Introduction Has parental care evolved by natural selection?

Introduction Has parental care evolved by natural selection?
Consider parental investment, the time and energy expended on offspring. The females of some species carry the embryos internally and give birth after they have hatched. Some fish do not provide any form of parental care and instead invest in quantity, producing millions of eggs at a time.

Introduction In a small Mediterranean species known as the peacock wrasse, the largest males build seaweed nests, where they court and mate with females, fertilize a nest full of eggs, and then lose interest in mating and guard the nest from predators. Females must search for a large nesting male, spending time and energy, to increase her chances of reproductive success.

Figure Figure Has parental care evolved by natural selection? (photo: peacock wrasse)

Population Structure and Dynamics
Figure Chapter 36: Big Ideas 1989 Male Female Figure Chapter 36: Big Ideas Population Structure and Dynamics The Human Population

Population Structure and Dynamics

36.1 Population ecology is the study of how and why populations change
A population is a group of individuals of a single species that occupy the same general area. Individuals in a population rely on the same resources, are influenced by the same environmental factors, and are likely to interact and breed with one another. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  A mark-recapture method not specifically addressed in this chapter can be used to estimate the size of a population. The following can serve as a demonstration or an activity for students working in small groups: a. Provide each group with an opaque bag (brown paper lunch bags work well) of about 200 dried lima beans (or any inexpensive small item that can be marked). b. Have each group draw out 40 beans. c. Mark each bean with a distinct pencil or ink mark. d. Return these marked beans back to the bag. e. Mix the beans in the bag by shaking or turning the bag. Note: Thorough mixing and random selection is essential to the mark-recapture method. You may wish to note here that this research method does not work well for wildlife populations that are territorial and thus do not mix. f. Draw out another 40 beans and count the number of marked beans in the sample. g. The formula for calculating the population size is as follows: The number of marked beans in the first sample  the total number in the second sample  the number of recaptures in the second sample = the population size. Thus, if you started out with exactly 200 beans, sampled 40, marked them, and resampled 40 beans, we would expect that you would recapture 8 marked beans, based on the equation 40  40  8 = 200. Active Lecture Tips  See the Activity The Capture-Recapture Method to Estimate Class Size on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

36.1 Population ecology is the study of how and why populations change
Population ecology is concerned with the changes in population size and factors that regulate populations over time. Populations increase through birth and immigration to an area and decrease through death and emigration out of an area. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  A mark-recapture method not specifically addressed in this chapter can be used to estimate the size of a population. The following can serve as a demonstration or an activity for students working in small groups: a. Provide each group with an opaque bag (brown paper lunch bags work well) of about 200 dried lima beans (or any inexpensive small item that can be marked). b. Have each group draw out 40 beans. c. Mark each bean with a distinct pencil or ink mark. d. Return these marked beans back to the bag. e. Mix the beans in the bag by shaking or turning the bag. Note: Thorough mixing and random selection is essential to the mark-recapture method. You may wish to note here that this research method does not work well for wildlife populations that are territorial and thus do not mix. f. Draw out another 40 beans and count the number of marked beans in the sample. g. The formula for calculating the population size is as follows: The number of marked beans in the first sample  the total number in the second sample  the number of recaptures in the second sample = the population size. Thus, if you started out with exactly 200 beans, sampled 40, marked them, and resampled 40 beans, we would expect that you would recapture 8 marked beans, based on the equation 40  40  8 = 200. Active Lecture Tips  See the Activity The Capture-Recapture Method to Estimate Class Size on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

36.2 Density and dispersion patterns are important population variables
Population density is the number of individuals of a species per unit area or volume. Examples of population density include the number of oak trees per square kilometer in a forest or the number of earthworms per cubic meter in forest soil. Ecologists use a variety of sampling techniques to estimate population densities. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  A simple application of the dispersion pattern of a population would be to apply the concept to the population of humans on your college or university campus. Would students consider the distribution of people to be clumped, uniform, or random? Most campuses would likely represent a clumped pattern. It might be fun to discuss when, if ever, the human population on your campus represents a uniform or random pattern. Active Lecture Tips  See the Activity The Capture-Recapture Method to Estimate Class Size on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Within a population’s geographic range, local densities may vary greatly. The dispersion pattern of a population refers to the way individuals are spaced within their area. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  A simple application of the dispersion pattern of a population would be to apply the concept to the population of humans on your college or university campus. Would students consider the distribution of people to be clumped, uniform, or random? Most campuses would likely represent a clumped pattern. It might be fun to discuss when, if ever, the human population on your campus represents a uniform or random pattern. Active Lecture Tips  See the Activity The Capture-Recapture Method to Estimate Class Size on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Video: Flapping Geese (clumped)

Video: Albatross Courtship (uniform)

Video: Prokaryotic Flagella (Salmonella typhimurium) (random)

Dispersion patterns can be clumped, uniform, or random. In a clumped dispersion pattern, resources are often unequally distributed and individuals are grouped in patches. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  A simple application of the dispersion pattern of a population would be to apply the concept to the population of humans on your college or university campus. Would students consider the distribution of people to be clumped, uniform, or random? Most campuses would likely represent a clumped pattern. It might be fun to discuss when, if ever, the human population on your campus represents a uniform or random pattern. Active Lecture Tips  See the Activity The Capture-Recapture Method to Estimate Class Size on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Figure 36.2a Figure 36.2a Clumped dispersion of ochre sea stars at low tide

