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0118_0132_bi_c07_te 3/10/06 6:57 PM Section 5–2 Page 124 5–2 Limits to Growth BI 6.c. Students know how fluctuations in population size in an ecosystem are determined by the relative rates of birth, immigration, emigration, and death. 1 FOCUS N Objectives 5.2.1 Identify factors that limit population growth. 5.2.2 Differentiate between density-dependent and density-independent limiting factors. Vocabulary Preview To prepare students for the new Vocabulary terms density-dependent and density-independent, ask them to recall the definition of population density that they learned in Section 5–1. (The number of individuals per unit of area) Key Concept • What factors limit population growth? Vocabulary limiting factor density-dependent limiting factor predator-prey relationship density-independent limiting factor ow that you know a few things about population growth, think again about the sea otter example in the beginning of the previous section. When a sea otter population declines, something has changed the relationship between the birthrate and the death rate, or between the rates of immigration and emigration. For instance, in part of the sea otter’s range, the death rate of sea otters is increasing because killer whales are eating the otters. Predation by killer whales creates a situation that reduces the growth of the sea otter population. Limiting Factors Reading Strategy: Predicting Before you read, preview the diagram below. Predict how each factor might limit the growth of a population. As you read, note whether your predictions were correct. Recall from Chapter 3 that the primary productivity of an ecosystem can be reduced when there is an insufficient supply of a particular nutrient. Ecologists call such substances limiting nutrients. A limiting nutrient is an example of a more general ecological concept: a limiting factor. In the context of populations, a limiting factor is a factor that causes population growth to decrease. Some of the limiting factors that can affect a population are shown in Figure 5–5. Reading Strategy Suggest that students record their predictions in writing, leaving space below each one to note whether the prediction was correct or incorrect. In the case of incorrect predictions, have students correctly note how those limiting factors affect a population’s growth. 왔 Figure 5–5 Many different factors can limit population growth. Some of these factors are shown below. Inferring How might each of these factors increase the death rate in a population? 2 INSTRUCT Limiting Factors Use Visuals Figure 5–5 For each of the limiting factors shown in the diagram, have students suggest examples that are already familiar to them—for example, weeds and crop plants competing for light, space, and nutrients in a vegetable garden; the predator-prey relationship of a toad eating a moth; and so on. As students cite examples, have a volunteer list them on a large sheet of paper. Save the list for use again later in Evaluate Understanding, page 127. SECTION RESOURCES Print: Tim 124 Chapter 5 r • Teaching Resources, Lesson Plan 5–2, Adapted Section Summary 5–2, Adapted ve Worksheets 5–2, Section SummarySa5–2, e Worksheets 5–2, Section Review 5–2, Enrichment • Reading and Study Workbook A, Section 5–2 • Adapted Reading and Study Workbook B, Section 5–2 • Issues and Decision Making, Issues and Decisions 48 Technology: • iText, Section 5–2 • Transparencies Plus, Section 5–2 0118_0132_bi_c07_te 3/10/06 6:57 PM Page 125 Density-Dependent Factors 왘 Figure 5– 6 The panda is one of the most critically endangered species in the world today. Populations are declining, in large part because pandas depend on bamboo for food, which grows only in certain forests because of habitat destruction. Inferring How might the panda population be saved? A resource base that is limited can also affect the long-term survival of a species. As shown in Figure 5–6, pandas depend on bamboo for food. Bamboo grows in certain kinds of temperate forests in China. Since the time that these forests have been cleared for timber and farmland, panda populations have fallen dramatically and have become isolated in small pockets of remaining forest. Density-Dependent Factors A limiting factor that depends on population size is called a density-dependent limiting factor. Density-dependent factors become limiting only when the population density—the number of organisms per unit area—reaches a certain level. These factors operate most strongly when a population is large and dense. They do not affect small, scattered populations as greatly. Density-dependent limiting factors include competition, predation, parasitism, and disease. Competition When populations become crowded, organisms compete with one another for food, water, space, sunlight, and other essentials. For example, puffins must compete for limited nesting sites. Competition among members of the same species is a density-dependent limiting factor. The more individuals living in an area, the sooner they use up the available resources. Likewise, the fewer the number of individuals, the more resources are available to them and the less they must compete with one another. Competition can also occur between members of different species. This type of competition is a major force behind evolutionary change. When two species compete for the same resources, both species are under pressure to change in ways that decrease their competition. Over time, the species may evolve to occupy separate niches. That is because, as you may recall, no two species can occupy the same niche in the same place at the same time. How does competition affect growth? Materials bean seeds, 2 paper cups, potting soil Procedure 1. Label two paper cups 3 and 15. Use a pencil to make several holes in the bottom of each paper cup. Fill each paper cup two-thirds full with potting soil. Plant 3 bean seeds in cup 3, and plant 15 bean seeds in cup 15. 