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 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.
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?
A population pyramid represents the age-sex structure of a population. What does a large base represent?
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.
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.
- Population structure and population pyramids at http://www.bbc.co.uk/schools/gcsebitesize/geography/population/population_change_structure_rev5.shtml .
- What does a population structure explain about a population?
- What information can be interpreted from a population pyramid?
- How do pyramids change over time?
- How does the age-sex structure of a population influence growth?
- Assume that a population pyramid has a very broad base. What does that tell you about the population it represents?
- Compare and contrast Type I and Type III survivorship curves.