The decomposers are living things which feed on the dead bodies of other living things. They rot them away gradually, until nothing is left except for parts such as bones and shells which cannot be decomposed.
Yeast is a decomposer.
It feeds on the sugars from crushed grapes or on solutions of sugar which we provide for it when making wine, bread or beer. Yeast is a useful decomposer, and so are several more which we shall meet in later chapters. We regard many other decomposers as dangerous. Examples are the dry rot fungus which attacks and destroys the wooden parts of buildings, and the many fungi and bacteria which spoil our food if we keep it too long. The decomposers play a very important part in nature.