Flow of energy through various trophic levels in an ecosystem is unidirectional and non-cyclic. Explain.

Trees $\rightarrow$ Birds $\rightarrow$ Lice $\rightarrow$ Bacteria. Thus food chain is

Distinguish between 

$(a)$ Litter and detritus

$(b)$ Primary and secondary productivity

Assertion : A network of food chains existing together in an ecosystem is known as food web.
Reason : An animal like kite cannot be a part of a food web.

Nutrient immobilisation