2. Water both cups so that the soil is moist but not wet. Put them in a location that receives bright indirect light. Water the cups equally as needed. 3. Count the seedlings every other day for 2 weeks. CAUTION: Wash your hands with soap and warm water before leaving the lab. Objective Students will be able to determine that crowding is a limiting factor in plant growth. Skill Focus Observing Materials bean seeds, 2 paper cups, potting soil Time 15 minutes for initial setup, followed by observation and recording for 2 weeks Strategy Remind students that the only variable that should be different between the two cups is the number of seeds planted. Ask: What variables should you keep the same in both cups? (The depth at which the seeds are planted, the amount of water the cups are given, the amount of sunlight the cups receive, temperature) Expected Outcome The uncrowded seedlings will thrive. The crowded seedlings will show limited growth, and some may die. Analyze and Conclude The seedlings in cup 15 will be smaller and less robust than those in cup 3, and some may die. Analyze and Conclude Observing What differences did you observe between the two cups? What is a density-dependent limiting factor? Answers to . . . A factor that limits population growth only when the population’s density reaches a certain level UNIVERSAL ACCESS Less Proficient Readers Have students make a compare/contrast table of the limiting factors discussed in the section. Column heads might include Factor, Definition, and Example. In the factor column, students should use subheads to divide the limiting factors into density-dependent and densityindependent factors. English Language Learners Have students pronounce the word dependent, and then discuss common usages of the word. Explain that the prefix inmeans “not,” and thus the word independent means “not dependent.” This section introduces factors that do and do not depend upon the density of a population. Advanced Learners Encourage interested students to further research the relationship between predator and prey on Isle Royale, as mentioned on page 126. These populations of wolves and moose have been studied extensively, and students should readily find relevant resources. Have them report their research to the class. Figure 5–5 Each of the factors that limit population growth could cause deaths of individuals. Accept all reasonable responses. Figure 5– 6 The panda population might be saved if the habitat on which they depend—bamboo in certain forests—is prevented from being destroyed or is restored in areas that had been cleared. Populations 125 3/10/06 6:57 PM Page 126 Wolf and Moose Populations on Isle Royale 5–2 (continued) Figure 5–7 Make sure students understand that two separate sets of data are plotted on the graph: the blue line represents the numbers of wolves labeled on the graph’s left vertical axis, and the red line represents the numbers of moose labeled on the right vertical axis. Number of Wolves Use Visuals 60 2400 50 2000 40 1600 30 1200 20 800 10 400 0 1955 0 1960 1965 1970 Make Connections Mathematics Present the following math problems: In order to survive, a 50-kilogram wolf needs to eat about 2700 kilograms of moose per year. The average mass of a moose is about 385 kilograms; males have more mass, females less. Ask: How many “average” moose does a wolf need to eat each year? (About 7) If there are 8 wolves in a pack, how many moose does the pack need to eat each year? (About 56) 왖 Figure 5–7 The relationship between moose and wolves on Isle Royale illustrates how predation can affect population growth. In this example, the moose population was also affected by changes in food supply, and the wolf population was also affected by disease. Interpreting Graphics How are the increases and decreases in the moose population related to the changes in the wolf population? 1985 1990 1995 Wolves Predation Populations in nature are often controlled by predation. The regulation of a population by predation takes place within a predator-prey relationship, one of the best-known mechanisms of population control. The relationships between sea otters and sea urchins and between sea otters and killer whales are examples of predator-prey interactions that affect population growth. A well-documented example of a predator-prey relationship is the interaction between wolves and moose on Isle Royale, an island in Lake Superior. The graph in Figure 5–7 shows how periodic increases in the moose population—the prey—on Isle Royale are quickly followed by increases in the wolf population—the predators. As the wolves prey on the moose, the moose population falls. The decline in the moose population is followed, sooner or later, by a decline in the wolf population because there is less for the wolves to feed upon. A decline in the wolf population means that the moose have fewer enemies, so the moose population rises again. This cycle of predator and prey populations can be repeated indefinitely. growth of a population. Parasitic organisms range in size from microscopic, disease-causing bacteria to tapeworms 30 centimeters or more in length. These organisms are similar to predators in many ways. Like predators, parasites take nourishment at the expense of their hosts, often weakening them and causing disease or death. The wasp cocoons in Figure 5–8, for example, can weaken or kill many caterpillars. Build Science Skills Chapter 5 1980 Parasitism and Disease Parasites can also limit the Density-Independent Factors 126 1975 Year Moose For: Population Dynamics activity