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36 CHAPTER 6 POPULATION BIOLOGY Chapter Objectives Describe the dynamics of population growth. Summarize the BIDE factors that increase or decrease populations. Compare and contrast the factors that regulate population growth. Identify some applications of population dynamics in conservation biology. Key Terms Boom and bust life cycle Density-dependent Density-independent Doubling time Influx Oscillation Proportion Rule of 70 Survivorship curves Viable Concept Review 6.1 Dynamics of Population Growth All the members of a species living in an area at the same time form a population. Unrestricted increase in population size is exponential growth. A graph of exponential population growth appears as a J curve. Biotic potential refers to population growth if there are no interfering factors. The Rule of 70 is used to determine the approximate doubling time of a population. Carrying capacity is the maximum number of individuals an ecosystem can support. Populations may experience oscillating cycles of population growth and decline. Some species regulate population growth according to the supply of resources. A logistic growth curve, which appears as an S, or sigmoid, illustrates population size limited by resource availability. Organisms with a high rate of reproduction followed by a high mortality rate are described as r-adapted species. Organisms with exponential growth patterns are generalists, depending upon quantity of numbers to ensure survival of some. 37 6.2 Complicating the Story: r = BIDE 6.3 Mortality is the death rate of a population. Major factors in mortality include predation, disease, accidents, and environmental influences. Growth= B+I-D-E Factors That Regulate Population Growth 6.4 Organisms that reproduce more slowly as the population approaches carrying capacity are K-adapted species. Organisms that are K-adapted produce fewer offspring, are larger in size, and live longer. Abiotic factors affect population growth independent of the population’s density. Natural disasters, weather, and climatic change over a period of time are density-independent factors. Biotic factors affect population growth in a manner that is dependent upon the density of the population. Predator-prey relationships and territoriality are biotic factors that are density-dependent. Stress and crowding affect the rate of reproduction in dense populations. Conservation Biology The minimum viable population size is the number of individuals needed for long-term survival of a population. Islands that are closer to the mainland will have larger populations and greater diversity than islands farther away. If there are only a few individuals in a population, genetic diversity will be limited—the founder effect. As individuals from a population migrate to new areas, the genetic makeup of the population changes—genetic drift. Populations that are geographically isolated from each other but have intermittent gene flow between them form metapopulations. Class Time: The Acorn outline suggests 10 to 15 percent of the class be used to study populations and demographic transition. Plan to spend 5–7 days on this chapter, emphasizing data interpretation. 38 Approach and Tips Students are not allowed to use calculators on the AP exam, so make sure they can perform all mathematical operations without this tool. Have students practice the Rule of 70 to learn the doubling time of different populations. Students need to remember to do the following for every problem: Write the equation. Write the information they are given. Place the numbers in the equation and perform the mathematical operations. Write the answer, including the appropriate label for the units. On the AP exam, points may be earned for each of these steps, even if the calculation is incorrect. Look for data and activities relating to predator-prey interactions. Project Learning Tree has several lesson plans dealing with deer populations. Table 6.1 39 Discuss the reproductive strategies of r-adapted and K-adapted species. Students should be able to provide information on at least one plant and animal of each type. Figure 6.8 Figure 6.8 illustrates the three basic types of survivorship curves. Discuss each curve using the specific organism given, or have students name a different organism that fits the pattern. Make sure students are able to give specific units of time for humans. Applications 1. Have the students plot exponential growth. Remember; do not let the students use a calculator to do this. The rule in my class is that if I find a calculator out, the student immediately loses 30 points on the activity or lab. If students do not practice doing the math without a calculator they will forget how to do it on the AP test where calculators are not allowed. Activity: Suppose you have $1,000 in the bank and it grows at 10% a year. To simplify the problem, assume that the interest is compounded only one time each year and you can ignore the cents and round to the nearest dollar. Graph (by hand) the growth of your account for 50 years and answer these 2 questions. 40 What is the doubling time? How much will be in your account after 50 years? Grading: • Use graph paper • Axes labeled • Scale and units indicated • Scale is consistent • Title the graph 2. Duckweed Lab A population count using Duckweed is an easy lab. Duckweed (Lemna major) is a small, plant that looks like a mini lily pad. It can be found in some local ponds and lakes. Duckweed can be ordered from supply catalogs or I find it by calling around to local plant nurseries that sell water garden supplies and have it taking over their ponds. These nurseries are usually happy to give you all that you can carry home for free. Some local fish supply stores will also get it in with their aquatic plant shipments and will allow you to remove it from the aquariums for them. Take large test tubes and have each group of students (2-4 in a group) pick out with forceps four individual duckweed. Duckweed reproduces both sexually and asexually and will reproduce until it completely covers the container. As it continues to reproduce, some will be forced down under the water and will die. This lab is perfect for demonstrating exponential growth and carrying capacity. Each day record the number of Duckweed found in the test tube. It will take about 2 to 3 weeks for the Duckweed to grow exponentially and then reach the carrying capacity. You can then have the students graph the data and have them answer conclusion questions. Date Days Elapsed Number of Individuals 41 Conclusion: 1. Has this population reached the carrying capacity of the test tube? 2. What are the limiting factors for population growth of Duckweed? 3. What would happen in a lake or pond if Duckweed completely covered the surface? What environmental effects might this have? 4. Can any organism exhibit exponential population growth forever? What happens to a population that is above its carrying capacity? 5. Do you believe there will be a point when the human population, which is growing exponentially, will reach the carrying capacity of the earth? Why or why not? Common Mistakes and Misconceptions Make sure that the students can solve the Rule of 70 without a calculator. The Rule of 70 is almost certain to be a question they will be asked on the AP exam. Have students solve it by giving them the growth rate and have them determine the doubling time and also by giving them doubling time and having them determine the growth rate. The formula is: 70/growth rate = doubling time Suggested Website A good website for population (though it is dealing more with human population issues which will be covered in the next chapter) is found at http://www.populationconnection.org/. This organization used to be called Zero Population Growth or ZPG but is now known as Population Connection. There are activities, free posters, and workshops, as well as information and ways to get involved. It is a good site to get facts on population and the environment, population and global warming, and how population affects water, women’s rights, and wildlife. 42 Questions 1. A female cockroach can have 80 offspring every 6 months. Calculate the population growth for five generations. What factors would limit the growth of the population? Is this population a K-adapted species or an r-adapted species? Explain why. 2. Abiotic factors tend to be density-independent. A collection of plants that is successful when receiving 10 hours of sunlight per day is placed in a location that receives only 6 hours of sunlight per day. Discuss what will happen to the population over time and explain your reasoning.