Why are decomposers important for ecosystems?

Decomposers break down feces and dead plant and animal matter into simple nutrients that can be used by primary producers like plants and algae.

Decomposers are an essential part of the ecosystem since they ensure the cycling of various nutrients, including carbon, nitrogen, phosphorus, calcium, and magnesium.

By allowing these nutrients to return to the soil, plants can reuse these nutrients for growth and reproduction and the animals that rely on plants for nutrition can continue to survive.

In this post I will dive deeper into the role of decomposers in ecosystems, we will see how they convert energy by extracting nutrients from dead organic matter to funnel it back into the food chain when “eaten” by animals or plants.

We will also try to convince you, that decomposers are really at the top of the food chain rather than, what is often depicted, at the bottom!

How do decomposers convert energy for an ecosystem?

Decomposers get their energy from dead organic matter that was not used by other organisms. This energy is then converted into nutrients that primary producers, like plants, can extract from the soil.

When animals consume plant or animal matter, not all the organic molecules get digested, and the undigested organic matter is excreted as feces.

Decomposers with the ability to break down organic compounds that are difficult for animals to digest, then eat these feces.

Similarly, lots of plants and animals die without being eaten by other animals. Without decomposers, the energy from feces and dead plant and animal matter would remain trapped inside organic waste and so be lost to the ecosystem.

Decomposers use the energy from organic waste to grow and reproduce. When other organisms eat the decomposers, they, in turn, gain energy from them.

There are many different decomposers, but microorganisms are by far the most influential in most ecosystems!
If you want to learn more about why decomposers are important – check out some of my other articles on decomposers on my blog!

But now you might think – if bacteria and fungi are the most abundant decomposers, who exactly are eating them to gain the energy they take up?

Well, the thing is, plants cannot break down and absorb fibrous components such as wood or the chitin of dying insects directly, however, bacteria and fungi can!

So the fungi and bacteria generate biomass when they grow by eating these, for many organisms indigestible substances.

And then the decomposers die themselves! Which, as you might have guessed, releases energy directly into the environment that can easily be taken up by plants and some animals!

This is how the energy is recycled by decomposers in most ecosystems!

How do decomposers contribute to the carbon cycle?

Decomposers can break down complex organic compounds that other organisms cannot digest, like lignin and cellulose into carbon dioxide and other simple compounds.

The carbon dioxide gets released into the air. When plants photosynthesize, they fix carbon dioxide from the air into organic molecules and biomass5.

The carbon-containing molecules from plants are consumed by animals, which are then consumed by other animals, and the carbon molecules are transferred from one organism to the next.

The carbon cycle is complex and involves many contributing factors including the decomposers that release carbon dioxide and frees up carbon by decomposing otherwise indigestible material into its more accessible components. Figure adapted from Amelese et al. 2020.

When plants or animals die, decomposers break down these organic molecules and the whole cycle continues from where it started.

Where do decomposers fit into the energy pyramid?

The short answer is that decomposers are consumers. Despite playing a critical role in the transfer of energy in the ecosystem, many diagrams of energy pyramids do not feature decomposers.

Other diagrams picture them separately, on the side or even at the bottom of the pyramid.

In an energy pyramid, the size of each level represents the amount of energy produced by the organisms that make up that level of the pyramid1.

Producers, such as plants and algae, produce the most energy, therefore they are at the bottom of the pyramid.

Decomposers
Decomposers

Then primary consumers (herbivores) produce the next biggest amount of energy, followed by secondary and tertiary consumers (predators), etc. The amount of energy generated by decomposers depends on the environment.

For example, in the rainforest, where it is hot and humid and there are lots of organic waste, decomposers produce more energy than primary consumers1, placing them low down in the pyramid.

However, in environments with fewer decomposers, like deserts2 or the arctic, the decomposers would produce less energy and they would be higher up in the pyramid.     

The classical view of decomposers at the bottom and predators at the top of the energy pyramid.

Are decomposers at the top or bottom of a food web?

Decomposers are often pictured at the bottom of the food chain. Probably because they are physically small and because a lot of other organisms feed on them.

For example, many animals, including humans, eat mushrooms. Birds and small mammals eat earthworms and other detritivores. But decomposers such as fungi also eat each other!

Apex predators are placed at the top of the food chain because they are not eaten by any other animals. But is that really reflecting reality?

If you consider that all animals, even apex predators, have parasites and bacteria that feed on them and that, when even the largest predator dies, its body is consumed by decomposers, then the concept of an apex predator at the top of the food chain becomes somewhat blurred!

In nature, everyone is someone else’s dinner.

McInerney, Reference no. 6

This holds true for even the deadliest of predators if you take decomposers into consideration!

If you consider that decomposers feed on all other organisms and therefore make up the end of the food chain, they should really be pictured at the top!

A food web, other than a food chain or an energy pyramid, has no top or bottom but is an interconnected web of complicated interactions between animals.

What is the trophic level of decomposers?

Decomposers are placed in their own trophic level. Bacteria and fungi that break down leaf litter, are primary consumers, just like herbivores, whereas decomposers breaking down the body of a predator, would be tertiary or quaternary consumers.

In food webs and energy pyramids, organisms are grouped into different trophic levels, based on where they fit in.

Organisms that can photosynthesize, such as plants and algae, are producers because they can make their own food.

All other organisms, including decomposers, are also consumers to some extent.  

Some decomposers can move between different levels. When an ant feeds on plant matter, it is a primary consumer, but if it feeds on a carcass, it is a secondary or tertiary consumer.

But fungi that specialize in decomposing wood would always be a primary consumer.

Fungi that only eat wood are primary decomposers, but they will in reality also eat other organisms like insects that die in their vicinity.

How do decomposers maintain the stability of an ecosystem?

Decomposers maintain the stability of the ecosystem by cycling nutrients. Without decomposers, nutrients would remain trapped inside the dead bodies of plants and animals, and eventually, the ecosystem would run out of limited nutrient resources.

The return of nutrients to the soil is essential for the continuity of life. For example, if leaf litter is removed from the forest floor before it can decompose, the soil loses its fertility7.

Reduced soil fertility leads to reduced plant production, which in turn, leads to reduced food availability for herbivores and, eventually, carnivores, which could lead to mass starvation and ecological collapse.

What happens if decomposers are removed from the ecosystem?

It is almost impossible to imagine a world without decomposers since they are constantly all around us. At first, a world without decay might seem attractive.

In theory, without decomposers, food would never go bad and many of the unpleasant smells surrounding feces and waste would disappear.

However, without decomposers, nutrients would not be extracted properly from food and feces would not exist (as it is mostly made up of bacteria!).

Fruit would not spoil due to microorganisms such as the yeast shown here if decomposers were not around.

Without decomposers, however, dead plant matter, animal carcasses, and feces would start to pile up.

Interestingly, ruminant animals like sheep or cows are completely dependent on decomposers to digest their food, so these would starve to death.

Inside the permafrost, where it is too cold for decomposers to function, animal carcasses from as far back as the ice age remain perfectly preserved8.

As the climate warms and the permafrost thaws, many of these animal remains are starting to decompose. Thawing carcasses could release diseases, such as Anthrax9, and revive long-extinct viruses that we are completely unprepared for, back into the environment!

In addition to having no space to grow due to all the waste build-up, the depletion of fixed nitrogen and non-renewable elements, like phosphorus, would be disastrous for plant life, because they need these nutrients from the soil to grow.

The process of photosynthesis is essential for life on earth, not only to produce food but also to release oxygen into the atmosphere.

When food and oxygen start to run low, life on earth, as we know it, would no longer exist and this would eventually mean the doom of humanity!

So next time you hear about dangerous bacteria or nasty fungi, pay a kind thought to those microscopic life forms actually running the ecosystems and keeping the world, you included, alive…

References

  1. Hickman, C.P., Roberts, L.S., Larson, A., I’Anson, H., Eisenhour, D.J. 2006. Integrated principles of zoology. McGraw Hill. New York, NY. 882 pp.
  2. Noy-Meir, I. 1974. Desert Ecosystems: Higher Trophic Levels. Annual Review of Ecology and Systematics, 5 (1), 195–214. doi:10.1146/annurev.es.05.110174.
  3. Blanchette, R. 1991. Delignification by wood-decay fungi. Annual Review of Phytopathology. 29: 281–403. doi:10.1146/annurev.py.29.090191.002121.
  4. Ingham, E.R.  Soil bacteria. United States Department of Agriculture website. https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/soils/health/biology/?cid=nrcs142p2_053862
  5. Carbon cycle (Ecology). Britannica. https://www.britannica.com/science/carbon-cycle
  6. McInerney, J.D. 1993. Animals in education: Are we prisoners of false sentiment? The American Biology Teacher, 55 (5): 276-280. https://doi.org/10.2307/4449659
  7. Kowalski, K. 2014. Recycling the dead. Science News for Students. https://www.sciencenewsforstudents.org/article/recycling-dead
  8. Luxmoore, M. 2021. As Siberia’s permafrost thaws, scientists marvel at the mammoth treasures beneath. Radio Free Europe Radio Liberty. https://www.rferl.org/a/siberia-permafrost-thaw-mammoth/31342051.html
  9. Goudarzi, S. 2016. What lies beneath. Scientific American, 15: 11-12. https://pubmed.ncbi.nlm.nih.gov/27918518/
  10. Gilbert, J.A. and Neufeld, J.D. 2014. Life in a world without microbes. PLoS Biol, 12 (12): e1002020. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4267716/