In a uniform dispersion pattern, individuals are most likely interacting and equally spaced in the environment. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  A simple application of the dispersion pattern of a population would be to apply the concept to the population of humans on your college or university campus. Would students consider the distribution of people to be clumped, uniform, or random? Most campuses would likely represent a clumped pattern. It might be fun to discuss when, if ever, the human population on your campus represents a uniform or random pattern. Active Lecture Tips  See the Activity The Capture-Recapture Method to Estimate Class Size on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Figure 36.2b Figure 36.2b Uniform dispersion of sunbathers at Coney Island

In a random dispersion pattern of dispersion, the individuals in a population are spaced in an unpredictable way, without a pattern, perhaps resulting from the random dispersal of windblown seeds. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  A simple application of the dispersion pattern of a population would be to apply the concept to the population of humans on your college or university campus. Would students consider the distribution of people to be clumped, uniform, or random? Most campuses would likely represent a clumped pattern. It might be fun to discuss when, if ever, the human population on your campus represents a uniform or random pattern. Active Lecture Tips  See the Activity The Capture-Recapture Method to Estimate Class Size on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Figure 36.2c Figure 36.2c Random dispersion of dandelions

36.3 Life tables track survivorship in populations
Life tables track survivorship, the chance of an individual in a given population surviving to various ages. Survivorship curves plot survivorship as the proportion of individuals from an initial population that are alive at each age. There are three main types of survivorship curves. Type I Type II Type III Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  The Centers for Disease Control provide information and life tables for people living in the United States at their website, Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Table 36.3 Table 36.3 Life table for the U.S. population in 2008

Percentage of survivors (log scale)
Figure 36.3b 100 I 10 Percentage of survivors (log scale) II 1 III Figure 36.3b Three types of survivorship curves 0.1 50 100 Percentage of maximum life span

36.4 Idealized models predict patterns of population growth
The rate of population increase under ideal conditions is called exponential growth. It can be calculated using the exponential growth model equation, G = rN, in which G is the growth rate of the population, N is the population size, and r is the per capita rate of increase (the average contribution of each individual to population growth). Eventually, one or more limiting factors will restrict population growth. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  Exponential growth in a population is like compounded interest on a bank account. The growth of the account is initially small, but as the interest earns interest, the growth expands. $1,000 invested at 7% interest is worth more than $30,000 in 50 years. Consider assigning students to calculate the value of a simple interest-bearing investment over a set period of years, as in the example just noted. Many online financial calculators can perform this task. Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Population size (N) Time (months) 500 450 400 350 300 250 200 150 100
Figure 36.4a-0 500 450 400 350 300 250 200 150 100 50 Population size (N) Figure 36.4a-0 Exponential growth of rabbits Time (months)

Population size (N) Time (months) 500 450 400 350 300 250 200 150 100
Figure 36.4a-1 500 450 400 350 300 250 200 150 100 50 Population size (N) Figure 36.4a-1 Exponential growth of rabbits (graph only) Time (months)

Table 36.4a Table 36.4a Exponential growth of rabbits, r = 0.3

The logistic growth model is a description of idealized population growth that is slowed by limiting factors as the population size increases. To model logistic growth, the formula for exponential growth, rN, is multiplied by an expression that describes the effect of limiting factors on an increasing population size. K stands for carrying capacity, the maximum population size a particular environment can sustain. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  Exponential growth in a population is like compounded interest on a bank account. The growth of the account is initially small, but as the interest earns interest, the growth expands. $1,000 invested at 7% interest is worth more than $30,000 in 50 years. Consider assigning students to calculate the value of a simple interest-bearing investment over a set period of years, as in the example just noted. Many online financial calculators can perform this task. Active Lecture Tips • See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Breeding male fur seals (thousands)
Figure 36.4b-0 10 8 6 4 2 Breeding male fur seals (thousands) Figure 36.4b-0 Growth of a population of fur seals Year Data from K. W. Kenyon et al., A population study of the Alaska fur-seal herd, Federal Government Series: Special Scientific Report—Wildlife 12 (1954).

Breeding male fur seals (thousands)
Figure 36.4b-1 10 8 6 4 2 Breeding male fur seals (thousands) Figure 36.4b-1 Growth of a population of fur seals (graph only) Year Data from K. W. Kenyon et al., A population study of the Alaska fur-seal herd, Federal Government Series: Special Scientific Report—Wildlife 12 (1954).

Number of individuals (N)
Figure 36.4c G = rN (K − N) Number of individuals (N) K G = rN K Figure 36.4c Logistic growth and exponential growth compared Time

Table 36.4B demonstrates how the expression (K – N)/K in the logistic growth model produces the S-shaped curve. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  Exponential growth in a population is like compounded interest on a bank account. The growth of the account is initially small, but as the interest earns interest, the growth expands. $1,000 invested at 7% interest is worth more than $30,000 in 50 years. Consider assigning students to calculate the value of a simple interest-bearing investment over a set period of years, as in the example just noted. Many online financial calculators can perform this task. Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Table 36.4b Table 36.4b Effect of K on growth rate as N approaches K, K = 1,000, r = 0.1

36.5 Multiple factors may limit population growth
The logistic growth model predicts that population growth will slow and eventually stop as population density increases. At higher population densities, density-dependent rates result in declining births and/or increases in deaths. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.  It is typically easier for students to understand a concept when the examples are familiar. Consider the biology of your region and identify a population that is likely to be well-known by your students, for instance, the population of squirrels on your campus. Challenge your students to work in class in pairs to identify limiting factors for that particular population.

