Consider the following statements about food chain

$I.$ The transfer of energy from producers to top consumers through a series of organisms is called food chain

$II.$ A food chain is always straight and proceeds in a progressive straight line

$III.$ In a food chain, there is unidirectional flow of energy from sun to producers and subsequently to series of different types of consumers

Which of the statements given above are correct?

The trophic level of mushroom and Monotropa is

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

Magnitude of primary productivity is affected by

What is primary productivity? Give brief description of factors that affect primary productivity.