The biotic community of any ecosystem can be divided simply into producers, consumers and decomposers.
Producers or autotrophs are organisms that make their own organic material from simple inorganic substances. For most of the biosphere the main producers are photosynthetic plants and algae that synthesis glucose from carbon dioxide and water. The microscopic algae and blue-green bacteria are the main producers in an aquatic ecosystem and are known as phytoplankton.
The glucose produced is used both as an energy source and combines with other molecules from the soil to build biomass. It is this biomass that provides the total theoretical energy available to all non photosynthesising organisms in the ecosystem.
Consumers or heterotrophs are organisms that obtain organic molecules by eating or digesting other organisms. These are the herbivores and carnivores of the ecosystem. By eating other organisms they gain both food as an energy supply and nutrient molecules from within the biomass ingested.
The organisms that feed on producers are called as primary producers or herbivores. An insect, deer or rabbit is a primary consumer in a terrestrial ecosystem, which has trees, grasses and herbs as producers. The secondary consumers feed on primary consumers or herbivores, and are thereby flesh eaters or carnivores,(eg. dog, fox, wolf, etc). The secondary carnivores or tertiary consumers feed upon the secondary consumers, (eg. lion feed upon the wolves).
Decomposers are the waste managers of any ecosystem. When organisms die, their bodies form a source of energy and raw materials for other organisms, called decomposers. They are the final link in a food web breaking down dead organic matter (DOM) from producers and consumers and ultimately returning energy to the atmosphere in respiration and inorganic molecules back to the soil during decomposition.
Decomposers can be divided into two groups based on their mode of nutrition.
- Detritivores; are an organisms that ingest non-living organic matter. These can include earthworms, beetles and many other invertebrates.
- Saprotrophs; are organism that lives on or in non- living organic matter, secreting digestive enzymes into it and absorbing the products of digestion. These include Fungi and bacteria.