The functional aspects of an ecosystem mainly include which of the following?

Which of the following statements is incorrect?

Biological equilibrium is found among the

If a lake is polluted by $DDT$,its highest concentration will be found in:

Which of the following statements regarding food chains are considered correct?
$(1)$ Removal of $80\%$ of tigers from an area results in greatly increased growth of vegetation.
$(2)$ Removal of most carnivores results in an increased population of deer.
$(3)$ The length of food chains is generally limited to $3-4$ trophic levels due to energy loss.
$(4)$ The length of food chains can vary from $2$ to $8$ trophic levels.

Which among the following is not a functional unit of the ecosystem?