Science · 7th Grade · Inheritance and Variation · Weeks 19-27

Food Webs and Energy Flow

Students analyze the flow of energy and cycling of matter through food chains and food webs within an ecosystem.

• Common Core State Standards
• MS-LS2-3

About This Topic

A food web maps the feeding relationships among organisms in an ecosystem, showing how energy captured by producers flows to consumers at successive trophic levels. At each transfer, roughly 90% of the energy is lost as heat, which means ecosystems can support far more herbivores than carnivores and far more plants than herbivores. The MS-LS2-3 standard asks students to develop models describing the cycling of matter and flow of energy among living and nonliving parts of an ecosystem.

Keystone species have ecological effects that far exceed what their population size would suggest. The reintroduction of wolves in Yellowstone National Park triggered a trophic cascade: wolf predation altered elk behavior, allowing streamside vegetation to recover, which stabilized riverbanks and changed the physical structure of waterways. Understanding these cascading effects prepares students to evaluate real conservation decisions.

Active learning tasks that ask students to build food webs from species data -- rather than fill in pre-drawn diagrams -- and then physically remove species to trace the effects give students genuine systems-level thinking about why ecosystem disruptions are difficult to predict.

Key Questions

1. Explain how energy flows through different trophic levels in a food web.
2. Analyze the impact of removing a keystone species from a food web.
3. Predict the consequences of a disruption to the energy flow in an ecosystem.

Learning Objectives

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

Before You Start

Producers, Consumers, and Decomposers

Why: Students need to identify the basic roles of organisms in an ecosystem before analyzing complex feeding relationships.

Basic Needs of Living Organisms

Why: Understanding that organisms require energy and matter for survival is fundamental to grasping how these are transferred through food webs.

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.

Watch Out for These Misconceptions

Common Misconception

Energy is recycled in a food web, just like matter.

What to Teach Instead

Matter cycles through ecosystems via decomposition and nutrient cycling, but energy does not cycle -- it flows in one direction and is lost as heat at each transfer. A demonstration where each 'consumer' keeps only 10% of paper money and passes the rest to a 'heat bucket' makes this one-way flow physically visible.

Common Misconception

Arrows in a food web point from predator to prey, showing what eats what.

What to Teach Instead

Arrows represent the direction of energy flow, pointing FROM prey TO predator -- from what is eaten toward what eats it. Consistently reinforcing this convention while building food webs prevents persistent diagram errors that interfere with analysis.

Common Misconception

Removing a predator is beneficial because it protects prey populations.

What to Teach Instead

Removing top predators typically causes prey populations to overshoot their food supply, degrading habitat and ultimately collapsing the prey population along with many dependent species. The Yellowstone wolf case is the most well-documented US example of this counterintuitive outcome.

Active Learning Ideas

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.

45 min·Small Groups

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.

20 min·Pairs

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.

40 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.

35 min·Small Groups

Real-World Connections

• Wildlife biologists use food web analysis to manage populations and restore habitats, such as monitoring the impact of sea otters on kelp forests in the Pacific Ocean.
• Conservationists assess the risks of invasive species by predicting how they might disrupt existing food webs and cause trophic cascades in ecosystems like the Florida Everglades.
• Fisheries managers study energy flow to set sustainable catch limits, understanding that overfishing top predators can destabilize the entire marine food web.

Assessment Ideas

Quick Check

Provide students with a list of 10-15 organisms from a specific ecosystem (e.g., a temperate forest). Ask them to draw arrows showing energy flow and label at least four trophic levels. Then, ask: 'What would happen to the insect population if the bird population decreased by half?'

Discussion Prompt

Present a scenario: 'A disease drastically reduces the population of rabbits in a grassland ecosystem. What are three potential consequences for other organisms in the food web, and why?' Facilitate a class discussion where students justify their predictions based on energy flow and trophic relationships.

Exit Ticket

On an index card, have students define 'keystone species' in their own words and provide one example. Then, ask them to explain how removing this species could impact at least two other organisms in its food web.

Frequently Asked Questions

What is the difference between a food chain and a food web?

A food chain shows a single linear pathway of energy flow (grass to grasshopper to frog to hawk). A food web shows the full network of feeding relationships, with multiple overlapping chains. Food webs are more realistic because most organisms eat more than one type of food and are eaten by more than one predator.

Why does energy decrease at each trophic level?

At each step, most energy is used by the organism for its own metabolism, movement, and maintaining body temperature. Only about 10% is stored in body tissue and available to the next trophic level. This is why food chains rarely have more than four or five links and why large predators are always less abundant than their prey.

What is a keystone species and why does it matter?

A keystone species has an ecological impact far greater than its population size suggests. Its removal causes disproportionately large disruptions to the food web. Sea otters, wolves, and fig trees are well-documented examples. Identifying keystone species is important for conservation because protecting one species can stabilize an entire ecosystem.

How does active learning help students understand food webs and energy flow?

When students remove a species card from a web they built themselves and trace the cascading effects, they grasp ecosystem interdependence in a way that reading a diagram cannot replicate. The 10% energy calculation is also more memorable when applied to a realistic scenario than when treated as an abstract formula to apply on a test.