Figure 36.5a-0 6 5 4 3 2 1 Mean number of offspring per female Figure 36.5a-0 Declining reproductive success of song sparrows (inset) with increasing population density Density of females Data from P. Arcese et al., Stability, Regulation, and the Determination of Abundance in an Insular Song Sparrow Population. Ecology 73: 805–882 (1992).

Mean number of offspring per female
Figure 36.5a-1 6 5 4 Mean number of offspring per female 3 2 1 Figure 36.5a-1 Declining reproductive success of song sparrows with increasing population density Density of females Data from P. Arcese et al., Stability, Regulation, and the Determination of Abundance in an Insular Song Sparrow Population. Ecology 73: 805–882 (1992).

Intraspecific competition is competition between individuals of the same species for limited resources and is a density-dependent factor that limits growth in natural populations. Limiting factors may include food, nutrients, or nesting sites. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.  It is typically easier for students to understand a concept when the examples are familiar. Consider the biology of your region and identify a population that is likely to be well-known by your students, for instance, the population of squirrels on your campus. Challenge your students to work in class in pairs to identify limiting factors for that particular population.

Proportional mortality
Figure 36.5b-0 Kelp perch 1.0 0.8 0.6 0.4 0.2 Proportional mortality Figure 36.5b-0 Increasing mortality of kelp perch (inset) with increasing density Kelp perch density (number/plot) Data from T. W. Anderson, Predator Responses, Prey Refuges, and Density-Dependent Mortality of a Marine Fish, Ecology 82: 245–257 (2001).

Proportional mortality
Figure 36.5b-1 1.0 0.8 0.6 Proportional mortality 0.4 0.2 Figure 36.5b-1 Increasing mortality of kelp perch with increasing density Kelp perch density (number/plot) Data from T. W. Anderson, Predator Responses, Prey Refuges, and Density-Dependent Mortality of a Marine Fish, Ecology 82: 245–257 (2001).

In many natural populations, abiotic factors such as weather may affect population size well before density-dependent factors become important. Density-independent factors are unrelated to population density. These may include fires, storms, habitat destruction by human activity, or seasonal changes in weather (for example, in aphids). Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.  It is typically easier for students to understand a concept when the examples are familiar. Consider the biology of your region and identify a population that is likely to be well-known by your students, for instance, the population of squirrels on your campus. Challenge your students to work in class in pairs to identify limiting factors for that particular population.

Exponential growth Number of aphids Sudden decline
Figure 36.5c-0 Exponential growth Sudden decline Number of aphids Figure 36.5c-0 Weather change as a density-independent factor limiting aphid (inset) population growth Apr May Jun Jul Aug Sep Oct Nov Dec Month

Number of aphids Exponential growth Sudden decline
Figure 36.5c-1 Exponential growth Sudden decline Number of aphids Figure 36.5c-1 Weather change as a density-independent factor limiting aphid population growth Apr May Jun Jul Aug Sep Oct Nov Dec Month

36.6 SCIENTIFIC THINKING: Some populations have “boom-and-bust” cycles
Some populations fluctuate in density with regularity. Boom-and-bust cycles may be due to food shortages or predator-prey interactions. A striking example is shown in Figure 36.6, which shows estimated populations of the snowshoe hare and the lynx based on the number of pelts sold by trappers in northern Canada to the Hudson Bay Company over a period of nearly 100 years. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  Consider challenging your class to explain why the lynx and hare cycle does not result in the elimination of one or both of the species. Why don’t we see hares hunted to extinction? Students may not have considered that predators encounter greater difficulty in finding prey when prey populations are low. This permits the recovery of the hare population, which in turn supports the recovery of the lynx population. Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Hare population size (thousands) Lynx population size (thousands)
Figure 160 Snowshoe hare 120 9 Lynx Hare population size (thousands) Lynx population size (thousands) 80 6 Figure Population cycles of the snowshoe hare and the lynx 40 3 1850 1875 1900 1925 Year Data from C. Elton and M. Nicholson, The ten-year cycle in numbers of the lynx in Canada, Journal of Animal Ecology 11 : 215–244 (1942).

Hare population size (thousands) Lynx population size (thousands)
Figure 160 Snowshoe hare 120 9 Lynx Hare population size (thousands) Lynx population size (thousands) 80 6 40 3 1850 1875 1900 1925 Figure Population cycles of the snowshoe hare and the lynx (graph only) Year Data from C. Elton and M. Nicholson, The ten-year cycle in numbers of the lynx in Canada, Journal of Animal Ecology 11 : 215–244 (1942).

