This lesson is a continuation of the learning that took place during the Backyard Biodiversity lesson. It is recommended to complete that lesson, or a similar investigation, prior to teaching this lesson.
Begin by prompting students to consider the interactions present in an ecosystem. Have a class brainstorm framed around the question: How do living things interact with each other in an ecosystem? (i.e. compete for resources, predator/prey, symbiotic relationship, etc.) List these interactions and relationships on the board or chart paper. Encourage students to consider some of the living things they observed in their local ecosystems, and how they relate to one another. As a class, create a list of different trophic levels: consumers (tertiary, secondary, primary), producers, and decomposers. Ask students to identify some examples of organisms for each trophic level.
Next, lead the class in a discussion around the questions: What is energy? Where do living things get energy to live? Looking at our list of trophic levels, where does each organism get its food? Guide students to conclude that living things get their energy from the food that they eat or make. Chemical energy is stored in the molecular bonds of all organisms. When one organism eats another - such as a squirrel eating a berry - some of the energy stored in the bonds of the food is transferred to the organism eating it. We show the energy transfer in a diagram through the use of an arrow. The direction of the arrow is the direction of the energy transfer:
Next, lead the class in a discussion around the questions: What is matter? How does matter move through an ecosystem? (.e. when a primary consumer eats a producer, the matter in the producer is transferred to the primary consumer. When the squirrel eats the berry, the squirrel is ingesting the matter that makes up the berry.) We can also show the transfer of matter in a diagram through the use of an arrow. The direction of the arrow shows how matter is moving from one organism to another. Tell students they should use two different colors or patterns when drawing the arrows, to be able to distinguish between energy and matter flow.
Tell students that they will now be creating a non-traditional food web model demonstrating the transfer of matter and energy within the ecosystem they observed. Students should incorporate a number of different organisms and interactions in their food web model, but they do not need to include all of the living things they observed in the previous lesson. (If you noticed that students did not observe a significant number of different species at different trophic levels, provide students with other organisms in your local area, or have them research them ahead of time.) Let students know that this will be their first draft of their model, and we will continue to discuss these interactions, and build their web. Provide students with several Post-It notes (or index cards and tape), as well as a blank piece of paper. Instruct students to write each organism on a separate Post-it/index card, and arrange the cards on their paper (this will allow them to more easily make changes to their model). When arranging the cards, students should think about how the organisms interact with each other in the ecosystem (i.e. they can put the squirrel and berry close to each other since they know the squirrel eats the berry). Students will then use a pencil to draw arrows that represent the matter and energy transfers between organisms in their system. As students work on their initial model, gauge students’ understanding by having them explain what is happening in their model (i.e. Why are they drawing arrows between two organisms? What is the relationship between two organisms that are connected by an arrow?). Be aware of any misconceptions you observe; some may be best addressed one-on-one, while more widespread gaps of knowledge will inform your class discussion in the remainder of the lesson.
Next, bring students back together as a class and have students discuss their models. Ask students to consider and discuss any limitations of their models. Tell students that we know energy is never created or destroyed; it is simply converted from one form to another. Do our models show the complete path of energy in an ecosystem? If not, what is it missing? It is likely that students did not consider the input (sunlight) of energy into their ecosystem, since they were focusing on the organisms that they had observed. It is also likely that students did not include decomposers in their model, as many of these species live in the soil, or are microscopic and not easily observed. Some students may have also neglected to include dead organisms in their model. Below are some sample guiding questions to help students fill in gaps in their models:
- Sunlight (input of energy): Some basic living things I see in everyone’s food webs are plants. Where do plants get their energy from? (Plants produce their own food through photosynthesis. They need sunlight (light energy) for photosynthesis to take place). Students should understand that the sun provides the input of energy for an ecosystem.
- Decaying matter (dead organisms) & decomposers: We know all living things eventually die. Did anyone include dead organisms in their model? Where does their energy go? Let’s say a squirrel dies, what will happen to it? (Decomposers, such as bacteria, fungi and insects, break down the decaying organism for food (energy).) What role do decomposers play in the cycling of matter in an ecosystem? (Decomposers break down decaying matter into its essential nutrients. These nutrients are transferred to the soil, and are then used by plants to grow. Decomposers are key in recycling matter between non-living and living components of the ecosystem.)
