Which one of the following is not a functional unit of an ecosystem?

The following are the functional units or functional components of an ecosystem that work together:

• Productivity is the rate at which biomass is produced.
• Energy flow refers to the process through which energy moves from one trophic level to the next. The Sun's energy is transferred from producers to consumers, then to decomposers, and eventually back to the environment.
• Decomposition refers to the break down of detritus (complex organic matter) into simpler inorganic substances such as carbon dioxide and water. The topsoil is the primary breakdown location.
• Nutrient cycling refers to the recycling and movement of nutrient elements in various forms through various components of the ecosystem for use by different organisms.

Stratification is just the vertical distribution of different species occupying different levels. Hence, stratification is not a functional unit of an ecosystem.