To understand past and predict future changes in populations, ecologists use birth rates, expected longevity of individuals, and the proportion of individuals in a population that is able to reproduce. Insights into these parameters come from the age structure of a population, the number of individuals within each age group of the population studied. A population in which most individuals are past their age of greatest reproduction (because for some reason young are not surviving) or a population mostly composed of individuals that has not yet reached peak reproductive age (because older individuals are not surviving) will not increase as rapidly as one dominated by individuals capable of greatest reproductive output.

To reconstruct the age structure of a population, researchers commonly divide the population, or a sample of the population, into age classes, for example individuals born within a single year or within an interval of several years. Age can often be determined by measuring an aspect of the organism’s anatomy (for example, counting growth rings in fish scales or in a core drilled into a tree trunk) or estimated by overall size (for example, length of a fish or tree height).

Estimating the number of individuals of differing ages enables ecologists to predict whether the size of a population will increase or decrease. A growing population usually shows a pyramid-shaped age distribution, with the youngest classes much more abundant than older classes, whereas a stable population shows a more even distribution of age classes. Fig. 46.8 shows the difference in age structure between France, a country with a stable population, and India, a country with a growing population.

In humans, such differences reflect, in part, socioeconomic variation among countries, but differences in the age structure of other species can reveal threats from various sources. Fig. 46.9 shows the effects of overfishing on the age structure of a species of long-lived Pacific rockfish, determined by comparing the ages of fish sampled in unfished waters with those in heavily fished waters. In the unfished population, the curve shows many individuals close to and past the age of 10 years, when reproductive ability starts to climb. In the fished populations, the curve shows a sharp decline of individuals older than 6 years because mostly older and larger fish are caught. Because these older fish produce most of the eggs and young fish, the annual reproductive effort of fished populations is severely affected by their removal.