I. High birth rate and high immigration rate II. Low birth rate and high immigration rate III. Low mortality…

Analyze the question

Population size changes through four factors: birth rate and immigration (which add individuals), and mortality and emigration (which remove individuals). Over-crowding happens when a population grows rapidly, so more individuals must be added than removed.

Explain the concept

Population growth = (births + immigration) − (deaths + emigration)

• Gains: high birth rate, high immigration
• Losses: high mortality, high emigration

Evaluate each statement

• I. High birth rate and high immigration rate: Both add many individuals, so the population increases rapidly. This causes over-crowding.
• II. Low birth rate and high immigration rate: Only one factor adds individuals, and the other is weak. Growth is limited, so this is not the best cause.
• III. Low mortality rate and low emigration rate: Few individuals die and few leave. Almost everyone stays in the population, so numbers build up. This contributes to over-crowding.
• IV. High mortality rate and high emigration rate: Many individuals are lost, so the population shrinks. This reduces crowding.

So the combination that causes rapid over-crowding is I and III. Individuals are added quickly (high birth and immigration) and hardly any are lost (low mortality and emigration).

Check the options

• A. II and III: II has a low birth rate, so it is weaker.
• B. I and III: correct.
• C. I and IV: IV reduces the population, so this is wrong.
• D. I and II: II has a low birth rate, and this pair ignores the low losses.

Answer: B (I and III)

Exam tip: For population questions, ask two things: "Are many individuals coming in?" and "Are few leaving?" If both answers are yes, the population will grow rapidly and crowd. Remember "In-flow high, out-flow low = over-crowding."