Step 1: Write the population change equation in words.
Population change equals births minus deaths, plus in migrants minus out migrants, over a period of time.
Step 2: Pull out two named factors from this equation.
The birth rate and death rate together decide the natural growth component, while migration decides the mechanical or spatial growth component.
Final Answer:
Birth and death rates, and migration, are two key factors behind population change.$\boxed{\text{Birth/death rates; Migration}}$