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Population Structure

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Young vs. old. Does it matter?

When it comes to populations, yes it does. The age structure (and the sex structure) of a population influences population growth. Can you explain why?

Population Structure

Population growth is the change in the size of the population over time. An important factor in population growth is age-sex structure . This is the number of individuals of each sex and age in the population. The age-sex structure influences population growth. This is because younger people are more likely to reproduce, while older people have higher rates of dying.

Population Pyramids

Age-sex structure is represented by a population pyramid . This is a bar graph, like the one Figure below . In this example, the bars become narrower from younger to older ages. Can you explain why?

Population pyramid represents the age and sex structure of a population

A population pyramid represents the age-sex structure of a population. What does a large base represent?

Survivorship Curves

Another way to show how deaths affect populations is with survivorship curves . These are graphs that represent the number of individuals still alive at each age. Examples are shown in Figure below .

Survivorship curves reflect death rates at different ages

Survivorship curves reflect death rates at different ages.

The three types of curves shown in the figure actually represent different strategies species use to adapt to their environment:

  • Type I: Parents produce relatively few offspring and provide them with a lot of care. As a result, most of the offspring survive to adulthood so they can reproduce. This pattern is typical of large animals, including humans.
  • Type II: Parents produce moderate numbers of offspring and provide some parental care. Deaths occur more uniformly throughout life. This pattern occurs in some birds and many asexual species.
  • Type III: Parents produce many offspring but provide them with little or no care. As a result, relatively few offspring survive to adulthood. This pattern is typical of plants, invertebrates, and many species of fish.

See http://www.youtube.com/watch?v=xkAnO8VjCz0 for an overview of survivorship curves.

The Type I strategy occurs more often in stable environments. The Type III strategy is more likely in unstable environments. Can you explain why?


  • The age-sex structure of a population is the number of individuals of each sex and age in the population.
  • Age-sex structure influences population growth. It is represented by a population pyramid.
  • The number of survivors at each age is plotted on a survivorship curve.


Use this resource to answer the questions that follow.

  1. Describe the relationship between a life table and a survivorship curve.
  2. What is a major factor in determining if offspring survive?
  3. What is an age pyramid?


  1. How does the age-sex structure of a population influence growth?
  2. Assume that a population pyramid has a very broad base. What does that tell you about the population it represents?
  3. Compare and contrast Type I and Type III survivorship curves.

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