Age or size structure influences how rapidly populations grow

Members of a population whose ages fall within a specified range are said to be part of the same age class. Age class 1, for instance, might include all individuals who are at least 1 year old but who are not yet 2 years old.

Age structure is a key feature of populations, in part because it influences whether a population increases or decreases in size. Consider two human populations of the same size and with the same survival and reproduction rates, but with different age structures. If one of the populations had many people older than 55, while the other had many people between ages 15 and 30, we would expect the second population to grow more rapidly than the first because it contained more individuals of reproductive age. Indeed, human populations that

are growing rapidly typically have a greater percentage of people in younger age classes than do populations that are growing slowly or are in decline (FIGURE 11.16).

FIGURE 11.16 Age Structure Influences Growth Rate in Human Populations Population pyramids for Nigeria and Japan show age structures that are typical of human populations with rapidly growing populations (Nigeria) and with growth rates that are negative or close to zero (Japan). The main reproductive ages (15-44) are shown in green.

We have emphasized the importance of age because in many species, birth and death rates differ greatly among individuals of different ages. For other kinds of organisms, age is less important. In many plant species, for example, if conditions are favorable, a seedling may grow to full size relatively rapidly and reproduce at a young age. If conditions are not favorable, however, the plant may remain small for years and reproduce little or not at all; if conditions become favorable at a later time, the plant may then grow to full size and reproduce. For such species, whether an individual reproduces or not is more closely related to size than to age. When birth and death rates correlate poorly with age, or when age is difficult to measure, life tables based on the sizes or the life cycle stages

(e.g., newborn, juvenile, adult) of individuals in the population can be constructed.