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Biology
Ecology Food Webs
How Energy Moves Through an Ecosystem
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A food web shows how organisms in an ecosystem are connected by feeding relationships. Energy begins with sunlight, is captured by producers such as grasses, algae, and trees, and then moves to animals that eat them. These connections matter because they help explain population changes, ecosystem stability, and why the loss of one species can affect many others. In a meadow, pond, and forest edge, many food chains overlap to form one larger network of energy flow.
Key Facts
- Photosynthesis captures light energy: 6CO2 + 6H2O + light energy = C6H12O6 + 6O2
- A food chain is one path of energy flow, while a food web is many connected food chains.
- Producers form the first trophic level because they make chemical energy from sunlight.
- Only about 10% of energy is transferred from one trophic level to the next.
- Energy flow can be estimated as energy at next level = energy at previous level x 0.10
- Matter cycles through ecosystems, but energy flows one way and is eventually lost as heat.
Vocabulary
- Producer
- A producer is an organism, such as a plant or algae, that makes its own food using sunlight or chemical energy.
- Consumer
- A consumer is an organism that gets energy by eating other organisms.
- Decomposer
- A decomposer is an organism, such as a fungus or bacterium, that breaks down dead organisms and wastes.
- Trophic Level
- A trophic level is a feeding position in an ecosystem, such as producer, primary consumer, or secondary consumer.
- Food Web
- A food web is a diagram showing many feeding relationships and energy pathways in an ecosystem.
Common Mistakes to Avoid
- Thinking arrows point to what an organism eats. Arrows in a food web point in the direction energy moves, from the food to the eater.
- Assuming all energy is passed to the next trophic level. Most energy is used for life processes or lost as heat, so only about 10% usually transfers upward.
- Forgetting decomposers in the food web. Decomposers are essential because they recycle nutrients from dead organisms and wastes back into the environment.
- Treating a food web as a simple straight line. Real ecosystems contain many overlapping food chains, so one organism can affect several pathways.
Practice Questions
- 1 A meadow has 20,000 kJ of energy stored in grasses. Using the 10% rule, how much energy is available to grasshoppers, and how much is available to frogs that eat the grasshoppers?
- 2 In a pond food chain, algae contain 50,000 kJ of energy. Zooplankton eat the algae, small fish eat the zooplankton, and herons eat the small fish. Estimate the energy available to the herons.
- 3 A pesticide greatly reduces the insect population in a meadow-pond ecosystem. Explain how this could affect producers, insect-eating birds, frogs, and decomposers in the food web.