Identify the organisms $I, II, III$ and $IV$ in the given food web.
$I$ | $II$ | $III$ | $IV$

Consider the following statements about food chain:
$I.$ The transfer of energy from producers to top consumers through a series of organisms is called a food chain.
$II.$ $A$ food chain is always straight and proceeds in a progressive straight line.
$III.$ In a food chain,there is a unidirectional flow of energy from the sun to producers and subsequently to a series of different types of consumers.
Which of the statements given above are correct?

Assertion $A$: As a result of evolution,plant species have migrated from aquatic habitats to terrestrial habitats.
Reason $R$: Plants have undergone developmental changes while passing through various stages of succession.
Which option is correct for Assertion $A$ and Reason $R$?

Find out the correct order of succession levels in xerarch.

Which of the following animals occupies more than one trophic level in the same ecosystem at the same time?

Assertion $(A):$ $A$ natural ecosystem is usually self-sustainable.
Reason $(R):$ An ecosystem having producers,decomposers,and a source of energy is self-sustainable. Select the correct answer from the options given below: