Course: BIS 105 Environmental Biology

Unit 4 Learning Module

Population Pyramids

Age-Sex Structure of a Population

Grizzly bear populations include adults up to 25 or 30 years old, capable of reproducing, and young immature bears under 6 years old. Healthy populations include roughly equal proportions of each age group.

Density and dispersion describe a population’s size, but size is not everything. Consider three populations of endangered grizzly bears, each containing one individual per 20 km^2, and a total of 100 individuals in 2,000 km^2. These populations are “equal” with respect to size. One population, however, has 50 immature (non-reproducing young) bears and 50 adult bears able to reproduce. A second population has the same number of immature and adult bears, but of the 50 adults, 45 are male. The third population has 30 immature bears and 70 bears of reproductive age. Which population is healthiest?

The answer is not simple, but age and sex differences between populations are significant indicators of health. Biologists concerned about a population’s future study age and sex within the population and then graph the results to show the age-sex structure as a population pyramid, although the result does not always resemble a pyramid. The X-axis in this double bar graph indicates percentage of the population, with males to the left and females to the right. The Y-axis indicates age groups from birth to old age.

A generalized age-sex structure or population pyramid shows the proportion of males and females (X-axis) at each age level (Y-axis). This example shows a slightly higher proportion of females compared to males, and a much higher proportion of young individuals compared to old.

The population in the generalized example above contains a large proportion of young individuals, suggesting a relatively high birth rate (number of births per individual within the population per unit time). The bars narrow at each age interval, showing that a significant number of individuals die at every age. This relatively high death rate (number of deaths per individual within the population per unit time) indicates a short life expectancy, or average survival time for an individual. Note the slightly greater proportion of females compared to males at each age level. Careful study could determine whether the cause for this imbalance is the ratio of female to male births, or higher death rates for males throughout a shorter lifespan. You will learn in a later lesson that this pattern is characteristic of human populations in less developed countries.

Age structures can reveal a population’s health. The example on the left, with most individuals below reproductive age, often indicates a growing population. The middle example with roughly equal proportions of the population at each age level indicates a stable population. On the right there are more individuals at (or above!) reproductive age than young, which describes a declining population.

A population’s age structure may reveal its health. A growing population has more young individuals than adults at beyond reproductive age. A stable population often has roughly equal numbers of young members and adults. A declining population shows more adults and fewer young. Sex structure may also affect the health of a population. Sex determination in sea turtles, for example, is temperature-dependent; lower egg incubation temperatures produce males, while temperatures only 1-2^oC higher may produce females. Some biologists predict that climate change may result in sea turtle sex structure shifts toward females, which could further endanger already threatened species. Continued monitoring of age-sex structures among sea turtles might be able to detect such changes before they become irreversible.

The sex of a sea turtle is determined by the temperature at which it develops – males in cooler temperatures, and females in temperatures as little as 1-2^oC warmer. Climate change may threaten natural sex ratios. Such changes would be reflected in changing age-sex structure pyramids.