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The Chapter in Review
The Human Population

The human population exhibits clumped distribution (both on a large scale and a small scale) and is undergoing rapid growth.

Future Population Growth

At the present growth rate, the doubling time for the human population is estimated to be 35 years. This rate of growth will put extreme demands on resources, and growth will decline due to resource scarcity. Eventually the population will most likely level off at its carrying capacity.

  • More-Developed Countries (MDCs) The growth rate for MDCs is about 0.1%. The total MDC population between 2002 and 2050 will remain at around 1.2 billion.

  • Less-Developed Countries (LDCs) The growth rate for LDCs is about 1.6%. The LDC population between 2002 and 2050 is expected to increase from 5 billion to 8 billion.

Comparing Age Structures

The age structure of populations is divided into three age groups: dependency, reproductive, and postreproductive. The MDCs are approaching stabilization with just about equal numbers in each group. The LDCs will continue to expand because their prereproductive group is the largest.

Population Growth and Environmental Impact

Two types of environmental impact can occur: The LDCs put stress on the biosphere due to population growth, and the MDCs put stress on the biosphere due to resource consumption and waste production.

Characteristics of Populations

Distribution patterns and population density are both dependent on resource availability.

  • Distribution Patterns Clumped, random, uniform.

  • Population Density The number of individuals per unit area is higher in areas with abundant resources and lower when resources are limited.

  • Demographics and Population Growth Population growth can be calculated based on the annual birthrate and death rate. Population growth is determined by:

    • Resource availability

    • Demographics (age structures, survivorship, and biotic potential—the highest rate of increase possible)

Patterns of Population Growth

The two patterns of population growth are exponential growth and logistic growth.

  • Exponential Growth Exponential growth results in a J-shaped curve. The two phases of exponential growth are lag phase (slow growth) and exponential growth phase (accelerating growth).

  • Logistic Growth Logistic growth results in an S-shaped curve. The four phases of logistic growth are lag phase (slow growth), exponential growth phase (accelerating growth), deceleration phase (slowing growth), and stable equilibrium phase (relatively no growth).

Factors That Regulate Population Growth

The two factors that regulate population growth are density-independent and density-dependent factors.

  • Density-independent factors include abiotic factors, such as weather and natural disasters. The effect of the factor is not dependent on density.

  • Density-dependent factors include biotic factors, such as competition and predation. The effect of the factor is dependent on density.

Life History Patterns and Extinction

The two fundamental life history patterns are exhibited by opportunistic populations and equilibrium populations.

  • Opportunistic populations are characterized by individuals with short life spans, who mature quickly, produce many offspring, have strong dispersal ability, provide little or no care of offspring, and exhibit exponential growth.

  • Equilibrium populations are characterized by individuals with long life spans, who mature slowly, produce few offspring, provide much care of offspring, and exhibit logistic growth.


Extinction is the total disappearance of a species or higher group. Opportunistic populations are less likely than equilibrium populations to become extinct. Three factors in particular influence vulnerability of equilibrium populations to extinction: size of geographic range, degree of habitat tolerance, and size of local populations.

The Scope of Ecology

Ecology is an experimental science that studies the interactions of organisms with each other and with the physical environment. The levels of biological organization studied by ecologists are:

  • Organisms

  • Population

  • Community

  • Ecosystem

  • Biosphere

Key Terms
Testing Yourself

Choose the best answer for each question.

  1. Decreased death rate followed by decreased birthrate has occurred in

    1. MDCs.

    2. LDCs.

    3. MDCs and LDCs.

    4. neither MDCs nor LDCs.

  2. Human societies at present are characterized by

    1. ever-increasing population growth.

    2. unsustainable practices.

    3. concentrations along coastlines.

    4. All of these are correct.

  3. A population's maximum growth rate is also called its

    1. carrying capacity.

    2. biotic potential.

    3. growth curve.

    4. replacement rate.

  4. Which of these levels of ecological study involves both abiotic and biotic components?

    1. organisms

    2. populations

    3. communities

    4. ecosystem

    5. All of these are correct.

  5. The biological level of organization subject to evolution by natural selection is called

    1. an organism.

    2. a population.

    3. a community.

    4. a ecosystem.

    5. the biosphere.

  6. Globally, if a population lives only along a lake's shoreline, members of this population exhibit which type of spatial distribution?

    1. variable

    2. clumped

    3. random

    4. uniform

  7. Which are likely to have the highest population density?

    1. zebras on the African savanna during the dry season

    2. mice in the frozen tundra

    3. moose in a Canadian forest

    4. earthworms in an organically rich soil

  8. Calculate the growth rate of a population of 500 individuals in which the birthrate is 10 per year and the death rate is 5 per year.

