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Chapter 36 Population Ecology Lecture by Brian R. Shmaefsky

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**POPULATION STRUCTURE AND DYNAMICS**

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

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**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.

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**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.

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**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.

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**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.

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**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.

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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.

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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.

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**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

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**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.

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**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.

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**Rabbits Time (months) Population size (N) 1 2 3 4 5 6 7 8 9 10 11 12**

1 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.

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**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

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**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

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**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.

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**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.

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**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.

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**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.

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**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.

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**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.

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**per 1,000 population Birth or death rate**

50 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

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