Science · 7th Grade · Ecosystems: Interactions, Energy, and Dynamics · Quarter 4

Energy Flow in Ecosystems

Trace the path of energy as it moves from the sun to producers, consumers, and decomposers through food chains and complex food webs.

TL;DR:Explore the hidden connections that power life on Earth. We'll trace the incredible journey of energy as it flows from the sun to plants, animals, and back to the soil.

• Common Core State Standards
• NGSS: MS-LS2 - Ecosystems: Interactions, Energy, and Dynamics - Developing a Model to Describe the Flow of Energy

About This Topic

This topic aligns with the Next Generation Science Standards (NGSS), particularly MS-LS2-3, which focuses on developing a model to describe the cycling of matter and flow of energy in ecosystems. The core concept for students to grasp is that energy flows in one direction through an ecosystem, while matter is cycled. The journey begins with the sun, the ultimate source of energy for most life on Earth. Producers, or autotrophs, like plants and algae, capture this solar energy through photosynthesis and convert it into chemical energy in the form of glucose. This stored energy is then transferred to consumers, or heterotrophs, when they eat other organisms.

Students will explore the different levels of consumers: primary consumers (herbivores) that eat producers, secondary consumers (carnivores or omnivores) that eat primary consumers, and tertiary consumers that eat secondary consumers. The lesson should differentiate between a simple, linear food chain and a more complex, realistic food web, which shows the multiple feeding relationships within an ecosystem. Finally, the critical role of decomposers, such as bacteria and fungi, must be highlighted. They break down dead organic material from all trophic levels, returning essential nutrients to the soil for producers to use, thus completing the cycle of matter, but not the flow of energy. A key quantitative concept to introduce is the 10% rule, which states that only about 10% of the energy from one trophic level is transferred to the next, with the rest being lost primarily as heat during metabolic processes.

Key Questions

1. Explain the role of producers, consumers, and decomposers in an ecosystem's energy flow.
2. Analyze how the removal of a top predator would impact the structure of a food web.
3. Compare the efficiency of energy transfer between different trophic levels in an energy pyramid.

Learning Objectives

• Model the one-way flow of energy through producers, consumers, and decomposers in a food web.
• Differentiate between the roles of autotrophs and heterotrophs within an ecosystem.
• Explain how the amount of available energy decreases at each successive trophic level.
• Analyze and predict how a change in one population can affect other populations in a food web.
• Construct a food chain or food web to represent feeding relationships in a given ecosystem.

Key Vocabulary

Producer	An organism, such as a plant, that can make its own food using energy from the sun; also called an autotroph.
Consumer	An organism that obtains energy by feeding on other organisms; also called a heterotroph.
Decomposer	An organism, such as bacteria or fungi, that breaks down wastes and dead organisms to return nutrients to the ecosystem.
Food Chain	A series of steps in which organisms transfer energy by eating and being eaten.
Food Web	A complex model of how energy and matter move through an ecosystem, consisting of all the interconnected food chains.
Trophic Level	Each step in a food chain or food web, representing an organism's position in the sequence of energy transfers.
Energy Pyramid	A diagram that shows the amount of energy that moves from one feeding level to another in a food web, with producers at the base.

Watch Out for These Misconceptions

Common Misconception

Plants get their 'food' from the soil.

What to Teach Instead

Plants create their own food (glucose, a sugar) through photosynthesis using sunlight, water, and carbon dioxide. The soil provides essential nutrients and water, but not the energy-rich food itself.

Common Misconception

Energy is recycled in an ecosystem, just like nutrients.

What to Teach Instead

Energy flows in one direction through an ecosystem and is lost as heat at each trophic level. It is not recycled. Matter, such as carbon and nitrogen, is recycled by decomposers.

Common Misconception

Arrows in a food chain point from the eater to what it eats.

What to Teach Instead

The arrows in a food chain or food web represent the direction of energy flow. Therefore, they point from the organism being eaten to the organism that eats it.

Common Misconception

Decomposers are at the 'bottom' of the food chain and are not very important.

What to Teach Instead

Decomposers are essential to every ecosystem. They break down dead organic matter from all trophic levels, recycling nutrients back into the environment so producers can use them.

Active Learning Ideas

Simulation Game

Food Web Yarn Activity

Assign each student a role as a producer, consumer, or decomposer in a specific ecosystem. Students toss a ball of yarn to connect themselves to the organisms they eat or are eaten by, creating a physical representation of a food web.

30 min·Whole Class

Simulation Game

Energy Pyramid Construction

Using building blocks or paper cutouts, students construct a physical energy pyramid. Each level should be proportionally smaller to represent the 10% rule of energy transfer, with the largest base for producers.

45 min·Small Groups

Simulation Game

Owl Pellet Dissection Lab

Students carefully dissect sterilized owl pellets to identify and sort the bones of prey animals. They then use their findings to reconstruct the owl's food chain and make inferences about its role in the local food web.

60 min·Pairs

Real-World Connections

• Understanding biomagnification, where toxins like mercury or pesticides become more concentrated in organisms at higher trophic levels, affecting both wildlife and human health.
• Analyzing the impact of overfishing on marine ecosystems, which can cause the collapse of entire food webs and threaten the global food supply.
• Making informed consumer choices, such as supporting sustainable agriculture, which minimizes disruption to local food webs and nutrient cycles.
• The practice of composting food scraps and yard waste, which mimics the role of decomposers to create nutrient-rich soil for gardening.
• Conservation efforts for keystone species, like wolves in Yellowstone, whose presence has a cascading effect on the entire food web.

Assessment Ideas

Exit Ticket

Exit Ticket: Students draw a four-organism food chain, label each organism with its role (producer, primary consumer, etc.), and draw arrows showing the correct direction of energy flow.

Peer Assessment

Food Web Project: Students research a specific ecosystem (e.g., desert, rainforest, local pond) and create a detailed poster or digital presentation of its food web, explaining the roles of key organisms and the flow of energy.

Quick Check

Students use a rubric to review a food web they created, checking for correct arrow direction, inclusion of producers and decomposers, and at least two different trophic levels for consumers.

Frequently Asked Questions

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

A food chain shows a single, linear path of energy flow in an ecosystem (e.g., grass -> rabbit -> fox). A food web is more realistic and shows many interconnected food chains, representing the multiple feeding relationships between organisms.

Why is there less energy at the top of the energy pyramid?

At each step, or trophic level, in a food chain, about 90% of the energy is used by the organism for its own life processes (like moving, breathing, and keeping warm) or is lost as heat. Only about 10% of the energy is stored and passed on to the next level.

Are humans always at the top of the food web?

Humans are omnivores and can occupy several trophic levels. While we are often apex consumers in many food webs, we are not always at the very top. For example, in the ocean, a killer whale might be a higher-level consumer than a human who eats fish.

What would happen if all the decomposers disappeared?

Without decomposers, dead plants and animals would pile up, and the nutrients locked within them would not be returned to the soil. This would eventually cause the ecosystem to collapse, as producers would run out of the essential nutrients they need to grow.