Producers, Consumers, and Food Webs

Essential Question

How does energy move from one organism to the next in a community?

Every living thing needs energy, and almost all of it starts as sunlight. In this lesson, you’ll sort organisms by how they get energy, follow energy through food chains and food webs, and predict what happens to a community when one population changes.

Producers and Consumers

• Producers (autotrophs) make their own food
• Consumers (heterotrophs) eat other organisms
• Decomposers break down dead matter

Group	How it gets energy	Examples
Producer	Makes its own food, usually by photosynthesis	Grass, algae, oak trees
Herbivore	Eats only producers	Rabbits, grasshoppers, deer
Carnivore	Eats only other consumers	Snakes, hawks, sharks
Omnivore	Eats producers and consumers	Bears, raccoons, humans
Decomposer	Breaks down dead matter and wastes	Bacteria, fungi

Producers, also called autotrophs, capture energy to make their own food. Most use photosynthesis, but some bacteria near deep-sea vents use chemical energy instead (chemosynthesis). Consumers, or heterotrophs, get energy by eating other organisms. Herbivores eat producers, carnivores eat other consumers, and omnivores eat both. Decomposers, such as bacteria and fungi, get energy by breaking down dead organisms and wastes.

Food Chains and Trophic Levels

• A food chain shows one path of energy
• Arrows point from food to eater
• Each feeding step is a trophic level

A food chain shows one path that energy takes through a community. Each arrow points from the organism being eaten to the organism that eats it, so the arrows show the direction energy moves. Each step is a trophic level. Producers are always the first trophic level. A herbivore that eats a producer is a primary consumer, an animal that eats a primary consumer is a secondary consumer, and so on.

Food Webs

• A food web links many food chains
• Most consumers eat more than one food
• An organism can feed at more than one level

Real communities are more tangled than a single chain. A food web shows all the feeding connections in a community. In this meadow web, the hawk eats sparrows, snakes, and rabbits, so it gets energy from several chains. Because the hawk eats a primary consumer (the rabbit) and secondary consumers (the sparrow and the snake), it is both a secondary and a tertiary consumer. The fox eats berries and animals, so it is an omnivore.

Energy Flows, Matter Cycles

• Energy enters as sunlight and leaves as heat
• Energy flows one way through a food web
• Decomposers return nutrients to the soil
• Matter is recycled again and again

Energy and matter move differently. Energy enters a community as sunlight, passes from organism to organism, and is lost as heat at every step. So energy flows one way and must be resupplied by the Sun all the time. Matter, such as carbon and nitrogen atoms, is recycled. Decomposers break down dead organisms and wastes and return nutrients to the soil and water, where producers take them up again.

Changes Ripple Through a Food Web

• Change one population and others change too
• What it eats may increase
• What eats it loses food and may decrease

Because organisms are linked, a change in one population affects others. In this sample data, a disease cut the mouse population in a field by 75%. With fewer mice eating it, the clover increased. Snakes and hawks, which depend on mice for food, declined. The graph uses a log scale so that thousands of clover plants and a handful of hawks fit on one graph.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Reading a Food Web

Use the meadow food web. (a) Name the producers. (b) Write a food chain with four organisms that ends with the hawk. (c) What trophic level is the snake?

Show the solution

1. (a) Producers: grass, clover, and berries make their own food, and no arrows point into them.
2. (b) One chain: grass → grasshopper → sparrow → hawk
3. (c) Snake: it eats mice, which eat clover and berries. Mice are primary consumers, so the snake is a secondary consumer, trophic level 3.
4. Answer: (a) Grass, clover, berries. (b) Grass → grasshopper → sparrow → hawk. (c) Secondary consumer, level 3.

Example 2: Predicting a Change

Use the meadow food web. Suppose a disease kills most of the rabbits. Predict how the grass, the fox, and the hawk populations will change, and explain each prediction.

Show the solution

1. Grass: fewer rabbits eat it, so the grass increases.
2. Fox: it loses one of its main foods, so the fox population may decrease, or foxes may eat more mice and berries.
3. Hawk: it also loses rabbits, so it may decrease, or eat more sparrows and snakes.
4. Pattern: consumers with several food sources are hurt less when one food disappears.
5. Answer: Grass increases. Foxes and hawks likely decrease and shift to their other foods.

Example 3: What Does a Fox Eat?

Scientists studied what red foxes eat in a meadow. Is the fox a herbivore, carnivore, or omnivore? What percent of its diet comes from animals? On which trophic levels does it feed?

Show the solution

1. Type: the fox eats berries (a producer) and animals, so it is an omnivore.
2. Animal part of the diet: 40% + 35% + 10% = 85%
3. Trophic levels: eating berries makes it a primary consumer (level 2). Eating rabbits, mice, and plant-eating insects makes it a secondary consumer (level 3).
4. Answer: Omnivore; 85% of its diet is animals; it feeds as a primary and a secondary consumer.

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Practice

Classify organisms by how they get energy, read a pond food web, predict population changes from data, and explain how energy and matter move. Try it on your own first, then check your work with the answer key.

• Producers, Consumers, and Food Webs Practice Worksheet (PDF)
• Producers, Consumers, and Food Webs Practice Answer Key (PDF)

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Lesson Slides

• 1.5 Producers, Consumers, and Food Webs Slides (PDF)

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Big Ideas

• Producers make food; consumers eat; decomposers recycle
• Arrows point from food to eater
• Food webs link many food chains
• Energy flows one way; matter cycles
• A change in one population affects the web

Back to the essential question: energy enters a community when producers capture sunlight, then moves to consumers through the feeding relationships shown in food chains and food webs. Arrows show energy moving from food to eater, one trophic level at a time. Energy is lost as heat at each step, while decomposers recycle matter. Next, you’ll measure how much energy is lost at each trophic level.

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