Question

the graph below shows the population growth for paramecia (a freshwater unicellular organism) kept under controlled laboratory conditions for 18 days. which of the following statements explains what is happening in the region of the curve labeled \x\? a the paramecia are in water that is too warm. b the populations birth rate equals the death rate. c the populations birth rate is zero. d the paramecia have used up their food supply.

Explanation:

In a population growth curve, the plateau phase (labeled "X" here) occurs when the population has reached the carrying capacity of the environment. This means resources like food supply have been used up, limiting further growth. Option A about water temperature is not indicated by the graph. Option B (birth rate equals death rate) is more characteristic of a stable - equilibrium phase in some models but not the main reason for the plateau here. Option C (birth rate is zero) is incorrect as there would be a decline if that were the case. Option D (food supply used up) aligns with the concept of carrying capacity and the plateau in population growth.

Answer:

D. The paramecia have used up their food supply.