Lesson Objectives

  • Define population size, density, and dispersion.
  • Relate population pyramids and survivorship curves to population structure.
  • Identify factors that determine population growth rate.
  • Compare and contrast exponential and logistic growth.


  • age-sex structure
  • carrying capacity (K)
  • dispersal
  • emigration
  • exponential growth
  • immigration
  • K-selected
  • logistic growth
  • migration
  • population density
  • population distribution
  • population growth rate (r)
  • population pyramid
  • r-selected
  • survivorship curve


Communities are made up of populations of different species. In biology, a population is a group of organisms of the same species that live in the same area. The population is the unit of natural selection and evolution. How large a population is and how fast it is growing are often used as measures of its health.

Population Size, Density, and Distribution

Population size is the number of individuals in a population. For example, a population of insects might consist of 100 individual insects, or many more. Population size influences the chances of a species surviving or going extinct. Generally, very small populations are at greatest risk of extinction. However, the size of a population may be less important than its density.

Population Density

Population density is the average number of individuals in a population per unit of area or volume. For example, a population of 100 insects that live in an area of 100 square meters has a density of 1 insect per square meter. If the same population lives in an area of only 1 square meter, what is its density? Which population is more crowded? How might crowding affect the health of a population?

Population Distribution

Population density just gives the average number of individuals per unit of area or volume. Often, individuals in a population are not spread out evenly. Instead, they may live in clumps or some other pattern (see Figure below). The pattern may reflect characteristics of the species or its environment. Population distribution describes how the individuals are distributed, or spread throughout their habitat.

Patterns of Population Distribution. What factors influence the pattern of a population over space?

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. A population pyramid represents the age-sex structure of a population.

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. 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.

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

Population Growth

Populations gain individuals through births and immigration. They lose individuals through deaths and emigration. These factors together determine how fast a population grows.

Population Growth Rate

Population growth rate (r) is how fast a population changes in size over time. A positive growth rate means a population is increasing. A negative growth rate means it is decreasing. The two main factors affecting population growth are the birth rate (b) and death rate (d). Population growth may also be affected by people coming into the population from somewhere else (immigration, i) or leaving the population for another area (emigration, e). The formula for population growth takes all these factors into account.

r = (b + i) - (d + e)

  • r = population growth rate
  • b = birth rate
  • i = immigration rate
  • d = death rate
  • e = emigration rate

Two lectures on demography are available at http://www.youtube.com/watch?v=3diw1Hu3auk (50:36) and http://www.youtube.com/watch?v=Wg3ESbyKbic (49:38).

Dispersal and Migration

Other types of movements may also affect population size and growth. For example, many species have some means of dispersal. This refers to offspring moving away from their parents. This prevents the offspring from competing with the parents for resources such as light or water. For example, dandelion seeds have “parachutes.” They allow the wind to carry the seeds far from the parents (see Figure below).

Dandelion Seeds. These dandelion seeds may disperse far from the parent plant. Why might this be beneficial to both parents and offspring?

Migration is another type of movement that changes population size. Migration is the regular movement of individuals or populations each year during certain seasons. The purpose of migration usually is to find food, mates, or other resources. For example, many Northern Hemisphere birds migrate thousands of miles south each fall. They go to areas where the weather is warmer and more resources are available (see Figure below). Then they return north in the spring to nest. Some animals, such as elk, migrate vertically. They go up the sides of mountains in spring as snow melts. They go back down the mountain sides in fall as snow returns.

Swainson’s hawks migrate from North to South America and back again each year. This map shows where individual hawks have been identified during their migration.

Patterns of Population Growth

Populations may show different patterns of growth. The growth pattern depends partly on the conditions under which a population lives.

Exponential Growth

Under ideal conditions, populations of most species can grow at exponential rates. Curve A in Figure below represents exponential growth. The population starts out growing slowly. As population size increases, the growth rate also increases. The larger the population becomes, the faster it grows.

Exponential and Logistic Growth. Curve A shows exponential growth. Curve B shows logistic growth.

Logistic Growth

Most populations do not live under ideal conditions. Therefore, most do not grow exponentially. Certainly, no population can keep growing exponentially for very long. Many factors may limit growth. Often, the factors are density dependent. These are factors that kick in when the population becomes too large and crowded. For example, the population may start to run out of food or be poisoned by its own wastes. As a result, population growth slows and population size levels off. Curve B in Figure above represents this pattern of growth, which is called logistic growth.

At what population size does growth start to slow in the logistic model of growth? That depends on the population’s carrying capacity (see Figure above). The carrying capacity (K) is the largest population size that can be supported in an area without harming the environment. Population growth hits a ceiling at that size in the logistic growth model.

K-Selected and r-Selected Species

Species can be divided into two basic types when it comes to how their populations grow.

  • Species that live in stable environments are likely to be K-selected. Their population growth is controlled by density-dependent factors. Population size is generally at or near the carrying capacity. These species are represented by curve B in Figure above.
  • Species that live in unstable environments are likely to r-selected. Their potential population growth is rapid. For example, they have large numbers of offspring. However, individuals are likely to die young. Thus, population size is usually well below the carrying capacity. These species are represented by the lower part of curve A in Figure above.

Lesson Summary

  • Population size is the number of individuals in a population. Population density is the average number of individuals per unit of area or volume. The pattern of spacing of individuals in a population may be affected by characteristics of a species or its environment.
  • 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.
  • Population growth rate is how fast a population changes in size over time. It is determined by rates of birth, death, immigration, and emigration.
  • Under ideal conditions, populations can grow exponentially. The growth rate increases as the population gets larger. Most populations do not live under ideal conditions and grow logistically instead. Density-dependent factors slow population growth as population size nears the carrying capacity.

Lesson Review Questions


1. What is population density?

2. Define immigration and emigration.

3. What is migration? Give an example.

4. Write the formula for the population growth rate. Identify all the variables.

5. State why dispersal of offspring away from their parents might be beneficial.

6. Describe exponential population growth.

7. What are K-selected and r-selected species?

Apply Concepts

8. A population of 820 insects lives in a 1.2-acre area. They gather nectar from a population of 560 flowering plants. The plants live in a 0.2-acre area. Which population has greater density, the insects or the plants?

9. Assume that a population pyramid has a very broad base. What does that tell you about the population it represents?

Think Critically

10. What can you infer about a species that has a random pattern of distribution over space? A uniform pattern?

11. Compare and contrast Type I and Type III survivorship curves.

Points to Consider

Human populations have an interesting history that you will read about in the next lesson. You just read about population dispersion and growth. Make some predictions about dispersion and growth in human populations:

  • Do you think human populations have a clumped, random, or uniform dispersion?
  • How fast do human populations grow? What might limit their growth?