Science · Grade 7 · Interactions within Ecosystems · Term 1

Energy Flow: Food Chains and Food Webs

Investigating how energy moves from the sun through producers, consumers, and decomposers in a food web.

• Ontario Curriculum Expectations
• MS-LS2-3

About This Topic

Energy flow in ecosystems traces how solar energy moves from producers, such as plants that capture sunlight through photosynthesis, to consumers like herbivores, carnivores, and omnivores, and finally to decomposers that break down dead matter. In Grade 7, students differentiate food chains, which show simple linear paths, from food webs, which represent interconnected relationships in real habitats. They identify trophic levels and note that only about 10 percent of energy transfers between levels due to losses as heat.

This topic aligns with Ontario's Interactions within Ecosystems unit by building skills in analyzing stability, such as the effects of removing a keystone species like sea otters, which control urchin populations and protect kelp forests. Students construct food webs for local habitats, like Ontario wetlands, fostering understanding of biodiversity and human impacts.

Active learning shines here because students manipulate physical or digital models to simulate disruptions, making invisible energy transfers and complex interconnections visible and testable through group predictions and revisions.

Key Questions

1. Analyze the impact of removing a keystone species from a food web.
2. Differentiate between a food chain and a food web using specific examples.
3. Construct a complex food web for a given habitat, identifying trophic levels.

Learning Objectives

• Construct a food web for a specific Ontario habitat, identifying producers, primary consumers, secondary consumers, tertiary consumers, and decomposers.
• Analyze the impact of removing a keystone species from a constructed food web, predicting the cascading effects on other populations.
• Compare and contrast the flow of energy in a simple food chain versus a complex food web, explaining the concept of energy loss at each trophic level.
• Explain the role of decomposers in nutrient cycling within an ecosystem, relating their function to the overall health of the food web.

Before You Start

Photosynthesis and Plant Needs

Why: Students must understand how plants create their own food using sunlight to grasp the role of producers in energy flow.

Basic Needs of Living Things

Why: Understanding that all organisms need energy and nutrients to survive is fundamental to comprehending feeding relationships.

Key Vocabulary

Producer	An organism, typically a plant or alga, that produces its own food using light, water, carbon dioxide, or other chemicals. Producers form the base of a food chain or food web.
Consumer	An organism that obtains energy by feeding on other organisms. Consumers can be herbivores (plant-eaters), carnivores (meat-eaters), or omnivores (eating both).
Trophic Level	The position an organism occupies in a food chain or food web. Each level represents a step in the transfer of energy, from producers to various levels of consumers and decomposers.
Food Web	A complex network of interconnected food chains showing the feeding relationships within an ecological community. It illustrates the flow of energy through multiple pathways.
Keystone Species	A species that has a disproportionately large effect on its environment relative to its abundance. Its removal can cause significant changes to the structure of the ecosystem.

Watch Out for These Misconceptions

Common Misconception

Food chains represent all energy flow equally at every level.

What to Teach Instead

Energy decreases by 90 percent per trophic level due to respiration and waste. Hands-on pyramid building with diminishing block sizes or candy transfers lets students quantify losses, correcting overload ideas through measurement and peer review.

Common Misconception

Food webs are just multiple separate chains.

What to Teach Instead

Webs show branching and shared links for realism. Yarn simulations reveal overlaps as groups tug strings to show multiple paths, helping students visualize complexity over linear thinking.

Common Misconception

Decomposers are outside food webs.

What to Teach Instead

Decomposers recycle nutrients back to producers. Card sorts including decomposers in cycles demonstrate closure, with discussions clarifying their role in sustaining energy flow.

Active Learning Ideas

Card Sort: Building Food Webs

Provide cards with local organisms, arrows for energy flow, and habitat labels. In small groups, students first arrange into simple food chains, then connect them into a food web, labeling trophic levels. Discuss interconnections and predict effects of species removal.

35 min·Small Groups

Yarn Web Simulation

Students stand in a circle holding cards for organisms. Toss yarn balls to represent feeding links while naming trophic roles. Remove a 'keystone' student and observe web collapse, then reconstruct and analyze stability.

25 min·Whole Class

Energy Pyramid Layers

Groups stack foam blocks or draw pyramids representing trophic levels, adding organism cutouts and calculating 10% energy rule with sample data. Compare pyramids before and after removing a producer.

40 min·Small Groups

Digital Food Web Builder

Use free online tools like Food Web Designer. Individually or in pairs, input a habitat's species, link them, and run removal scenarios to graph population changes.

30 min·Pairs

Real-World Connections

• Ecologists studying the Bay of Fundy use food web analysis to understand the impact of declining herring populations on marine mammals like right whales and seabirds.
• Conservationists working in Algonquin Provincial Park might map food webs to assess the health of the forest ecosystem and the effects of introducing or removing species like wolves or moose.
• Fisheries managers in Lake Ontario monitor the food web to ensure sustainable populations of fish, considering the impact of invasive species like zebra mussels on the food sources for native fish.

Assessment Ideas

Exit Ticket

Provide students with a list of organisms found in a specific habitat (e.g., a temperate forest). Ask them to draw a simple food chain with at least three trophic levels and identify the producer, primary consumer, and secondary consumer. Then, ask them to write one sentence explaining what would happen if the producer disappeared.

Quick Check

Display an image of a complex food web on the board. Ask students to identify one omnivore and one tertiary consumer. Then, pose a hypothetical: 'If the population of the main herbivore in this web decreased significantly, what are two other populations that would likely be affected and why?'

Discussion Prompt

Present a scenario: 'Imagine the sea otter, a keystone species in kelp forests, is removed from the ecosystem. Discuss in small groups: What other organisms rely on kelp? What might happen to the sea urchin population? How could this impact the entire kelp forest ecosystem?' Facilitate a brief whole-class share-out of key ideas.

Frequently Asked Questions

How do food chains differ from food webs in Grade 7 science?

Food chains depict straight-line energy paths, like grass to rabbit to fox, while food webs interconnect multiple chains to show real ecosystem complexity, such as foxes also eating birds that consume insects. Students use examples from Ontario habitats, like deciduous forests, to construct both and analyze stability differences. This builds toward evaluating keystone species impacts.

What is the role of a keystone species in a food web?

A keystone species disproportionately affects ecosystem structure despite low numbers, like beavers creating wetlands that support diverse life. Removing them cascades changes through trophic levels. Simulations let students test predictions, graphing biomass shifts to grasp indirect effects on energy flow.

How can active learning help teach energy flow in food webs?

Active methods like yarn webs or pyramid builds make abstract 10% transfer rule concrete as students handle materials, predict disruptions, and revise models collaboratively. These approaches boost retention by linking actions to outcomes, such as web instability from keystone removal, while discussions refine scientific explanations over passive reading.

What activities construct food webs for local habitats?

Use organism cards from Ontario ecosystems, like Great Lakes shores, for sorting into webs with trophic labels. Add extensions like human impact scenarios. These 30-40 minute group tasks align with curriculum expectations, emphasizing interconnections and energy dynamics through hands-on creation and analysis.