When plants and animals die, and when animals produce waste such as faeces, the materials in them are not lost. Decomposers break them down. Decomposers are microorganisms, mainly bacteria and fungi. They live in places such as soil, leaf litter, compost heaps and ponds, so every ecosystem has them.
Decomposers release enzymes onto the dead material. The enzymes break large molecules into small soluble ones, which the microorganism absorbs. This process is called decay or decomposition.
Decay returns materials to the ecosystem. Decomposers respire, so they release carbon dioxide into the air. They also release minerals such as nitrates into the soil, where plants can take them up through their roots. Without decomposers, dead material would build up and the materials needed for growth would stay locked inside it.
Decay is fastest when it is warm, moist and there is plenty of oxygen, because these are the conditions where microorganisms work best.