    1. 5%

    2. -5%

    3. 1%

    4. -1%

  9. If the human birthrate was reduced to 15 per 1,000 per year and the death rate remained the same (9 per 1,000), what would be the growth rate?

    1. 9%

    2. 6%

    3. 10%

    4. 0.6%

    5. 15%

  10. Replacement reproduction in a population with a pyramid-shaped age structure diagram results in

    1. no population growth.

    2. population growth.

    3. a decline in the population.

  11. Label the following age structure diagrams to indicate whether the population is stable, increasing, or decreasing.

  12. If the number of prereproductive and reproductive members of a population exceeds the number of postreproductive members, the population will

    1. grow.

    2. remain stable.

    3. decline.

  13. An S-shaped growth curve indicates

    1. logistic growth.

    2. logarithmic growth.

    3. exponential growth.

    4. additive growth.

  14. Exponential growth occurs

    1. when population size is increasing to an ever-higher amount.

    2. at the carrying capacity of the environment.

    3. when people are poor and don't have enough to eat.

  15. For statements 15–18, indicate the interaction in the key exemplified by the scenario.


    a. density-independent factor

    b. competition

    c. predation

    d. predator-prey cycle

  16. A severe drought destroys the entire food supply of a herd of gazelle.

  17. A population of feral cats increases in size as the mouse population increases and then crashes regularly after the mouse population enters periods of decline.

  18. Swift coyotes are able to catch rabbits and live to reproduce. Other coyotes lack a good food source and are not strong enough to reproduce.

  19. Deer prefer to feed on a dense thicket of oak saplings in preference to more widely spaced young oak trees.

  20. Which of the following is not an adaptive feature of an opportunistic life history pattern?

    1. many offspring

    2. little or no care of offspring

    3. long life span

    4. small individuals

    5. fast to mature

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  22. Natural selection favors a population of organisms living in a frequently disturbed environment to

    1. have a long life.

    2. invest much time and energy into parental care for young.

    3. have many offspring.

    4. mature slowly.

  23. When the carrying capacity of the environment is exceeded, the population typically

    1. increases, but at a slower rate.

    2. decreases and stabilizes at the highest level possible.

    3. reverts to an opportunistic life history strategy.

    4. experience extinction immediately.

  24. A pyramid-shaped age distribution means that

    1. the prereproductive group is the largest group.

    2. the population will grow for some time in the future.

    3. the country is more likely an LDC than an MDC.

    4. fewer women are leaving the reproductive years than entering them.

    5. All of these are correct.

  25. Which of these is a density-independent factor?

    1. competition

    2. predation

    3. weather

    4. resource availability

  26. Which of the following would not limit the biotic potential of an organism?

    1. a decrease in predators of this population

    2. an increase in parasites of this population

    3. maintaining a high degree of competition between this population and other species

    4. an decrease in natural resources available to this population

    5. All of these would limit a population's biotic potential.

Thinking Scientifically
  1. In Sri Lanka, the death rate is 6 per 1,000, while the birthrate is 19 per 1,000. Calculate the current population growth rate. The formula for the doubling time of a population is:

    t = 0.69/r, where t is the doubling time and r is the growth rate.

    Determine the doubling rate (the number of years it will take to double the population size) in Sri Lanka. Be sure to convert your growth rate from a percentage (e.g., 2.1%) to a decimal value (0.021). If the birthrate drops to 10 per 1,000, what is the doubling rate?

  2. Assume that a population of dandelions grows exponentially so that it doubles in size every week. The population (beginning with one plant) expands to fill a field in 20 weeks. How many weeks will it take to fill one-quarter of the field? How long will it take to fill half the field?

  3. What type data would you collect to show that a population is undergoing stabilizing selection, as per Figure 16.9?

Bioethical Issue
Saving Species from Extinction

Species that are more prone to certain risk factors are more likely than others to become extinct. For example, species with a unique lineage, such as the giant panda, are likely to be at severe risk for extinction. Should our limited resources for species protection be focused on species that are at the highest risk for extinction? Some argue that high-risk species are less successful products of evolution and should not receive extraordinary protection. Consequently, all species at risk for extinction should be equally protected. Which camp are you in? Support your position.

Essentials of Biology Website

The companion website for Essentials of Biology provides a wealth of information organized and integrated by chapter. You will find practice tests, animations, videos, and much more that will complement your learning and understanding of general biology.