Based on ecological principles and the evaluation of each statement:

. All ecosystems are constant in size: This is incorrect. Ecosystems vary greatly in size, ranging from a small pond or a single log to an entire ocean or forest.

. In nature food chains exist, not food web: This is incorrect. In nature, simple linear food chains are rare; instead, multiple food chains are interconnected to form a food web, which provides greater stability to the ecosystem.

. Flow of energy in ecosystem is linear: This is correct. Energy flow is described as linear or unidirectional because it moves in one direction from producers to consumers and cannot be recycled back to the sun.

. Gene flow occurs between 2 different population (of 2 species): This is incorrect. Gene flow typically occurs between different populations of the same species. By definition, members of different species are generally reproductively isolated and do not exchange genetic material.