What Is a Decomposer? The Organisms That Return Everything to Circulation
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Every ecosystem depends on organisms that break dead material down and release the elements in it for reuse. Without them nothing would be recycled and the nutrients supporting all growth would be locked up in accumulated remains within a few decades.
What they do
Decomposers obtain energy by breaking down dead organic material, and in doing so they release the nitrogen, phosphorus, sulphur and other elements bound up in it back into forms that plants can take up. That closes the loop, since plants build tissue from those elements, animals eat plants, and everything eventually dies, and without the return step the elements would accumulate in dead material and become unavailable. The quantities are large, since the great majority of the energy captured by photosynthesis passes through decomposition rather than through grazing, which means decomposer communities process more biological material than all the herbivores combined. The process also releases carbon dioxide, making decomposition a major term in the carbon cycle and one that responds to temperature, which is why warming soils are a concern.
Who does the work
The community is layered and different organisms handle different stages:
- •Bacteria, which dominate in wet conditions and process simple compounds rapidly
- •Fungi, which are the main agents on land for tough material, since they alone break down lignin efficiently
- •Detritivores including earthworms, woodlice, millipedes and springtails, which fragment material and expose more surface
- •Insect larvae, which consume specific material including wood and carrion
- •Protists and microscopic animals, which graze the bacteria and fungi and release nutrients in doing so
- •Scavengers, which are distinguished from decomposers proper by eating material before it has broken down
The wood problem
Lignin, which gives wood its rigidity, is a complex and irregular polymer that resists nearly everything, and only a limited set of fungi can break it down completely, mainly the white rot group which secrete enzymes that attack it outside their own cells. That capability is unusual and is why fallen wood persists for years while leaves disappear in months. The evolution of lignin-degrading fungi has been proposed as the reason coal formation declined after the Carboniferous, on the argument that wood accumulated undecomposed before such fungi existed, and that claim has been substantially revised, since the timing does not fit cleanly and since burial conditions alone account for much of the accumulation. The general point stands that wood decomposition is chemically difficult and is performed by a narrow set of organisms whose absence changes everything.
The soil they build
Decomposition does more than release nutrients, since it also constructs the physical material plants grow in. Partly decomposed organic matter becomes humus, a complex and stable mixture that binds mineral particles into crumbs, which creates the pore structure that holds water and admits air, and a soil without it is either compacted or loose and holds neither. That organic fraction also holds nutrients against leaching by binding them until roots take them up. Earthworms and other burrowing detritivores mix organic material downward and mineral material upward, which builds soil profiles over centuries and which Darwin studied in detail in his last book, calculating the quantity of soil worms pass through their bodies annually and concluding that they had shaped the landscape more than anyone credited.
What happens where they are absent
Conditions that suppress decomposition produce recognisable and consequential results. Waterlogging excludes oxygen and slows the process enormously, which is how peat forms, accumulating as partly decomposed plant material over thousands of years and storing an enormous quantity of carbon that drainage and burning release. Cold does the same, which is why permafrost holds vast amounts of undecomposed organic material and why thawing it is a serious concern. Acidity suppresses decomposer activity, which is why acidic bogs preserve bodies and artefacts for millennia. Aridity preserves by removing the water decomposers need. Each of those conditions produces a store of material that would otherwise have been recycled, and each is now being altered by drainage, by warming and by land use in ways that release what was stored.
The takeaway
Breaking down dead material releases the elements plants need, and most of the energy captured by photosynthesis passes through decomposition rather than grazing. Fungi handle the tough material on land, since they alone break lignin down efficiently, which is why wood persists and leaves do not. Waterlogging, cold, acidity and aridity suppress the process and produce carbon stores now being released.