Visit: PHSchool.com Web Code: cbe-2059 Students can interact with the art online. Applying Concepts Ask: Does the graph in Figure 5–7 show a crash in either population? (Yes; the wolf population from 1980 to 1982 and the moose population from 1995 to 1996) Read the caption and then ask: What combination of density-dependent and density-independent factors may have caused the crash in the wolf population? (Decline in the moose population; unusually deep winter snows could have made it difficult for the wolves to hunt; parasites or disease could have weakened or killed the wolves.) Number of Moose 0118_0132_bi_c07_te 126 Chapter 5 왗 Figure 5–8 This larval sphinx moth has been attacked by a parasitic wasp. The wasp inserted its eggs beneath the moth’s skin. After hatching, the wasp larvae fed on their host internally until they appeared as white cocoons on its back. Predicting How might the wasp larvae affect the sphinx moth population? TEACHER TO TEACHER Use a short video clip of a forest fire, such as the National Park Service’s video of fires in Yellowstone Park. Then, read one or two actual news articles that describe the fire and its effects. Divide the class into groups of three or four, and assign each group the task of developing a management plan to study the ecological damage caused by the fire and to help reestablish plant and animal populations in the burned area. Students should research the following information: the types of populations living in the area before and after the fire; the initial and current size of each population; resources now available to the populations and resources no longer available; and possible steps to help reintroduce populations. Give each group an opportunity to explain its plan to the class. —Brenda Waldon Biology Teacher Clayton County Public Schools Morrow, Georgia 0118_0132_bi_c07_te 3/10/06 6:58 PM Page 127 3 ASSESS Density-Independent Factors Evaluate Understanding Density-independent limiting factors affect all populations in similar ways, regardless of the population size. Unusual weather, natural disasters, seasonal cycles, and certain human activities—such as damming rivers and clear-cutting forests—are all examples of density-independent limiting factors. In response to such factors, many species show a characteristic crash in population size. After the crash, the population may soon build up again, or it may stay low for some time. For some species, storms or hurricanes can nearly extinguish a population. For example, thrips, aphids, and other insects that feed on plant buds and leaves might be washed out by a heavy rainstorm. Extremes of cold or hot weather also can take their toll on a population, regardless of the population’s density. A severe winter frost, for example, can kill giant saguaro cactuses in the Arizona desert. In some areas, periodic droughts can affect entire populations of vegetation, as shown in Figure 5–9. Such events can, in turn, affect the populations of consumers within the food web. Environments are always changing, and most populations can adapt to a certain amount of change. Populations often grow and shrink in response to such changes. Major upsets in an ecosystem, however, can lead to long-term declines in certain populations. Human activities have caused some of these major upsets, as you will soon read. Display the list of examples that the class created at the beginning of this section (Use Visuals, page 124). Call on students at random to identify each example as density-dependent or density-independent. Reteach Have students work in groups of three, with each student responsible for writing a brief description of how one of the three density-dependent limiting factors discussed in the text can limit a population’s growth. Let the group members share their descriptions and offer corrections and improvements. 왘 Figure 5–9 A drought can result in the abrupt decrease of a population, regardless of its size. Droughts and other natural disasters are density-independent limiting factors. 5–2 Section Assessment 1. Key Concept List three density-dependent factors and three density-independent factors that can limit the growth of a population. 2. What is the relationship between competition and population size? 3. If an entire lynx population disappears, what is likely to happen to the hare population on which it preys? 4. Identify how a limited resource can affect the size of a population. Give an example that illustrates this situation. 5. Critical Thinking Applying Concepts Give an example of a density-independent limiting factor that has affected a human population. Describe how this factor changed the human population. Interdependence in Nature Study the factors that limit population growth as shown in Figure 5–5. Classify each factor as either biotic or abiotic. Refer to the information on biotic and abiotic factors in Section 4–2. Density-dependent limiting factors, such as competition, predation, parasitism, and disease, can be classified as biotic factors. Density-independent limiting factors, such as drought and other climate extremes as well as human disturbance to ecosystems, can be classified as abiotic factors. Some students may argue that human disturbance should be considered a biotic factor for the reason that humans are organisms interacting with other organisms. At the same time, however, human disturbances such as building roads, filling wetlands, or clearing forests cause large-scale changes in the physical environment that should be considered abiotic factors. If your class subscribes to the iText, use it to review the Key Concepts in Section 5–2. 