But what causes the boom-and-bust cycles of snowshoe hares? One hypothesis proposed that when hares are abundant, they overgraze their winter food supply, resulting in high mortality. Another hypothesis attributed hare population cycles to excessive predation. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  Consider challenging your class to explain why the lynx and hare cycle does not result in the elimination of one or both of the species. Why don’t we see hares hunted to extinction? Students may not have considered that predators encounter greater difficulty in finding prey when prey populations are low. This permits the recovery of the hare population, which in turn supports the recovery of the lynx population. Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Using radio collars to track individual hares, researchers determined that 90% of hares had been killed by predators, and none had died of starvation. These results support the predation hypothesis. However, further experiments showed that although fluctuating food availability is not the primary factor controlling hare population cycles, it does play an important role. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  Consider challenging your class to explain why the lynx and hare cycle does not result in the elimination of one or both of the species. Why don’t we see hares hunted to extinction? Students may not have considered that predators encounter greater difficulty in finding prey when prey populations are low. This permits the recovery of the hare population, which in turn supports the recovery of the lynx population. Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

36.7 EVOLUTION CONNECTION: Evolution shapes life histories
The traits that affect an organism’s schedule of reproduction and death make up its life history. Key life history traits include age of first reproduction, frequency of reproduction, number of offspring, and amount of parental care. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  Compromise is a key principle of biology. No adaptation can be perfect, and no reproductive strategy can maximize all types of efforts. As the text notes, an organism cannot have a great number of offspring and invest great amounts of parental care in each one. Resources, including time, are limited. Have students imagine how different their lives would have been if they had been born as one of a set of quadruplets—or if they were faced with the task of rearing four children at once! Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Populations with r-selected life history traits grow rapidly in unpredictable environments, where resources are abundant, have a large number of offspring that develop and reach sexual maturity rapidly, and offer little or no parental care. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  Compromise is a key principle of biology. No adaptation can be perfect, and no reproductive strategy can maximize all types of efforts. As the text notes, an organism cannot have a great number of offspring and invest great amounts of parental care in each one. Resources, including time, are limited. Have students imagine how different their lives would have been if they had been born as one of a set of quadruplets—or if they were faced with the task of rearing four children at once! Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Populations with K-selected traits tend to be long-lived animals (such as bears and elephants) that develop slowly and produce few, but well-cared-for, offspring and maintain relatively stable populations near carrying capacity. Most species fall between these two extremes. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  Compromise is a key principle of biology. No adaptation can be perfect, and no reproductive strategy can maximize all types of efforts. As the text notes, an organism cannot have a great number of offspring and invest great amounts of parental care in each one. Resources, including time, are limited. Have students imagine how different their lives would have been if they had been born as one of a set of quadruplets—or if they were faced with the task of rearing four children at once! Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

A long-term project in Trinidad studied guppy populations, provided direct evidence that life history traits can be shaped by natural selection, and demonstrated that questions about evolution can be tested by field experiments. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape. Teaching Tips  Compromise is a key principle of biology. No adaptation can be perfect, and no reproductive strategy can maximize all types of efforts. As the text notes, an organism cannot have a great number of offspring and invest great amounts of parental care in each one. Resources, including time, are limited. Have students imagine how different their lives would have been if they had been born as one of a set of quadruplets—or if they were faced with the task of rearing four children at once! Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Mass of guppies at maturity (mg) Age of guppies at maturity (days)
Figure Pool 1 Experiment: Transplant guppies Predator: Killifish; preys mainly on small guppies 200 185.6 161.5 160 Mass of guppies at maturity (mg) 120 76.1 Results 80 67.5 Guppies: Larger at sexual maturity than those in pike-cichlid pools 40 Males Females Pool 3 100 85.7 92.3 Pools with killifish, but no guppies prior to transplant 80 Age of guppies at maturity (days) 58.2 60 48.5 40 Pool 2 20 Predator: Pike cichlid; preys mainly on large guppies Males Females Control: Guppies from pools with pike-cichlids as predators Guppies: Smaller at sexual maturity than those in killifish pools Figure The effect of predation on the life history traits of guppies Experimental: Guppies transplanted to pools with killifish as predators Hypothesis: Predator feeding preferences caused difference in life history traits of guppy populations. Data from D. N. Reznick and H. Bryga, Life-History Evolution in Guppies (Poecilia reticulata): 1. Phenotypic and genetic changes in an introduction experiment, Evolution 41: 1370–1385 (1987).

Experiment: Transplant guppies
Figure Pool 1 Experiment: Transplant guppies Predator: Killifish; preys mainly on small guppies Guppies: Larger at sexual maturity than those in pike-cichlid pools Pool 3 Pools with killifish, but no guppies prior to transplant Pool 2 Predator: Pike cichlid; preys mainly on large guppies Figure The effect of predation on the life history traits of guppies (part 1) Guppies: Smaller at sexual maturity than those in killifish pools Hypothesis: Predator feeding preferences caused difference in life history traits of guppy populations. Data from D. N. Reznick and H. Bryga, Life-History Evolution in Guppies (Poecilia reticulata): 1. Phenotypic and genetic changes in an introduction experiment, Evolution 41: 1370–1385 (1987).

Mass of guppies at maturity (mg) Age of guppies at maturity (days)
Figure 200 185.6 161.5 160 Mass of guppies at maturity (mg) 120 76.1 80 67.5 40 Males Females 100 92.3 85.7 80 58.2 Age of guppies at maturity (days) 60 48.5 40 20 Males Females Figure The effect of predation on the life history traits of guppies (part 2) Control: Guppies from pools with pike-cichlids as predators Experimental: Guppies transplanted to pools with killifish as predators

36.8 CONNECTION: Principles of population ecology have practical applications
Sustainable resource management involves harvesting crops without damaging the resource. In terms of population growth, this means maintaining a high population growth rate to replenish the population. According to the logistic growth model, the fastest growth rate occurs when the population size is at roughly half the carrying capacity of the habitat. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape.  Students often expect that spraying insecticides or using various killing devices (such as bug zappers) will make a significant impact in a pest population. As noted in Module 36.8, many pesticides kill both pests and their predators. Furthermore, most pest populations are r-selective and capable of recovering quickly, perhaps more quickly than their predators. Such considerations provide a classic illustration of the complexities inherent in biological systems and the unexpected consequences of change. Teaching Tips  The sustainable management of the Alaskan fisheries is a good lesson in responsible resource management. Consider learning more about the management of halibut populations around Alaska as a positive example. The “Monterey Bay Aquarium Seafood Watch” addresses pacific halibut at Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

36.8 CONNECTION: Principles of population ecology have practical applications
Fish are hunted on a large scale and are particularly vulnerable to overharvesting. The northern Atlantic cod fishery was overfished, collapsed in 1992, and still has not recovered. Resource managers may try to provide additional habitat or improve the quality of existing habitat to raise the carrying capacity and thus increase population growth. Student Misconceptions and Concerns  Many students who are not biology majors have trouble thinking about the evolution of systems. One analogy that can be developed, especially for economically minded students, is the parallels to the “evolution” of businesses. Consider the introduction and expansion of McDonald’s restaurants in the United States over the last 60 years. When McDonald’s restaurants were just starting out, they experienced little competition, with access to many customers. The “population” of McDonald’s restaurants in the United States grew exponentially (or nearly so), with few density-dependent factors. However, today McDonald’s restaurants in the U.S. must compete with each other, as well as with many other fast-food restaurants, such as Burger King, Taco Bell, and Subway. The population of McDonald’s restaurants in the United States has stabilized because of this competition for customers, a density-dependent factor. A graph of the growth of McDonald’s restaurants in the United States would likely resemble the lazy “S” shape.  Students often expect that spraying insecticides or using various killing devices (such as bug zappers) will make a significant impact in a pest population. As noted in Module 36.8, many pesticides kill both pests and their predators. Furthermore, most pest populations are r-selective and capable of recovering quickly, perhaps more quickly than their predators. Such considerations provide a classic illustration of the complexities inherent in biological systems and the unexpected consequences of change. Teaching Tips  The sustainable management of the Alaskan fisheries is a good lesson in responsible resource management. Consider learning more about the management of halibut populations around Alaska as a positive example. The “Monterey Bay Aquarium Seafood Watch” addresses pacific halibut at Active Lecture Tips  See the Activity Calculations to Show How Deer Growth Is Unchecked in the Suburbs on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Yield (thousands of metric tons)
Figure 36.8 900 800 700 600 Yield (thousands of metric tons) 500 400 300 200 100 Figure 36.8 Collapse of northern cod fishery off Newfoundland 1960 1970 1980 1990 2000 Data from Stock Assessment of Northern (2J3KL) Cod, Science Advisory Report 2011/041, Fisheries and Oceans Canada (2011).

The Human Population

36.9 The human population continues to increase, but the growth rate is slowing
grew rapidly during the 20th century and currently stands at about 7 billion. An imbalance between births and deaths is the cause of population growth (or decline). The human population is expected to continue increasing for at least the next several decades. Student Misconceptions and Concerns  Some students may not understand the impact of delayed reproduction on population growth. Working through the following example in class might help. Refer back to the text example of exponential growth in a population of bacteria (Module 36.4). What if one population reproduced every 20 minutes and another population reproduced every 40 minutes? Clearly, the 20-minute cycle would increase the population faster. Teaching Tips  The U.S. government’s Census Bureau sponsors a U.S. and World Population Clock at There are many interesting aspects of human populations at this site. If your facilities and technology permit, this would be a great screen to have up before class starts!

Total population (in billions) Annual increase (in millions)
Figure 36.9a 100 10 Population increase 80 8 Total population (in billions) Annual increase (in millions) 60 6 40 4 Total population size 20 2 Figure 36.9a Five centuries of human population growth, projected to 2050 Year Adapted from The World at Six Billion, United Nations Publications (1999).

The demographic transition is a shift from zero population growth, in which birth rates and death rates are high but roughly equal, to zero population growth, characterized by low but roughly equal birth and death rates. Figure 36.9B shows the demographic transition of Mexico, which is projected to approach zero population growth with low birth and death rates in the next few decades. Student Misconceptions and Concerns  Some students may not understand the impact of delayed reproduction on population growth. Working through the following example in class might help. Refer back to the text example of exponential growth in a population of bacteria (Module 36.4). What if one population reproduced every 20 minutes and another population reproduced every 40 minutes? Clearly, the 20-minute cycle would increase the population faster. Teaching Tips  The U.S. government’s Census Bureau sponsors a U.S. and World Population Clock at There are many interesting aspects of human populations at this site. If your facilities and technology permit, this would be a great screen to have up before class starts!

