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# Population Structure

## Gender ratios and reproductive age heavily influence the growth of populations.

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Population Structure

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?

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.

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?

### Summary

• 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.

### Practice

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/Biology $\rightarrow$ Biology for AP* $\rightarrow$ Search: Population Distribution
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?

### Review

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.

### Vocabulary Language: English Spanish

age-sex structure

age-sex structure

Number of individuals of each sex and age in a population.
population growth

population growth

The change in the size of the population over time.
population pyramid

population pyramid

Bar graph that represents the age-sex structure of a population.
survivorship curve

survivorship curve

Graph that represents the individuals still alive at each age in a population.