Birth Rate, Death Rate, and Population Change

The core idea is bookkeeping. Every individual born adds to a population, every individual that dies subtracts. Over any stretch of time, net change equals births minus deaths, and if you want to count migration too, you add immigrants and subtract emigrants. This simple subtraction is why a population can still be growing even while its birth rate is falling: as long as births stay above deaths, the population keeps adding individuals, just more slowly than before. Growth rate and population size are different things — a slowing rate can still mean a rising total, the same way a car still moves forward even as it decelerates.

Age structure diagrams make this concrete by showing how many individuals occupy each age group, split by sex, stacked from youngest at the bottom to oldest at the top. A wide base packed with children and teenagers signals a population about to grow, because a large cohort is about to enter its reproductive years, even if current birth and death rates look stable. A diagram that narrows sharply at the bottom, with more adults and elderly than children, signals decline, since there are not enough young people coming up to replace those who will eventually die. The shape predicts the future, not just the present.

This connects directly to how ecologists describe population change mathematically — birth rate minus death rate is literally the building block of the growth rate equations that follow, including how that rate gets scaled by population size itself and how it eventually gets limited by resources and carrying capacity.