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pre-AP Bio Chapter 3 part 3
- 1. Energy Flow inEcosystems
- 2. Learning Objectives Trace the flow of energy through living systems. Explain how ecological pyramids show the flow of energy through trophic levels.
- 3.
- 4. Decomposers and Detritivores Decomposer Primary producer
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- 6. Food Webs andDisturbance
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- 8. Pyramid of Energy First-level consumer Primary producer Third-level consumer Second-level consumer Heat Heat Heat Heat 0.1% 1% 10% 100%
- 9. Pyramids of Biomassand Numbers
- 10.
Editor's Notes
- #2 Read the lesson title aloud to students.
- #3 Click to reveal each of the learning objectives. Ask: Where do you get the energy you need? Answer: from food Have students name some of the foods they eat, focusing on the foods that contain beef and/or chicken Ask: Where do cows and chickens get the energy they need? Sample answer: from the grass and grains they eat Ask: Where do the grasses get the energy they need? Answer: from the sun
- #4 Ask: What is the source of energy for algae? Answer: the sun Ask: What are organisms that use light energy called? Answer: primary producers Ask: The flagfish eats the algae. What is this type of organism called? Answer: herbivore Ask: The other organisms on this chart are labeled with blue circles. What type of organisms are they? Answer: carnivores Click to reveal the labels. Explain that energy flows through an ecosystem in a one-way stream, from primary producers to various consumers; this is called a food chain. A food chain is a series of steps in which organisms transfer energy by eating and being eaten. Food chains can vary in length. For example, in a prairie ecosystem, an herbivore, such as a grazing antelope, feeds on a primary producer, such as grass. A carnivore, such as a coyote, in turn feeds on the antelope. In this simple food chain, the carnivore is only two steps removed from the primary producer. In the longer food chain shown on the slide, the top carnivore is four steps removed from the primary producer.
- #5 Ask students to describe the food sources of decomposers and detritivores. Answer: Both of these feed on dead organic material. Ask students to identify which machine in the image might represent a decomposer. Click to reveal the answer. Explain that decomposition releases nutrients that can be used by primary producers. Click to reveal the primary producer. Ask: What do the individual blocks in this analogy represent? Answer: nutrients Ask: What would happen if decomposers were absent from an ecosystem? Answer: Nutrients would not be released from dead and decaying matter, so there might not be enough nutrients to support the primary producers in an ecosystem.
- #6 Explain that in most ecosystems, feeding relationships are much more complicated than those described in a simple chain. One reason for this is that many animals eat more than one kind of food and are eaten by more than one type of consumer. This is an expansion of the earlier food chain that outlined the flow of energy from algae, to flagfish, to largemouth bass, to anhinga, to alligator. Click to reveal in turn the arrows from algae to flagfish, flagfish to largemouth bass, largemouth bass to anhinga, and anhinga to alligator. Ask students to trace different pathways from the algae to the alligator. Sample answer: algae shrimp killifish pig frog alligator Ask: What primary producers are shown in this food web? Click to reveal the answer. Answer: plants, leaves, seeds, fruits, and algae Click to highlight the vulture. Ask: What can you infer about vultures from this food web? Click to reveal the answer. Answer: They eat animals that are already dead. Explain that each path through the food web is a food chain. So a food web is a network that contains all the food chains in an ecosystem. Realize, however, that this is a highly simplified representation of this food web, in which many species have been left out. Explain that food chains and food webs are types of models. Remind students that modeling is one of three methods used in ecological studies. Ask: Why might scientists study feeding relationships using a model, such as a food chain or food web? Sample answer: Feeding relationships may be difficult to observe, and it may be unethical to experiment with them. Ask: What type of predictions could be made using a food chain or food web? Sample answer: A food web could be used to predict what would happen if a certain population in an ecosystem increased or decreased in size.
- #7 Explain that food webs are complex, so it is often difficult to predict exactly how they will respond to any particular kind of environmental change. Ecologists use these models to ask questions about changes in feeding relationships following a disturbance. Click to highlight detritus, bacteria, and associated fungi. Ask: What if an oil spill caused a serious decline in the number of the bacteria and fungi that break down detritus? Sample answer: Nutrients would be recycled more slowly and the organisms that consume these organisms would have less food. Ask: What effect do you think that might have on populations of shrimp? Answer: Shrimp populations would decline because they feed on these bacteria. Ask: How about the effects on killifish and catfish? Answer: They would also decline because of decreased food. Ask: How might alligators and egrets change their feeding behavior? Answer: They would likely feed more on other organisms instead. Ask: How might changes in carnivore behavior affect other species on which they feed? Answer: This would cause a decrease in the number of those other species. Point out that all of these effects came about from the change in population of a single part of the food web.