Birth or death rate per 1,000 population
Figure 36.9b 50 40 Rate of increase 30 Birth or death rate per 1,000 population 20 Birth rate Death rate 10 Figure 36.9b Demographic transition in Mexico Year Adapted from Transitions in World Population, Population Bulletin 59: 1 (2004).

In the developing world death rates have dropped, but high birth rates persist, and these populations are growing rapidly. Student Misconceptions and Concerns  Some students may not understand the impact of delayed reproduction on population growth. Working through the following example in class might help. Refer back to the text example of exponential growth in a population of bacteria (Module 36.4). What if one population reproduced every 20 minutes and another population reproduced every 40 minutes? Clearly, the 20-minute cycle would increase the population faster. Teaching Tips  The U.S. government’s Census Bureau sponsors a U.S. and World Population Clock at There are many interesting aspects of human populations at this site. If your facilities and technology permit, this would be a great screen to have up before class starts!

Table 36.9 Table 36.9 Population changes in 2012 (estimated)

The age structure of a population is the number of individuals in different age-groups and affects the future growth of the population. Student Misconceptions and Concerns  Some students may not understand the impact of delayed reproduction on population growth. Working through the following example in class might help. Refer back to the text example of exponential growth in a population of bacteria (Module 36.4). What if one population reproduced every 20 minutes and another population reproduced every 40 minutes? Clearly, the 20-minute cycle would increase the population faster. Teaching Tips  The U.S. government’s Census Bureau sponsors a U.S. and World Population Clock at There are many interesting aspects of human populations at this site. If your facilities and technology permit, this would be a great screen to have up before class starts!

The fertility rate is the average number of children produced by a woman over her lifetime. Population momentum is the continued growth that occurs despite reduction of the fertility rate to replacement level and is a result of girls in the 0–14 age-group of a previously expanding population reaching their childbearing years. Student Misconceptions and Concerns  Some students may not understand the impact of delayed reproduction on population growth. Working through the following example in class might help. Refer back to the text example of exponential growth in a population of bacteria (Module 36.4). What if one population reproduced every 20 minutes and another population reproduced every 40 minutes? Clearly, the 20-minute cycle would increase the population faster. Teaching Tips  The U.S. government’s Census Bureau sponsors a U.S. and World Population Clock at There are many interesting aspects of human populations at this site. If your facilities and technology permit, this would be a great screen to have up before class starts!

Figure 36.9c-0 Population momentum in Mexico
80+ 1989 2012 2035 75–79 70–74 65–69 Male Female Male Female Male Female 60–64 55–59 50–54 Age 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 Figure 36.9c-0 Population momentum in Mexico Population in millions Estimated population in millions Projected population in millions Total population size = 83,366,836 Total population size = 114,975,406 Total population size = 139,457,070 Adapted from International Data Base, U.S. Census Bureau (2013).

Population in millions Total population size = 83,366,836
Figure 36.9c-1 80+ 1989 75–79 70–74 Male Female 65–69 60–64 55–59 50–54 45–49 Age 40–44 35–39 30–34 25–29 20–24 15–19 10–14 Figure 36.9c-1 Population momentum in Mexico (part 1) 5–9 0–4 Population in millions Total population size = 83,366,836 Adapted from International Data Base, U.S. Census Bureau (2013).

Estimated population in millions Total population size = 114,975,406
Figure 36.9c-2 80+ 2012 75–79 70–74 Male Female 65–69 60–64 55–59 50–54 45–49 Age 40–44 35–39 30–34 25–29 20–24 15–19 10–14 Figure 36.9c-2 Population momentum in Mexico (part 2) 5–9 0–4 Estimated population in millions Total population size = 114,975,406 Adapted from International Data Base, U.S. Census Bureau (2013).

Projected population in millions Total population size = 139,457,070
Figure 36.9c-3 80+ 2035 75–79 70–74 Male Female 65–69 60–64 55–59 50–54 45–49 Age 40–44 35–39 30–34 25–29 20–24 15–19 10–14 Figure 36.9c-3 Population momentum in Mexico (part 3) 5–9 0–4 Projected population in millions Total population size = 139,457,070 Adapted from International Data Base, U.S. Census Bureau (2013).

36.10 CONNECTION: Age structures reveal social and economic trends
Age-structure diagrams reveal a population’s growth trends and social conditions. For instance, an expanding population has an increasing need for schools, employment, and infrastructure, and a large elderly population requires that extensive resources be allotted to health care. Student Misconceptions and Concerns  Some students may not understand the impact of delayed reproduction on population growth. Working through the following example in class might help. Refer back to the text example of exponential growth in a population of bacteria (Module 36.4). What if one population reproduced every 20 minutes and another population reproduced every 40 minutes? Clearly, the 20-minute cycle would increase the population faster. Teaching Tips  Module provides a wonderful opportunity to discuss the social impact of human population changes in the United States. As noted in Module 36.10, Medicare and Social Security will be increasingly impacted as the U.S. population ages. You might want to discuss the occupational outlook for professions that will address the needs of the growing elderly population, and the opportunity to invest in companies that will capitalize on these changes.