5–2 Section Assessment 1. Density-dependent: competition, predation, parasitism and disease; density-independent: unusual weather, natural disasters, seasonal cycles, human activities 2. When populations become larger and more crowded, organisms must compete with one another for food, water, space, sunlight, and other essential resources. 3. The hare population would probably undergo explosive growth. 4. Accept all reasonable responses. Students might mention any of the densityindependent factors as limiting a resource. A limited resource limits the size of a population. 5. Accept all reasonable responses. Sample answer: A prolonged drought, with its associated crop loss, could cause deaths, financial hardship, and emigration to other countries. Answers to . . . Figure 5–7 As the moose population increased, the wolf population increased. Decreases in the moose population were followed by decreases in the wolf population. Figure 5–8 Predation by wasp larvae would reduce the growth rate of the sphinx moth population. Populations 127 0118_0132_bi_c07_te 3/10/06 6:58 PM Page 128 Does the Gray Wolf Population Need Protection? Encourage students to research current nature publications to gather more detailed information on both sides of this issue. If students live in an area where ranchers or farmers suffer losses due to wolf predation, suggest that they interview affected people and research local newspaper articles to learn more. You may wish to let students work in several small groups, with some groups representing ranchers and others representing conservationists and other stakeholders. Give the groups time to discuss their points of view and their reasoning. Then, hold a mock meeting in which each side presents its view to the “government official.” (You may want to role-play the official yourself.) Research and Decide 1. Students’ lists will vary and may include researched information as well as information included in this feature. 2. Accept opinions on both sides of the issue so long as students defend their choices with reasonable explanations. Students can research gray wolf protection on the site developed by authors Ken Miller and Joe Levine. W olves were once widely distributed around the world, occupying almost every habitat except tropical jungles. Today, however, wolves occupy only a fraction of their former range. In 1973, the Endangered Species Act was passed by the U.S. Congress to protect declining populations of gray wolves from becoming extinct. At the time, there were only about 400 wolves in the lower 48 states. By 2002, the population had swelled to an estimated 4000 individuals scattered mostly throughout the Rocky Mountains and Great Lakes areas. Classifying the status of animals is a judgment call. In some cases, the judgment is easy. For instance, the California condor population now includes only a few remaining members and is clearly in great danger. With other species, such as the gray wolf, the situation is much more complex. How should the gray wolf be classified—and therefore managed—in the United States? The Viewpoints Ranchers are concerned that, at the current growth rate, wolves will encroach on their livestock. Many feel strongly that landowners should have the right to protect themselves from potential losses. The protection of wolves currently costs the U.S. government over $200,000 per year. If the wolves could be legally hunted and trapped, the money that would be saved could be used to help protect other, more endangered species. Research and Decide 1. Analyzing the Viewpoints To make an informed decision, learn more about this issue by consulting library or Internet resources. List the pros and cons of each option as they relate to both humans and wolves. Consider the different perspectives of landowners, conservationists, and other interested groups. 2. Forming Your Opinion Decide whether the federal government should change the status of the gray wolf. Write a persuasive statement to support your decision. Keep the Endangered Classification People who want to keep the gray wolf’s status as an endangered species say that most of its former habitat in the 48 contiguous states is unsuitable because of human encroachment. Proponents of this view cite the fact that only after gray wolves were given protection under the Endangered Species Act did the wolf population in the United States begin to increase. There is concern that persecution by people and loss of habitat will confine gray wolves to more remote areas, or reduce their habitat even further, unless federal protection continues. Reclassify the Wolf and Remove Federal Protection Opponents of the endangered species classification counter that in states like Minnesota, the gray wolf population is growing at a rate of 4 to 5 percent each year. These people are confident that, because the populations are increasing at a healthy rate, the wolves no longer need federal protection. For: Links from the authors Visit: PHSchool.com Web Code: cbe-2052 BACKGROUND The gray wolf profiled The gray wolf is a subspecies of Canis lupus and a member of the dog family, Canidae. Adult gray wolves are 1.5–1.8 meters long and stand 66–81 centimeters at the shoulder. Adult males average 31.8–45.4 kilograms; adult females average 24.9–38.6 kilograms. Wolves have long legs, and their bodies are suited for traveling great distances. Wolves prey on everything from large ungulates, such as moose and elk, to small 128 Chapter 5 rodents, such as field mice. Although wolves were once common in what is now the United States, populations of gray wolves are now found only in Alaska, Upper Michigan, Wisconsin, Minnesota, Wyoming, Montana, Idaho, and Washington State. Their decline was a result of loss of habitat, loss of prey, and hunting by humans. Bounties on wolves were once common. Wolf recovery in the United States is primarily the result of the Endangered Species Act.