Decomposers are an essential part of this model, also. Their primary function is to cycle nutrients. They are responsible for the breaking down of organisms into organic compounds which are added to the nutrient reservoir which contains, among others, the limited nutrient, phosphorus. Biomass, or living weight, is formed by a process of production. An organism must have energy for this to occur.
Primary production is an example.
This energy is then used by a consumer and the result is growth. The first law of thermodynamics states that energy can neither be created nor destroyed. In other words, the amount is constant. Energy can, however, be converted. Energy will be lost here in the form of heat.