific food preferences, while others are generalists. Some examples are earthworms, termites, and crabs.

Animal decomposers are called detritivores. Detritivores are animals that feed on decaying plants, animal matter, and dung.

For example, termites only feed on plant matter and carrion fly larvae only eat rotting meat, while ants and cockroaches will eat anything.

Termites are a good example of animals that function as decomposers of wood and other plant matter (cellulose), although it is actually bacteria in their guts doing the job!

Scavengers, such as raptors, carnivores, and rodents, reduce the size of organic waste and speed up the process of decomposition.

Scavengers usually consume larger pieces of organic material compared to detritivores. However, some opportunistic feeders, like crabs, can act as either scavengers or detritivores, depending on the type of food available in their environment.

Different decomposers and scavengers are found in different habitats. Detritivores such as earthworms, millipedes, and woodlice live in the soil, while many insects prefer to live between the leaf litter or inside rotting logs.

In aquatic environments, many fish species are scavengers, and crabs and snails can act as both scavengers and detritivores. In marine environments, hagfish, sea urchins, and sea stars act as scavengers, while sea worms and sea cucumbers are detritivores.

Fiddler and Porcelain crabs are examples of animals that sometimes function as decomposers by filtering out dead organic matter from the environment, and other times like scavengers by eating dead animals.

Filter feeders, such as barnacles, some crabs, and clams, collect and feed on small particles of organic matter floating in the water. In freshwater habitats, many insect larvae act as detritivores.

What are examples of other decomposers?

True decomposers consist of bacteria and fungi. The difference between detritivores and true decomposers is that detritivores digest organic waste internally, whereas true decomposers digest organic waste externally by releasing enzymes into the environment.

Detritivores consume organic waste, but true decomposers turn organic waste into inorganic nutrients that are accessible to plants.

Bacterial decomposers can be divided into aerobes and anaerobes. Aerobes are bacteria that need oxygen to function.

Anaerobic bacteria don’t need oxygen to break down organic waste, but they are less efficient than aerobes and can produce substances that are foul-smelling or toxic, such as hydrogen sulfide.

Most fungi, such as mushrooms, molds, yeast, and rusts, are decomposers. Fungi have hyphae (almost like roots) that can penetrate large pieces of organic waste to digest them, while bacteria can only grow on exposed surfaces of organic waste.

What is the role of decomposers in the ecosystem?

Decomposers are the recyclers of the ecosystem. They have two critical functions in the environment: Firstly, they remove rotting and decaying matter, and, secondly, they put primary nutrients back into the environment by making them available to plants and algae.

A familiar example of decomposition is the process of compost making. Inside a compost heap, small animals like earthworms, centipedes, insect larvae, and even scavengers like rats and mice, will eat large pieces of garden waste and kitchen scraps, reducing the size of the organic material. The true decomposers do the rest.

Worms, bacteria, fungi, and insects are examples of decomposers in compost.

80 to 90 % of the microorganisms in a compost heap are bacteria, and the rest are fungi such as molds and yeasts.

The microorganisms then break down the inedible organic matter and the indigestible organic matter in the feces of the scavengers and detritivores, until the organic waste is turned into compost. When you add the compost to the soil in your garden, nutrients from the compost get released into the soil and absorbed by plants through their roots.

Another important example of decomposition in the wild is the decomposition of animal carcasses that is carried out by everything from larger scavenger birds and bottom dwelling fish to the smallest of microorganisms.

When an animal dies, the microbes and enzymes that were already present inside the animal’s body while it was alive, immediately start decomposing the body.

Additionally, microbes present on their skin and in the surrounding environment decompose the body from the outside and enter the carcass through openings such as the mouth, eyes, and wounds.

The smell from these microbial processes is what attracts detritivores and larger scavengers. Insects like carrion flies, beetles, and ants, can detect a carcass within minutes and play a big role in decomposing carcasses.

While flies and many beetle species lay eggs or larvae in the rotting flesh, some beetles feed of the flesh themselves, while others predate on the flies and fly larvae. A large carcass can contain hundreds of thousands of insect larvae.

Large scavengers such as vultures play a big role in reducing the mass of the carcass (Parmenter) and damage from scavengers creates openings in the carcass, exposing a greater surface area of the carcass for detritivores and microbes to reach, speeding up the decomposition process.

In addition to animal carcasses, animal dung makes up a large part of organic waste in the environment. Many organisms contribute to removing dung from the environment, including dung beetles, flies, and microbes.

Flies lay eggs in the dung and their larvae then consume the dung. Dung beetles serve a similar function, but in addition to their larvae consuming the dung, the adults distribute dung.

By rolling pieces of dung away from the dung pile, they distribute nutrients throughout the environment. Microbes break down organic and inorganic matter in dung into substances that are accessible to plants.  

For example, Ammonium in dung needs to be broken down into nitrite and nitrate by bacteria, before they are accessible to plants. When large piles or mass storage of dung leads to the exclusion of oxygen, anaerobic bacteria take over the degradation process, leading to the production of smelly and toxic gasses.

Decomposers are the real heroes of the ecosystem!

In this way, decomposers create the important link between dead and living that makes ecosystems functional!

Why are decomposers important?

Decomposers and scavengers prevent the build-up of organic waste and provide nutrients to the soil, promoting new plant growth and contributing to the food chain.

Areas with a thick layer of dead plant matter permit little to no new growth. In contrast, after decomposition, areas where carcasses lay, see increased plant growth due to the high nutrient content of the soil.

