Assuming that a period of stable population size is followed by a period when natality increases by ten (1O)…

Analyze the question: The population starts stable, meaning births plus immigration equal deaths plus emigration. Then natality (birth rate) and immigration both increase. We need to decide what happens to the population size.

Concept: Population change is given by:

Population change = (Natality + Immigration) − (Mortality + Emigration)

• Natality and immigration add individuals to the population.
• Mortality and emigration remove individuals from it.

Solution:

1. At the stable stage, (Natality + Immigration) = (Mortality + Emigration), so the net change is zero.
2. Natality increases by 10% and immigration increases, so the "gain" side becomes larger.
3. Nothing is said to change on the "loss" side (mortality and emigration stay the same).
4. Gains now exceed losses, so the population begins to increase.

Why not the other options?

• A (equilibrium): Equilibrium needs gains to equal losses. That balance has been broken.
• B (increase by 40%): Adding 10% and 20% does not give 40%. The percentages apply to different quantities (a birth rate and an immigration figure), so they can't be summed to 30%, let alone 40%. The question only asks about the direction of change.
• D (decrease): Gains have risen, so the population cannot fall.

Exam tip: Remember the four factors as "B-I-D-E" (Birth, Immigration, Death, Emigration). Birth and Immigration make the population go up. Death and Emigration make it go down. If the "up" factors increase and the "down" factors stay the same, the population grows.