When food is abundant animals generally increase in number. When food is scarce animals decrease in abundance. A lag usually exists between the peak in food abundance and the peak in consumer abundance. Bacterial decomposition of dead organic matter returns nutrients to the system so that they can be used again to generate new organic matter. Without decomposition the generation of new organic matter would cease.
Cyanobacteria however are photosynthetic and can produce new organic matter.
The amount of plant or animal matter in a region is referred to as biomass. The biomass at lower trophic levels provides food for higher trophic levels. Most of the food consumed by animals is assimilated and is used first to satisfy metabolic demands. The excess can be used for growth and reproduction. Energy used for metabolic processes is lost as heat and will not be available to higher trophic levels.