they live everywhere if food is side out too long, mold will get on it also it when it is full leaves fall from the ground, and that is where  Mushrooms and other funguses

A decomposer is a mechanism that decomposes plants and animals.  

If we did not have decomposers, there would be leaves piling up to the sky because every time it becomes so many leaves fall from the trees, we need decomposers to breakdown all of the leaves. They also break down other waste and bring nutrients to plants and trees.