Plants are producers which can prepare their food by the process of photosynthesis. Zooplanktons, fish and man are primary, secondary and tertiary carnivores respectively.

In an ecosystem, producers (e.g., plants/phytoplankton)belong to first trophic level, herbivores or primary consumer (e.g., grasshopper) to the second and carnivores or secondary consumer (e.g., lion, wolf) to the third trophic level.

The rate of resynthesis of organic matter by the consumers is known as secondary productivity . It depends upon the loss while transferring energy containing organic matter from the previous trophic level plus the consumption due to respiration and predation.Respiration loss is about 20% for autotrophs, 30% for herbivores and upto 60%in case of carnivores. Therefore net productivity decreases with each trophic level.

Humification, catabolism, leaching and fragmentation are the steps of decomposition which operate simultaneously on the detritus. Fragmentation is breaking down detritus into smaller particles by detritivores like earthworm. By the process of leaching, water soluble in organic nutrients go down into soil horizon and get precipitated as unavailable salts. Humification occurs at a very slow rate.

The natural reservoir of phosphate is rock which contains phosphorus in the form of phosphates when rocks are weathered, minute amounts of these phosphate dissolved in soil solution and are absorbed by the roots of the plants. Herbivores and other animals obtain this element from plants.

Four important functional aspects of the ecocystem are
(i)Productivity
(ii) Decomposition,
(iii) Energy flow and
(iv) Nutrient cycling.

Phosphorus is mostly used as phosphate. Its reservoir pool is phosphate rocks while cycling pool is soil for terrestrial ecosystems and water for aquatic ecosystems.