Food Webs and Energy Flow

Active learning works for this topic because students need to visualize how energy moves through ecosystems, not just memorize terms. By building models and testing disruptions, they see firsthand why energy limits food chain length and how small changes ripple through a web.

• 7th Grade
• Science
• 4 activities
• 20 min–45 min

Learning Objectives

1. Construct a model food web illustrating energy transfer between producers, primary consumers, secondary consumers, and decomposers.
2. Analyze the impact of introducing or removing a specific organism on the stability of a given food web.
3. Predict the cascading effects on an ecosystem when a keystone species is removed.
4. Explain the 90% energy loss at each trophic level and its effect on biomass distribution within an ecosystem.
5. Compare and contrast the roles of producers, consumers, and decomposers in nutrient cycling within a food web.

Inquiry Circle

⏱ 45 min·Small Groups

Inquiry Circle: Build-a-Food-Web

Groups receive species cards with diet, habitat, and trophic level information for a specific ecosystem (grassland, coral reef, or deciduous forest). Students arrange the cards and draw arrows showing energy flow direction to construct a complete food web, then identify producers, primary and secondary consumers, and the most likely keystone species based on their web's structure.

Prepare & details

Explain how energy flows through different trophic levels in a food web.

Facilitation Tip: During Build-a-Food-Web, circulate with a red pen to correct arrow directions in real time, reinforcing that arrows show energy flow toward the eater, not the eaten.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Think-Pair-Share

⏱ 20 min·Pairs

Think-Pair-Share: The 10% Energy Rule

Present a specific food chain and ask students to calculate the energy available at each trophic level if 1,000 calories are fixed by producers. Students work individually, compare with a partner, and discuss why top predators have large territories and small populations compared to the prey species below them.

Prepare & details

Analyze the impact of removing a keystone species from a food web.

Facilitation Tip: In the 10% Energy Rule activity, have students physically count and move paper money to make the loss of energy tangible and memorable.

Setup: Standard classroom seating; students turn to a neighbor

Materials: Discussion prompt (projected or printed), Optional: recording sheet for pairs

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Stations Rotation

⏱ 40 min·Small Groups

Stations Rotation: Disrupting the Web

At three stations, students encounter a scenario where a species has been removed or dramatically reduced: overhunting of a predator, disease wiping out a primary consumer, invasive plant crowding out native producers. At each station, groups trace the ripple effects through a provided food web and predict which species increase and which decrease.

Prepare & details

Predict the consequences of a disruption to the energy flow in an ecosystem.

Facilitation Tip: At Disrupting the Web stations, provide a timer so groups rotate quickly and stay focused on the impact of each disruption within the allotted time.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Gallery Walk

⏱ 35 min·Small Groups

Gallery Walk: Trophic Cascade Case Studies

Post four documented trophic cascade examples around the room (wolves in Yellowstone, sea otters and sea urchins, sharks in Atlantic estuaries, elephants in African savannas). Groups rotate and annotate each case with the mechanism of the cascade and the unexpected species it affected, then the class compares patterns across all four cases.

Prepare & details

Explain how energy flows through different trophic levels in a food web.

Facilitation Tip: For the Gallery Walk, assign each case study a specific question to answer, such as ‘How did the wolf’s return change energy flow in Yellowstone?’ to guide analysis.

Setup: Wall space or tables arranged around room perimeter

Materials: Large paper/poster boards, Markers, Sticky notes for feedback

• Understand
• Apply
• Analyze
• Create
• Relationship Skills
• Social Awareness

Teaching This Topic

Teachers approach this topic by treating it as a system, not a list of terms. Start with concrete examples students know, like a backyard food web, then generalize rules. Avoid overloading students with vocabulary upfront; let them discover the 10% rule through data. Research shows students retain these concepts better when they create, test, and revise models than when they passively receive information.

What to Expect

Successful learning looks like students correctly drawing energy arrows from prey to predator, explaining why only 10% of energy transfers between trophic levels, and predicting ecosystem changes when key species are removed. Their models should reflect real-world relationships and energy constraints.

Watch Out for These Misconceptions

Common Misconception

During Build-a-Food-Web, watch for students who draw arrows pointing from predator to prey, indicating what eats what rather than the direction of energy flow.

What to Teach Instead

Pause the activity and ask students to trace the path of energy with their fingers, starting with grass and moving to the grasshopper. Reinforce: arrows go from the organism being eaten to the organism that eats it.

Common Misconception

During the 10% Energy Rule activity, watch for students who assume energy is recycled like matter because both are essential for life.

What to Teach Instead

Ask students to set aside 90% of their paper money in a ‘heat bucket’ after each transfer and explain why that energy is lost. Emphasize that energy flows in one direction and is not reused.

Common Misconception

During Disrupting the Web, watch for students who believe removing a top predator benefits prey by reducing predation.

What to Teach Instead

Point to the station’s graph showing prey population spikes followed by crashes. Ask students to explain why the initial spike leads to overgrazing and habitat degradation.

Assessment Ideas

Quick Check

After Build-a-Food-Web, collect student diagrams and provide feedback focusing on correct arrow directions and labeled trophic levels. Ask students to predict how a 50% decrease in the bird population would affect the insect population and justify their answer.

Discussion Prompt

During the 10% Energy Rule Think-Pair-Share, listen for students to connect the 10% loss to real-world limits on carnivore populations. After the share, ask three groups to present their reasoning and call on others to add to or challenge their ideas.

Exit Ticket

After the Gallery Walk, have students write a paragraph explaining the role of a keystone species in a trophic cascade, using evidence from at least one case study they observed.

Extensions & Scaffolding

• Challenge students to design a food web for an ecosystem with extreme energy scarcity, such as a desert, and explain how adaptations affect trophic levels.
• Scaffolding: Provide pre-printed organism cards with trophic level labels for students who struggle to classify producers and consumers independently.
• Deeper exploration: Have students research and present on a real-world trophic cascade, such as the reintroduction of sea otters in kelp forests, and connect it to the Yellowstone wolf case.

Key Vocabulary

Trophic Level	The position an organism occupies in a food chain or food web, indicating its feeding relationship and energy source.
Keystone Species	A species that has a disproportionately large effect on its environment relative to its abundance, playing a critical role in maintaining ecosystem structure.
Trophic Cascade	An ecological process that starts at the top of a food chain and tumbles down to lower levels, often triggered by the addition or removal of a top predator.
Biomass	The total mass of organisms in a given area or volume, which decreases significantly at higher trophic levels due to energy loss.
Decomposer	An organism, such as bacteria or fungi, that breaks down dead organic material, returning essential nutrients to the ecosystem.