Just like the major parts of an ecosystem, the smaller parts also play important and indispensable roles in the function of the larger system. Take an oak tree in a deciduous forest, for example.
It harnesses energy from sunlight and by utilizing the products in the soil produced by decomposers, it creates plant matter in the form of woody growth and leaves. The tree also produces acorns which are dropped to the forest floor and consumed by various consumers. The leaves also drop each season to return the various nutrients to the soil to be decomposed and passed back to the tree or to other organisms and plants that require them.
The tree also acts as habitat for various birds and other animals that need space off the forest floor to protect them from various predators, etc.