If students are not familiar with decomposers, take some time to explore this concept with them by watching a video or reading an article. As they watch or read, instruct students to take note of different types of decomposers. Afterwards, lead the class in a discussion about the different types of decomposers they discovered, and how they break down dead and decaying matter.
Discuss with students how at every trophic level, organisms use some of the energy they obtained from their food to carry out the necessary processes of life (growth, reproduction, respiration, etc.). A lot of the energy is released into the surroundings at every level. Ask students if they can think of ways that energy is released out into the ecosystem at every level. (Energy is lost through waste/feces. Energy is released as heat; heat is a form of energy that organisms cannot use.) Ask students to think about energy as flowing through a system. Ask them what the inputs and outputs of energy are in their system. (Sunlight is the input and energy released in waste or as heat is the outputs.)
Next, tell students that they will be revising their models to incorporate any missing components such as sunlight, dead organisms, and decomposers. They should also include soil and nutrients on the side to help tie in the non-living components of matter transfer and cycling. You should tell students that normally only organisms are components of a traditional food web (and in that case only one set of arrows is sufficient to illustrate both matter and energy transfer). However, they are creating a modified food web model so they can tie in the non-living components and show the inputs and outputs of energy and matter distinctively in the system. As students work, the teacher should walk around and check for student understanding: Explain what is happening in your model. Show me one path that energy may follow through the ecosystem. Show me one cycle that matter may follow through the ecosystem, including both living and non-living components.
Tell students that they have been referring to energy as flowing through the system and matter as cycling through the system. Ask them to look at their models to think about what the difference between energy flow and matter cycling is. (Students should understand that energy enters and exits the system, is transferred within certain components of the system, but that it never cycles completely in the system. Once that energy is released as heat, we do not see any arrows or connections bringing the energy back to other components in the system. Students should understand that matter cycles within the ecosystem – decomposers have a key role in the cycling, because they cycle atoms between the living and non-living components of an ecosystem.)
Finally, students will use their food web to make a prediction about possible impacts on an ecosystem. Ask students: What are some things that might make an organism population decrease? i.e. What might kill off a lot of the bird population? (Illness/virus, more predators, hunting, cutting down trees/habitat destruction, etc.) Remind students to include the concept of energy in their answers.
Tell students that they will be looking at their food web, and asking themselves: What if one of the populations of living things dramatically decreased in population? How would the ecosystem be affected over time? Remind them to think about the transfer of energy and how that would be impacted in the ecosystem. Instruct students to choose one organism that has at least two others connected to it. They will be imagining that this population has been dramatically reduced. Students will then use their model to predict what impact this will have on the ecosystem over time. It may be helpful to have students discuss this in pairs or small groups in order to broaden their ideas. Tell students that once they have thought of at least 2-3 impacts, they will then write out a prediction of what will happen to their ecosystem, including how the flow of energy will be affected. With their written prediction, students should include evidence from their models, observations, readings, and videos, and they should describe the reasoning that connects their evidence to their prediction.
Once students have formulated their predictions, have students perform a Gallery Walk around the room: half of the students will present their predictions, while the other half mingle, and engage in discussions with the students presenting their predictions. After a few minutes, students will switch roles.
Wrap up the lesson by having a few students share their predictions with the class.
You may wish to provide some or all of your students with a word bank to create or revise their food web models. This would contain vocabulary words you want to emphasize, such as producer, consumer, symbiotic relationship, etc.
- Formative: Students’ understanding will be assessed by their answers to questions, both during class discussions, as well as one-on-one. Students’ initial food web model will also assess students’ prior knowledge. Teacher should look for misconceptions to address in students’ work, and focus the instruction and revisions phase on correcting these gaps in understanding.
- Summative: Students’ understanding of food webs and using models to make predictions will be assessed by their final, revised food web and their prediction.
Students can research real examples of human-caused impacts on the biodiversity in ecosystems, and then predict how these changes may impact the system as a whole by examining the energy transfers. Students can then create an action plan for how to reduce human impacts, and maintain an ecosystem’s natural balance.