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Food-web energy transfer practice
Energy flows through ecosystems while matter cycles. A food-web arrow usually points from a resource to the organism that consumes it, but always check the diagram’s legend.
By ExamCat · Original study lesson · Published September 24, 2026
Understand the method
Only part of the energy available at one trophic level becomes production at the next. Transfer efficiency varies; a ten-percent assumption is a classroom model, not a universal law for every species or ecosystem.
Worked example, step by step
In a simplified chain, producers capture 20,000 energy units and each transfer has an assumed efficiency of 10%. Estimate energy available to primary and secondary consumers.
- Primary consumers receive 20,000 × 0.10 = 2,000 units under the model.
- Secondary consumers receive 2,000 × 0.10 = 200 units.
- Two transfers multiply the original amount by 0.10², not by 0.20.
- The remaining energy is not destroyed; it is used in metabolism, dissipated as heat or remains in material not transferred along this chain.
A mistake to watch for
Do not claim that energy cycles back to producers in the same way nutrients can. Ecosystems require continuing energy inputs, and a food web can contain multiple feeding pathways.
Try it yourself: three questions
Write an answer before opening the explanation. For calculations, keep your units and intermediate steps; for concepts, state the reason for your choice.
1. With 15% transfer from 1,000 units, how much reaches the next level?
Show answer and explanation
150 units. Use the efficiency provided instead of automatically assuming ten percent.
2. Why can an omnivore occupy more than one trophic position?
Show answer and explanation
It eats resources from different levels, so a single linear-chain label may not describe all of its feeding relationships.
3. Does fewer predators always mean more producers?
Show answer and explanation
Not necessarily. Indirect interactions and alternative pathways can change the outcome; inspect the specific web before predicting a cascade.
Build your next practice session
Mark the first step where your reasoning differed from the explanation. Turn that step into a question, answer it without looking, then change one condition in the example and explain what would change. A correct guess is a reason to revisit the method.
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Further reading
OpenStax Biology 2e provides textbook background. The worked scenario and practice questions on this page were written independently for ExamCat.
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