Figure 1989 2012 2035 Birth years Male Female Birth years Male Female Birth years Male Female 85+ before 1905 before 1928 before 1951 80–84 1905–1909 1928–32 1951–55 75–79 1910–14 1933–37 1956–60 70–74 1915–19 1938–42 1961–65 65–69 1920–24 1943–47 1966–70 60–64 1925–29 1948–52 1971–75 55–59 1930–34 1953–57 1976–80 Age 50–54 1935–39 1958–62 1981–85 45–49 1940–44 1963–67 1986–90 40–44 1945–49 1968–72 1991–95 35–39 1950–54 1973–77 1996–2000 30–34 1955–59 1978–82 2001–05 25–29 1960–64 1983–87 2006–10 20–24 1965–69 1988–92 2011–15 15–19 1970–74 1993–97 2016–20 10–14 1975–79 1998–2002 2021–25 5–9 1980–84 2003–2007 2026–30 0–4 1985–89 2008–2012 2031–35 Figure Age structures for the United States in 1989, 2012 (estimated), and 2035 (projected) Population in millions Estimated population in millions Projected population in millions Total population size = 246,819,230 Total population size = 313,847,465 Total population size = 389,531,156 Data from International Data Base, U.S. Census Bureau website, (2013).

Population in millions Total population size = 246,819,230
Figure 1989 Birth years Male Female 85+ before 1905 80–84 1905–1909 75–79 1910–14 70–74 1915–19 65–69 1920–24 60–64 1925–29 55–59 1930–34 50–54 1935–39 Age 45–49 1940–44 40–44 1945–49 35–39 1950–54 30–34 1955–59 25–29 1960–64 20–24 1965–69 15–19 1970–74 Figure Age structure for the United States in 1989 (part 1) 10–14 1975–79 5–9 1980–84 0–4 1985–89 Population in millions Total population size = 246,819,230 Data from International Data Base, U.S. Census Bureau website, (2013).

Estimated population in millions Total population size = 313,847,465
Figure 2012 Birth years Male Female 85+ before 1928 80–84 1928–32 75–79 1933–37 70–74 1938–42 65–69 1943–47 60–64 1948–52 55–59 1953–57 50–54 1958–62 Age 45–49 1963–67 40–44 1968–72 35–39 1973–77 30–34 1978–82 25–29 1983–87 20–24 1988–92 15–19 1993–97 Figure Age structure for the United States in 2012 (estimated) (part 2) 10–14 1998–2002 5–9 2003–2007 0–4 2008–2012 Estimated population in millions Total population size = 313,847,465 Data from International Data Base, U.S. Census Bureau website, (2013).

Projected population in millions Total population size = 389,531,156
Figure 2035 Birth years Male Female 85+ before 1951 80–84 1951–55 75–79 1956–60 70–74 1961–65 65–69 1966–70 60–64 1971–75 55–59 1976–80 50–54 1981–85 Age 45–49 1986–90 40–44 1991–95 35–39 1996–2000 30–34 2001–05 25–29 2006–10 20–24 2011–15 15–19 2016–20 Figure Age structure for the United States in 2035 (projected) (part 3) 10–14 2021–25 5–9 2026–30 0–4 2031–35 Projected population in millions Total population size = 389,531,156 Data from International Data Base, U.S. Census Bureau website, (2013).

36.11 CONNECTION: An ecological footprint is a measure of resource consumption
The U.S. Census Bureau projects a global population of 8 billion people within the next 20 years and 9.5 billion by the mid-21st century. Do we have sufficient resources to sustain 8 or 9 billion people? To accommodate all the people expected to live on our planet by 2025, the world will have to double food production. Student Misconceptions and Concerns  Some students may not understand the impact of delayed reproduction on population growth. Working through the following example in class might help. Refer back to the text example of exponential growth in a population of bacteria (Module 36.4). What if one population reproduced every 20 minutes and another population reproduced every 40 minutes? Clearly, the 20-minute cycle would increase the population faster.  Students are often frustrated by the long list of environmental problems caused by humans, and students may begin to feel helpless as coverage of the issues goes on. You might consider directing them to specific websites for basic suggestions on what they can do to make a difference. A Google search of “what you can do environment” should yield many potentially good sites. Teaching Tips  Module notes that the United States has an ecological footprint greater than the land area of the United States. Consider asking your class to explain how this is possible and what this means to other countries. The authors further note that the world’s richest countries, with 15% of the global population, account for 36% of humanity’s total footprint. Active Lecture Tips  See the Activity The Carbon Footprint on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

