Backyard Biodiversity: Food Web
Next Generation Science Standards:
●Science & Engineering Practice: Developing and Using Models Modeling in 6–8 builds on K–5 experiences and progresses to developing, using, and revising models to describe, test, and predict more abstract phenomena and design systems. (MS-LS2-3)
●DCI: LS2.B: Cycle of Matter and Energy Transfer in Ecosystems: Food webs are models that demonstrate how matter and energy is transferred between producers, consumers, and decomposers as the three groups interact within an ecosystem. Transfers of matter into and out of the physical environment occur at every level. Decomposers recycle nutrients from dead plant or animal matter back to the soil in terrestrial environments or to the water in aquatic environments. The atoms that make up the organisms in an ecosystem are cycled repeatedly between the living and nonliving parts of the ecosystem. (MS-LS2-3)
○Energy and Matter: The transfer of energy can be tracked as energy flows through a natural system. (MS-LS2-3)
○Systems and System Models: Systems may interact with other systems; they may have sub-systems and be a part of larger complex systems. (MS-LS1-3)
Working towards Performance Expectation MS-LS2-3: Develop a model to describe the cycling of matter and the flow of energy among living and nonliving parts of an ecosystem.
Driving Question: How does energy move within an ecosystem?
●Construct a model to explain the energy transfers within an ecosystem.
●Predict what would happen to an ecosystem if one population dramatically increased or decreased.
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.
This lesson encourages students to use scientific vocabulary words regarding interspecies relationships (i.e. consumer, competition, etc.). However, the focus of this lesson is on how energy cycles through an ecosystem, and how a change in the population of a single species can impact many aspects of the ecosystem. Thus, students should encouraged to use vocabulary words, but will be assessed on their understanding of energy flow in a food web, and interconnectedness of an ecosystem.
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? (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 ecosystem, and how they relate to one another. As a class, create a list different trophic levels: consumers (tertiary, secondary, primary), producers, 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 - most of the energy stored in the bonds of the food is transferred to the organism eating it. We show this in a diagram using an arrow to show the transfer of energy:
Berry ------> Squirrel
Tell students that they will now be creating a food web model demonstrating the flow of energy within the ecosystem they observed. Before they begin constructing their model, ask students to Think-Pair-Share to come up with a definition of a system or a cycle. If students are struggling to define these concepts, you may ask them to recall the Water Cycle or the Rock Cycle in order to apply their prior knowledge. Bring the class back together to create a working definition of a system or cycle (i.e. many different parts/components, all relate to one another; transitions through each part).
Students should incorporate a number of different organisms and interactions, but do not need to include all of the living things they observed. 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 them on their paper (this will allow them to more easily make changes to their model). They will then use a pencil to draw the energy transfers in their system. As students work on their initial model, gauge students’ understanding by having them explain what is happening in their model. 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 review their models and its limitations. Tell students that we know that energy is never created or destroyed, simply transferred or recycled. Do our models show a complete cycle of energy? If not, what is it missing? It is 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, as well as sunlight. Below are some sample guiding questions to help students fill in gaps in their models:
●Sunlight: 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 for photosynthesis to take place - light energy)
●Decaying matter & decomposers: We know all living things eventually die, plants and animals alike. 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 matter into its essential nutrients. These nutrients are transferred to the soil, and used by plants to grow.)
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.
Next, tell students that they will now be revising their models to incorporate any missing components such as sunlight, dead organisms, and decomposers. As students work, the teacher should walk around a check for student understanding: Explain what is happening in your model. Show me one path energy may follow throughout the cycle.
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.)
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? 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 has 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 an explanation of what they predict will happen to their ecosystem. Encourage students to draw a diagram or chart if it helps them to explain the changes that they predict.
Once students have formulated their predictions and explanations, have students perform a Gallery Walk around the room: half of the students will present their predictions and explanations, 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 and explanations 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, i.e. 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’ prior knowledge will also be assessed by their initial food web model. 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 explanation of 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.