Additionally, rapid removal and decomposition of feces and carcasses are important to prevent the spread of diseases and toxins in the environment.

For example, the highly acidic stomachs of vultures can kill most viruses and bacteria.

Therefore, when vultures feed on carcasses, they can prevent the spread of diseases like rabies, anthrax, and foot-and-mouth disease.

Dung beetles can reduce the spread of parasites of humans, livestock, and wildlife.

By eating the dung, dung beetles kill many parasites that are normally spread by dung, such as internal worms. Furthermore, by reducing the amount of dung, dung beetles reduce the reproduction of many problematic fly species that lay their eggs or larvae in dung (see Nichols et al).

In streams, a group of organisms known as shredders, prevent water streams from getting clogged up by large pieces of organic waste, by feeding on leaf litter that falls into the water from surrounding trees (Graca et al.).

Shredders include many of the marine decomposers such as small shrimps, crabs, snails, and various insect larvae, including those of stoneflies, caddisflies, and crane flies.

Shredders cannot produce the enzymes necessary to break down many of the structural compounds of leaves. Therefore, they prefer to consume leaves that have been colonized by aquatic fungi!

These fungi initiate the breakdown process by transforming inedible plant material into edible compounds. The introduction of pesticides into streams can reduce the abundance of shredders, increasing the time leaf litter takes to decompose.   

True decomposers like fungi are important for breaking down organic substances that are inedible or undigestible for animals. For example, lignin is a compound found in plants that make them rigid and woody and is very difficult to break down.

Animals cannot digest lignin, so most animals will avoid eating plant parts, like bark, that contain high amounts of lignin. This is where fungi and bacteria come in.

Fungi play a major role in decomposing old organic matter in the forest as well as in your kitchen!

Wood-rotting fungi (white rot and brown rot) are the only organisms that can produce the enzymes that decompose lignin, making them very important in forests and other ecosystems with high loads of plant matter.

Without them, the forest floor would be filled with tree skeletons of lignin.

In deserts, however, there are very few true decomposers in the soil since it is too hot and dry for them to function.

In these environments, detritivores, and the microbes in their gut, play a very important role in the breakdown of organic material (Crawford and Taylor).

Microbe and detritivore activity in deserts often follow seasonal or rainfall patterns. For example, microbes and detritivores are often more abundant and more active after a rainfall event, when humidity levels and air temperatures are ideal (Crawford and Taylor).  

The slow rate at which organic waste is broken down in deserts contributes to the low fertility of the soil.

The circle of life (conclusion)

Decomposers are often considered to be at the bottom of the food chain, with predators being at the top of the food chain.

However, if you consider that, in the end, even the strongest predator will become food for decomposers – then we might ask ourselves if maybe, they are on the top of the food chain?

The energy pyramid is a bit deceptive in that it depicts the flow unidirectionally, whereas it really is a circle!

Of course, in truth, such a question is nonsense because the food chain is not a linear, hierarchical system, but rather a circular one, and thus there is no top or bottom. Nutrients just keep cycling and changing from one form to another and each organism has its place in this circle of life.  


If you’re interested in learning more about decomposers and diving into the science behind this article, feel free to explore the references below:

Blanchette, Robert (September 1991). “Delignification by Wood-Decay Fungi”. Annual Review of Phytopathology. 29: 281–403. doi:10.1146/annurev.py.29.090191.002121.

Mondor, E. B., Tremblay, M. N., Tomberlin, J. K., Benbow, E. M., Tarone, A. M. & Crippen, T. L. (2012) The Ecology of Carrion Decomposition. Nature Education Knowledge 3(10):21. https://www.nature.com/scitable/knowledge/library/the-ecology-of-carrion-decomposition-84118259/

Parmenter, R. R., & MacMahon, J. A. (2009). Carrion decomposition and nutrient cycling in a semiarid shrub-steppe ecosystem. Ecological Monographs, 79(4), 637-661. https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/08-0972.1

Crawford, CS and Taylor, E. C. (1984). Decomposition in arid environments: role of the detritivore gut. South African Journal of Science, 80(4), 170. https://journals.co.za/doi/pdf/10.10520/AJA00382353_4742

Biodegradation in Animal Manure Management By Matthieu Girard, Joahnn H. Palacios, Martin Belzile, Stéphane Godbout and Frédéric Pelletier. 2013. DOI: 10.5772/56151https://www.intechopen.com/chapters/45120

Pecenka JR, Lundgren JG. 2018. The importance of dung beetles and arthropod communities on the degradation of cattle dung pats in eastern South Dakota. PeerJ 6:e5220 https://doi.org/10.7717/peerj.5220

Graça, M. A. S. (2001). The Role of Invertebrates on Leaf Litter Decomposition in Streams – a Review. International Review of Hydrobiology, 86(4-5), 383–393. doi:10.1002/1522-2632(200107)86:4/5<383::aid-iroh383>3.0.co;2-d

Vultures: the collapse of critical scavengers Evan R. Buechley* and Cagan H. Sekercioglu Current biology 26: 2016 R560 – R561  DOI: 10.1016/j.cub.2016.01.0. https://www.researchgate.net/profile/Cagan-Sekercioglu/publication/305277605_Vultures_the_collapse_of_critical_scav

Nichols, E., Spector, S., Louzada, J., Larsen, T., Amezquita, S., & Favila, M. E. (2008). Ecological functions and ecosystem services are provided by Scarabaeinae dung beetles. Biological Conservation, 141(6), 1461–1474. doi:10.1016/j.biocon.2008.04.011

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