- #8 Ask students to identify the primary producer in this food web. Click to highlight the answer Answer: algae Click to highlight krill. Explain that all animals in this food web depend directly or indirectly on these organisms. Krill are one example of a diverse group of small, swimming animals, called zooplankton, that feed on marine algae. Ask: What do ecologists mean when they say that killer whales indirectly depend on krill for survival? Answer: Krill are the only herbivores in this food web; without krill, the animals the killer whale eats would not survive. In recent years, krill populations have dropped substantially. Over that same period, a large amount of sea ice around Antarctica has melted. With less sea ice remaining, there are fewer of the algae that grow beneath the ice. Ask: What do you predict what will happen if the krill population decreases? Answer: A drop in the krill population can cause drops in the populations of all other members of the food web. Ask: How might an increase in the herring population affect the emperor penguin population? Answer: Emperor penguins eat herring. An increase in the emperor penguin’s food source would likely lead to an increase in the emperor penguin population. Ask: Suppose the killer whale population is significantly reduced by disease. How might this affect the leopard seal population? Answer: A decrease in the killer whale population might lead to an increase in the leopard seal population, because killer whales are the leopard seal’s only predator.
- #9 Explain that each step in a food chain or food web is called a trophic level. Primary producers make up the first trophic level. Click to reveal primary producers. Various consumers occupy other levels. Click to reveal all other consumers. Explain that one way to illustrate the trophic levels in an ecosystem is with a model called an ecological pyramid. Ecological pyramids are models that show the relative amount of energy or matter contained within each trophic level in a given food chain or food web. There are three different types of ecological pyramids: pyramids of energy, pyramids of biomass, and pyramids of numbers. Explain that only a small portion of the energy that enters any trophic level is available to organisms at the next level. This is because organisms expend much of the energy they acquire on life processes, such as respiration, movement, growth, and reproduction. Most of the remaining energy is released into the environment as heat. Click to reveal the heat labels. Pyramids of energy show the relative amount of energy available at each trophic level of a food chain or food web. On average, about 10 percent of the energy available within one trophic level is transferred to the next trophic level. To outline this decrease in energy, start with 100 percent of the energy in primary producers. Click to reveal the starting energy. Ask: How much energy is available for first-level consumers? Click to reveal the answer. Answer: 10 percent Ask: How much energy is available for second-level consumers? Click to reveal the answer. Answer: 1 percent Ask: How much energy is available for third-level consumers? Click to reveal the answer. Answer: 0.1 percent Ask: If this food chain included fourth-level consumers, what percentage of the original energy would be available to those organisms? Answer: 0.01 percent Ask students to explain why a pyramid was chosen to represent flow through trophic levels. Answer: to show the decrease in energy at each level Ask: If there are 1,000 units of energy available at the producer level of the energy pyramid, approximately how many units of energy are available to the third-level consumer? Answer: 1 unit Ask: What is the original source of the energy that flows through most ecosystems? Why must there be a continuous supply of energy into the ecosystem? Answer: Solar energy is the original source of energy in most ecosystems. There must be a continuous supply of energy because energy, unlike matter, flows one way through ecosystems. Energy that is converted to heat is lost to the environment, so more energy must enter the ecosystem to replace it. Ask: Why are there usually fewer organisms in the top levels of an energy pyramid than in the bottom levels? Answer: There are usually fewer organisms in the top levels of the pyramid because there is much less energy available.
- #10 Explain that the total amount of living tissue within a given trophic level is called its biomass (measured in grams of organic matter per unit area). The amount of biomass a given trophic level can support is determined in part by the amount of energy available. A pyramid of biomass is a model that illustrates the relative amount of living organic matter available at each trophic level in an ecosystem. Ecologists interested in the number of organisms at each trophic level use a pyramid of numbers. A pyramid of numbers is a model that shows the relative number of individual organisms at each trophic level in an ecosystem. In most ecosystems, the pyramid of numbers is similar in shape to the pyramid of biomass. The numbers of individuals on each level decrease from the level below it. In some cases, however, consumers are much smaller in size and mass than the organisms they feed upon. Thousands of insects may graze on a single tree, for example, and countless mosquitoes can feed off a few deer. In such cases, the pyramid of numbers may be turned upside down, but the pyramid of biomass usually has the normal orientation. Ask: Which pyramid shows the amount of organic matter at each trophic level? Answer: pyramid of biomass
- #11 Create word strips with names of the organisms in the food web and give each student a strip. Have the students color-code their word strips by trophic levels. Provide lengths of yarn and have students connect a piece of yarn from their organism to the organism upon which their organism would feed. Ask all students to stand up. Click to reveal the food web. Discuss what would happen within the food web if each organism were not present or if there were too many of a certain organism. Have the student(s) representing that organism sit down (or add extra students to that group). The other students should rearrange the food web accordingly.
- #12 Hand out the worksheets and have students work in pairs to complete the Venn diagram. Then have students work in larger groups to check their answers. Create a master diagram on the board and have students each add one item. Worksheet Answers: The center of the diagram should include that both of these models describe how energy flows through the ecosystem. Food webs focus on which organisms feed on which other organisms. Energy pyramids focus on the flow and loss of energy through the ecosystem.