An ecological footprint is an estimate of the amount of land required to provide the raw materials an individual or a nation consumes, including food, fuel, and housing. Student Misconceptions and Concerns  Some students may not understand the impact of delayed reproduction on population growth. Working through the following example in class might help. Refer back to the text example of exponential growth in a population of bacteria (Module 36.4). What if one population reproduced every 20 minutes and another population reproduced every 40 minutes? Clearly, the 20-minute cycle would increase the population faster.  Students are often frustrated by the long list of environmental problems caused by humans, and students may begin to feel helpless as coverage of the issues goes on. You might consider directing them to specific websites for basic suggestions on what they can do to make a difference. A Google search of “what you can do environment” should yield many potentially good sites. Teaching Tips  Module notes that the United States has an ecological footprint greater than the land area of the United States. Consider asking your class to explain how this is possible and what this means to other countries. The authors further note that the world’s richest countries, with 15% of the global population, account for 36% of humanity’s total footprint. Active Lecture Tips  See the Activity The Carbon Footprint on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Comparing our demand for resources with Earth’s capacity to renew these resources, or biocapacity, gives us a broad view of the sustainability of human activities. When the total area of ecologically productive land on Earth is divided by the global population, we each have a share of about 1.8 global hectares (1 hectare, or ha, = 2.47 acres; a global hectare, or gha, is a hectare with world-average ability to produce resources and absorb wastes). Student Misconceptions and Concerns  Some students may not understand the impact of delayed reproduction on population growth. Working through the following example in class might help. Refer back to the text example of exponential growth in a population of bacteria (Module 36.4). What if one population reproduced every 20 minutes and another population reproduced every 40 minutes? Clearly, the 20-minute cycle would increase the population faster.  Students are often frustrated by the long list of environmental problems caused by humans, and students may begin to feel helpless as coverage of the issues goes on. You might consider directing them to specific websites for basic suggestions on what they can do to make a difference. A Google search of “what you can do environment” should yield many potentially good sites. Teaching Tips  Module notes that the United States has an ecological footprint greater than the land area of the United States. Consider asking your class to explain how this is possible and what this means to other countries. The authors further note that the world’s richest countries, with 15% of the global population, account for 36% of humanity’s total footprint. Active Lecture Tips  See the Activity The Carbon Footprint on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Figure compares the ecological footprints of several countries to the world average footprint (orange line) and Earth’s biocapacity. Student Misconceptions and Concerns  Some students may not understand the impact of delayed reproduction on population growth. Working through the following example in class might help. Refer back to the text example of exponential growth in a population of bacteria (Module 36.4). What if one population reproduced every 20 minutes and another population reproduced every 40 minutes? Clearly, the 20-minute cycle would increase the population faster.  Students are often frustrated by the long list of environmental problems caused by humans, and students may begin to feel helpless as coverage of the issues goes on. You might consider directing them to specific websites for basic suggestions on what they can do to make a difference. A Google search of “what you can do environment” should yield many potentially good sites. Teaching Tips  Module notes that the United States has an ecological footprint greater than the land area of the United States. Consider asking your class to explain how this is possible and what this means to other countries. The authors further note that the world’s richest countries, with 15% of the global population, account for 36% of humanity’s total footprint. Active Lecture Tips  See the Activity The Carbon Footprint on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

Ecological Footprint (number of Earths)
Figure 36.11a 1.5 Key Built-up land Carbon Footprint 1.0 World biocapacity Ecological Footprint (number of Earths) 0.5 Figure 36.11a Humanity’s ecological footprint, 1961–2007 0.0 Year Data from B Ewing et al., The Ecological Footprint Atlas, Oakland: Global Footprint Network (2010).

Ecological Footprint (global hectares per person)
Figure 36.11b 8 7 6 5 Ecological Footprint (global hectares per person) 4 World average 3 2 Earth’s biocapacity 1 Figure 36.11b Ecological footprints of several countries Haiti Mexico China India Australia Argentina Colombia South Africa Uzbekistan Afghanistan United States United Kingdom Adapted from Living Planet Report 2012: Biodiversity, Biocapacity, and Better Choices, World Wildlife Fund (2012).

Figure 36.11c Figure 36.11c American consumers shopping for electronics

You should now be able to
Define a population and population ecology. Define population density and describe different types of dispersion patterns. Explain how life tables are used to track mortality and survivorship in populations. Compare Type I, Type II, and Type III survivorship curves. Describe and compare the exponential and logistic population growth models, illustrating both with examples.

Explain the concept of carrying capacity. Describe the factors that regulate growth in natural populations. Define boom-and-bust cycles, explain why they occur, and provide examples. Explain how life history traits vary with environmental conditions and with population density. Compare r-selection and K-selection and indicate examples of each.

Describe the major challenges inherent in managing populations. Explain how the structure of the world’s human population has changed and continues to change. Explain how the age structure of a population can be used to predict changes in population size and social conditions. Explain the concept of an ecological footprint. Describe the uneven reliance upon natural resources in the world.

Few large offspring, low mortality until old age
Figure 36.UN01 Few large offspring, low mortality until old age I Percentage of survivors Many small offspring, high mortality II III Figure 36.UN01 Reviewing the concepts, 36.3 Percentage of maximum life span

Population in millions Estimated population in millions
Figure 36.UN02 80+ 1989 2012 75–79 70–74 Male Female Male Female 65–69 60–64 55–59 50–54 Age 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 Figure 36.UN02 Reviewing the concepts, 36.10 5–9 0–4 Population in millions Estimated population in millions Total population size = 83,366,836 Total population size = 114,975,406

Figure 36.UN03 (K − N) G = rN K Figure 36.UN03 Connecting the concepts, question 1

I II III IV Birth or death rate Time Figure 36.UN04
Figure 36.UN04 Connecting the concepts, question 2 Time

Figure 36.UN05 Figure 36.UN05 Testing your knowledge, question 12

Introduction Has parental care evolved by natural selection?

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