views updated


Decomposers are the choppers, shredders, plowers, and dissolvers of the biological world. They break down tree leaves, dead flowers, grass blades, old logs in forests, and plant roots into small parts, and, finally, into carbon dioxide, water, and numerous basic chemical compounds in soils, water bodies, and sediments. Organisms involved in decomposition vary from earthworms that drag leaves into their burrows, chew up parts of the leaves, and pass them through their guts to microscopic bacteria that make the final breakdown of fragments into basic chemicals. Some decomposers are specialized and act most effectively on only, for example, oak leaves or maple seeds. Others decompose parts of many plant or animal remains that fall on the soil or into a stream or lake. Most decomposers are often not visible, but in some lawn areas, especially under deciduous trees, we can see little volcano-like earthworm mounds. Mushrooms in our gardens and forests are the visible parts of fungi that are decomposing plant and animal remains in the soil.

Decomposers are the ultimate recyclers of land and water ecosystems . As byproducts of their actions in breaking down organic matter, decomposers obtain (and release) nutrients and energy-yielding compounds. And decomposers leave behind simpler fragments for other decomposers along with simple forms of nitrogen, phosphorus, calcium, and other plant nutrients. Plant roots then can take up these nutrients to sustain new plant growth, and insects and other animals can eat the plants. So, the cycles continue. These cycles from plant organic matter, sometimes to animal tissues, then to decomposers and basic chemical compounds are essential to maintaining the world's ecosystems. Some of the residues of decomposition, and some byproducts of decomposer processes, serve to glue together mineral soil particles. This gives soils the porosity that allows roots to grow and water and air to enter and leave soils. These cycles maintain soil fertility in grasslands, forests, lakes, and agricultural lands.

Many decomposers are partners in interesting biological systems. Microscopic bacteria in the rumens"first stomachs"of cows decompose grass that cows eat and pass on more easily digestible substances to the real stomachs. Other bacteria in the gut "tubes" of earthworms partially decompose plant fragments, making elements and compounds available to the worms and yield nutrient-rich residues that are passed back into the soil. Some mushroom parts of wood-decomposing fungi are important foods for some insects and forest animals, including deer and small rodents. In some cases insects or animals then carry fungal parts or spores to other spots where they form new fungal decomposing systems. Many small insects and other arthropods are important first-stage shredders and partial decomposers of plant remains. In soils where such decomposers are excluded by intensive cultivation or excess chemicals, the natural recycling of organic matter is slowed down. This can lead to decreased soil fertility and plant growth; farmers or gardeners are then forced to add fertilizers or mulches. Good ecosystem stewardship includes keeping active populations of decomposers of all sizes to keep the systems productive.

see also Biogeochemical Cycles; Carbon Cycle; Compost; Fungi.

James R. Boyle