1. Chapter 36
Population Ecology
Lecture by Brian R. Shmaefsky

2. POPULATION STRUCTURE AND DYNAMICS
POPULATION STRUCTURE AND DYNAMICS
Copyright © 2009 Pearson Education, Inc.

3. 36.1 Population ecology is the study of how and why populations change
Population
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
Are likely to interact with one another
Described by the
Number of individuals
Distribution of individuals
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. Use of the mark-recapture method to estimate the size of a population is discussed in the Applying the Concepts section at the end of Chapter 36. The following can serve as a demonstration of the mark-recapture method 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.
Copyright © 2009 Pearson Education, Inc.

4. 36.1 Population ecology is the study of how and why populations change
Population dynamics is the interactions between
Biotic and abiotic factors
It is the cause of variation in population sizes
A population increases through birth and immigration
Death and emigration out of an area decrease the population
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. Use of the mark-recapture method to estimate the size of a population is discussed in the Applying the Concepts section at the end of Chapter 36. The following can serve as a demonstration of the mark-recapture method 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.
Copyright © 2009 Pearson Education, Inc.

5. 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
Within a population’s geographic range, local densities may vary
The dispersion pattern of a population refers to the way individuals are spaced within their area
Clumped
Uniform
Random
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. 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.
Copyright © 2009 Pearson Education, Inc.

6. In a clumped pattern individuals are grouped in patches
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. 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.
Possibly due to:
Unequal distribution of resources
Social behavior advantage
Copyright © 2009 Pearson Education, Inc.

7. In a uniform pattern individuals are equally spaced in the environment
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. 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.
Possibly due to:
- Interactions between individuals
- Social behavior (territoriality)
Copyright © 2009 Pearson Education, Inc.

8. In a random pattern of dispersion, the individuals in a population are spaced in an unpredictable way
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. 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.
Possibly due to:
Lack of interactions
Random or even distribution of resources
Copyright © 2009 Pearson Education, Inc.

9. Life tables track survivorship over the life span of individuals in a population
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. The Centers for Disease Control provide information and life tables for people living in the United States at their website,
Copyright © 2009 Pearson Education, Inc.

10. Survivorship curves plot the proportion of individuals alive at each age
100
Type I 10
Type II
Percentage of survivors (log scale) 1
Figure 36.3 Three types of survivorship curves.
Type III
0.1 50
100
Percentage of maximum life span

11. 36.4 Idealized models predict patterns of population growth
Two models used to describe population growth:
Exponential growth model
Logistic growth model
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. 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.
Copyright © 2009 Pearson Education, Inc.

12. 36.4 Idealized models predict patterns of population growth
Exponential growth model
The rate of population increases under ideal conditions
Calculated using the equation:
G = rN
G is the growth rate of the population
N is the population size
r is the per capita rate of increase
Rabbits
Time (months)
Population size (N) 1 2 3 4 5 6 7 8 9 10 11 12 50
100
150
200
250
300
350
400
450
500
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. 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.
Copyright © 2009 Pearson Education, Inc.

13. Rabbits Time (months) Population size (N) 1 2 3 4 5 6 7 8 9 10 11 121 2 3 4 5 6 7 8 9 10 11 12 50
100
150
200
250
300
350
400
450
500
Table 36.4A Exponential Growth of Rabbits, r = 0.3.

14. Bacteria and opportunistic microorganisms follow an Exponential Growth model
Lag phase: recovery from low metabolic state, cells “gear up” their enzymes
Log phase: exponential expansion when nutrients are not limiting and conditions are right
Stationary phase: growth slows due to lower nutrients or oxygen, or buildup of waste products
Death phase: cells die more rapidly with toxic wastes or acidic pH

15. Exponential growth Sudden decline
Number of aphids
Figure 36.5B The effect of an abiotic factor (climate) on aphid population size.
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec

16. 36.4 Idealized models predict patterns of population growth
Logistic growth model
This growth model takes into account limiting factors
Limiting factors are environmental factors that restrict population growth
Formula
where K = Carrying Capacity
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. 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.
Copyright © 2009 Pearson Education, Inc.

17. 36.4 Graphs depicting exponential and logistic growth models
Time
Number of individuals (N) K
G = rN
(K – N)
Figure 36.4C Logistic growth and exponential growth compared.

18. 36.5 Multiple factors may limit population growth
Abiotic factors may reduce population size before other limiting factors become important
Weather, fires, floods
Biotic factors often play major role in limiting population size
Competition, predation, internal regulations,
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. 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 identify limiting factors for that particular population.
Copyright © 2009 Pearson Education, Inc.

19. 36.5 Multiple factors may limit population growth
Most populations fluctuate in numbers
(Song sparrow population drops suddenly with severe winter weather)
Time (years)
Number of females
1975
1980
1985
1990
1995
2000 20 40 60 80
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. 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 identify limiting factors for that particular population.
Copyright © 2009 Pearson Education, Inc.

20. 36.6 Some populations have “boom-and-bust” cycles
Lynx
Snowshoe hare
Lynx population size
(thousands)
Hare population size
Year
1850
1875
1900
1925 40 80
120
160 3 6 9
Some populations fluctuate in density with regularity
Boom-and-bust cycles
Food shortages
Predator-prey interactions
For snowshoe hare, it’s likely a combination of both these
For the BLAST Animation Population Dynamics, go to Animation and Video Files.
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. 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.
Copyright © 2009 Pearson Education, Inc.

21. 36.7 EVOLUTION CONNECTION: Evolution shapes life histories
r /K selection
r-selective traits
Small, short-lived organisms (insects, small rodents, weeds)
Where disturbances make new opportunities for rapid growth
Fastest to reproduce has advantage
K-selective traits
Larger, long-lived organisms (bear, elephants, trees)
Nutrient- or density-limited stable climate environments
Highest efficiency in resource competition has advantage
Student Misconceptions and Concerns
1. 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 50 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 and Taco Bell. 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
1. 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 both 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 themselves were faced with the task of rearing four children at once!
Copyright © 2009 Pearson Education, Inc.

22. per 1,000 population Birth or death rate50 40 30
per 1,000 population
Birth or death rate
Rate of increase (r) 20
Birth rate
Death rate 10
Figure 36.9B Demographic transition in Mexico.
1900
1925
1950
1975
2000
2025
2050
Year