Energy Flow in Ecosystems: Food Webs and Trophic Levels

Energy Flow in Ecosystems: Food Webs and Trophic Levels

Grade 7 · Science · NYSSLS MS-LS2-3 · 55 Minutes

NYS-Aligned Standard

MS-LS2-3 — Develop a model to describe the cycling of matter and flow of energy among living and nonliving parts of an ecosystem. NYS P-12 Science Learning Standards / NYSSLS (2017)

Three-Dimensional Alignment

Learning Objectives — “I Can” Statements

• I can distinguish between producers, primary consumers, secondary consumers, and decomposers.
• I can construct a food web model for a local NYS ecosystem (e.g., Hudson Valley forest).
• I can explain how energy decreases as it moves up trophic levels (10% rule).
• I can describe what happens to the ecosystem if one organism is removed (cascade effects).

Essential Question

What happens to energy as it moves through a food web — and what breaks the chain?

Materials & Prep

• Food Web Organism Cards (teacher-prepared — see organisms list below)
• Large construction paper (1 per group)
• String/yarn for food web connections
• Food Web Analysis Worksheet (1 per student)
• Energy Pyramid graphic organizer

NYS-Relevant Organisms (Hudson Valley Forest Ecosystem — original list):

• Producers: oak trees, ferns, grass, wildflowers
• Primary consumers: white-tailed deer, mice, rabbits, caterpillars
• Secondary consumers: red foxes, black bears (omnivore), owls, hawks
• Tertiary consumers: bald eagles
• Decomposers: mushrooms, bacteria, earthworms

Lesson Sequence

Hook (8 min)

Show the prompt: “A pesticide kills all the caterpillars in a forest. What happens next? And next? And next?” Students predict in writing (2 minutes), then share.

Direct Instruction (12 min)

1. Review: producer → primary consumer → secondary consumer → tertiary consumer → decomposer
2. Introduce the energy pyramid: energy decreases by ~90% at each trophic level (10% rule)
3. Model one food chain: oak tree → caterpillar → warbler → hawk
4. Expand to food WEB: add multiple connections; point out that organisms can be in multiple chains

Investigation / Model Building (18 min)

1. Groups receive organism cards and string; build a food web on construction paper
2. Each group: (a) connects organisms with string to show who eats whom, (b) adds arrows showing energy direction, (c) identifies producers, consumers by level, decomposers
3. Teacher circulates; ask: “Which direction do the arrows point — toward the eater or the eaten?”

Analysis (12 min)

Students complete the Food Web Analysis Worksheet:

1. Name 2 producers in your web
2. Trace one complete food chain from producer to tertiary consumer
3. If deer disappeared, what would happen to the fox population? Explain.
4. Where do decomposers fit in the energy cycle?

Closure (5 min)

Exit ticket: “Draw a 3-level energy pyramid for one food chain in your web. Label the amount of energy at each level (use percentages or relative amounts).”

SDI & Differentiation Block

Supports for MLLs/ELLs

Entering/Emerging (NYSESLAT Levels 1–2):

• Bilingual organism cards with pictures AND names in English and L1
• Pre-built partial food web; student adds 2–3 organisms and connections
• Sentence frame: ”___ eats ___. Energy moves from ___ to ___.”
• Allow drawing of the food web with labeled pictures

Transitioning/Expanding (NYSESLAT Levels 3–4):

• Vocabulary support: producer, consumer, decomposer, trophic level, energy pyramid
• Academic sentence frames: “If ___ were removed from this ecosystem, ___ would happen because ___.”

Supports for Students with IEPs

SDI Adaptation Dimensions: content, methodology, delivery

• Content: Provide a simplified food web with 5 organisms only (1 producer, 2 consumers, 1 apex predator, 1 decomposer); reduce analysis questions to 2
• Methodology: Use physical string on the floor (kinesthetic) to build the food web; color-code organism cards by trophic level
• Delivery: Allow verbal response to analysis questions; extended time per IEP; provide digital graphic organizer

Suggested Placement: ICT, Resource Room

Answer Key / Model Response

Sample food chain: oak tree → caterpillar → warbler → hawk → (decomposers when hawk dies) If deer disappeared: fox population would likely decrease (less prey); vegetation would increase (less grazing) Decomposers fit in the cycle by breaking down dead matter, returning nutrients to the soil for producers.

Alignment Record