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Summary of Key Concepts
Understanding Populations
  • Population ecology studies how populations grow and what factors promote and limit growth.

  • Ecologists measure population density, the numbers of organisms in a given unit area, in many ways, including the mark-capture technique. (Figures 56.1, 56.2)

  • Individuals within populations show different patterns of dispersion, including clumped (the most common), uniform, and random. Individuals also exhibit different reproductive strategies, and populations have different age classes. (Figures 56.3, 56.4, 56.5)

  • Life tables summarize the survival pattern of a population. (Table 56.1)

  • Survivorship curves illustrate life tables by plotting the numbers of surviving individuals at different ages. (Figures 56.6, 56.7, 56.8)

  • Age-specific fertility and survivorship data help determine the overall growth rate per generation, or the net reproductive rate (R0).

How Populations Grow
  • The per capita growth rate (r) helps determine how populations grow over any time period.

  • When r is > 0, exponential (J-shaped) growth occurs. Exponential growth can be observed in an environment where resources are not limited. (Figures 56.9, 56.10)

  • Logistic (S-shaped) growth takes into account the upper boundary for a population, called carrying capacity, and occurs in an environment where resources are limited. (Figures 56.11, 56.12)

  • Variations in temperature, rainfall, or resource quantity or quality cause changes in carrying capacities, and thus the idealized logistic growth model does not describe all populations.

  • Density-dependent factors are mortality factors whose influence varies with population density. Density-independent factors are those whose influence does not vary with density. (Figure 56.13)

  • Life history strategies are a set of features including reproductive traits, survivorship and length of life characteristics, and competitive ability.

  • Life history strategies can be viewed as a continuum, with r-selected species (those with a high rate of population growth but poor competitive ability) at one end and K-selected species (those with a lower rate of population growth but better competitive ability) at the other. (Figures 56.14, 56.15, Table 56.2)

Human Population Growth
  • Up to the present, human population growth has fit an exponential growth pattern. (Figure 56.16)

  • Human populations have been moving from states of high birth and death rates to low birth and death rates, a shift called the demographic transition. (Figures 56.17, 56.18)

  • Differences in the age structure of a population, the numbers of individuals in each age group, are also characteristic of the demographic transition. (Figure 56.19)

  • Although they have been declining worldwide, total fertility rates (TFRs) differ markedly in less-developed and moredeveloped countries. Predicting the growth of the human population depends on the total fertility rate that is projected. (Figures 56.20, 56.21)

  • The ecological footprint refers to the amount of productive land needed to support each person on Earth. Because people in many countries live in a nonsustainable manner, globally we are already in an ecological deficit. (Figure 56.22)

Assess and Discuss
Test Yourself
  1. A student decides to conduct a mark-recapture experiment to estimate the population size of mosquitofish in a small pond near his home. In the first catch, he marked 45 individuals. Two weeks later, he captured 62 individuals, of which 8 were marked. What is the estimated size of the population based on these data?

    1. 134

    2. 349

    3. 558

    4. 1,016

    5. 22,320

    Page 1203

    Questions 2–4 refer to the table that follows:

    Age nx dx lx mx lxmx
    0 100 35 1.00 0 0
    1 65 ? 0.65 0 0
    2 45 15 ? 3 1.35
    3 30 20 0.30 1 ?
    4 10 10 0.10 1 0.10
    5 0 0 0.00 1 0.0
  2. How many individuals die between their first and second birthday?

    1. 65

    2. 45

    3. 35

    4. 25

    5. 20

  3. What proportion of newborns survive to age 2?

    1. 0.55

    2. 0.45

    3. 0.35

    4. 0.20

    5. 0.15

  4. What is the net reproductive rate?

    1. 5

    2. 2.5

    3. 1.75

    4. 1.45

    5. 0.80

  5. survivorship curves are usually associated with organisms that have high mortality rates in the early stages of life.

    1. Type I

    2. Type II

    3. Type III

    4. Types I and II

    5. Types II and III

  6. If the net reproductive rate (R0) is equal to 0.5, what assumptions can we make about the population?

    1. This population is essentially not changing in numbers.

    2. This population is in decline.

    3. This population is growing.

    4. This population is in equilibrium.

    5. none of the above

  7. The maximum number of individuals a certain area can sustain is known as

    1. the intrinsic rate of growth.

    2. the resource limit.

    3. the carrying capacity.

    4. the logistic equation.

    5. the equilibrium size.

    Questions 8 and 9 refer to the following generalized growth patterns as plotted on arithmetic scales. Match the following descriptions with the patterns indicated below.

    Each pattern may be used once, more than once, or not at all.

  8. Which pattern is found where a population exhibits a constant per capita growth rate?

    1. A

    2. B

    3. C

    4. D

    5. none of the above

  9. Which pattern is found where a population is heading toward extinction?

    1. A

    2. B

    3. C

    4. D

    5. none of the above

  10. The amount of land necessary for survival for each person in a sustainable world is known as

    1. the sustainability level.

    2. an ecological impact.

    3. an ecological footprint.

    4. survival needs.

    5. all of the above.


Conceptual Questions
  1. Define population and population ecology.


  2. Describe and list the assumptions of the mark-recapture technique.


  3. Using the logistic equation, calculate population growth when K = 1,000, N = 500, and r = 0.1 and when K = 1,000, N = 100, and r = 0.1. Compare the results to those shown in Section 56.3, where K = 1,000, N = 900, and r = 0.1.


Collaborative Questions
  1. Discuss the two main extremes of life history strategies.


  2. Describe where students on campus might show each type of dispersion pattern, and explain why this might occur.


Online Resource


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