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How does energy move through an ecosystem, and why is so little of it left at the top of a food chain?

Interpret food chains, food webs, and energy pyramids to explain how energy flows through an ecosystem and is lost at each trophic level (TEKS Biology, Reporting Category 5; energy and matter; using mathematics).

A TEKS-level answer on energy flow for the Texas STAAR Biology EOC: food chains and webs, producers and consumers, trophic levels, the energy pyramid and the 10 percent rule, and why energy is lost at each level.

Generated by Claude Opus 4.812 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this topic is asking
  2. Food chains and food webs
  3. Energy flows one way and is lost
  4. Producers, consumers, and decomposers
  5. Why energy must keep entering
  6. Try this

What this topic is asking

The Biology TEKS ask you to interpret food chains, food webs, and energy pyramids and explain how energy flows through an ecosystem and is lost at each level. For STAAR Reporting Category 5 you need to read these diagrams, use the about 10 percent rule to estimate energy (a using mathematics task), and explain why energy decreases up the levels. This is an energy and matter topic.

Food chains and food webs

Each step is a trophic (feeding) level: producers at the base (plants, which make their own food by photosynthesis), then primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), and so on. Decomposers (bacteria and fungi) break down dead organisms at every level. The arrows in a food chain point in the direction the energy flows, from the organism eaten to the organism that eats it.

Energy flows one way and is lost

This loss explains two things STAAR tests:

  • An energy pyramid (which shows the energy at each level) gets smaller toward the top, because less energy is available higher up.
  • Food chains are short (rarely more than four or five links), because after a few levels there is too little energy left to support another.

The "about 10 percent" figure lets you estimate energy down a pyramid:

10 000 kJ→1 000 kJ→100 kJ→10 kJ10\,000 \text{ kJ} \rightarrow 1\,000 \text{ kJ} \rightarrow 100 \text{ kJ} \rightarrow 10 \text{ kJ}

across four levels, with each level holding roughly a tenth of the one below.

Producers, consumers, and decomposers

The flow of energy starts with producers, which capture light energy (see photosynthesis). Consumers get their energy by eating other organisms. Decomposers release the energy and nutrients in dead matter, returning nutrients to the ecosystem. Because producers are the entry point of energy, removing them collapses the whole web, while changes higher up ripple through the connected chains, an idea food-web questions test by asking what happens if one species is removed.

Why energy must keep entering

Because energy is lost as heat at every transfer and cannot be reused, an ecosystem needs a continuous input of energy (sunlight). This contrasts with matter, which cycles and is reused (see the cycling of matter). Energy flows one way; matter cycles. This is the central idea of the topic and a frequent exam point.

Try this

Q1. Explain why an energy pyramid gets smaller toward the top. [2]

  • Cue. Only about 10 percent of the energy passes to each next level; the rest is used in life processes and lost as heat, so less energy is available higher up.

Q2. A food chain's producers store 5 0005\,000 kJ. Estimate the energy available to the primary consumers. [1]

  • Cue. About 10 percent, so about 500500 kJ.

Exam-style practice questions

Practice questions written in the style of TEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

STAAR Biology (2023 released style)1 marksIn a food chain, the arrows point in which direction? (A) From the organism that is eaten to the organism that eats it (the direction energy flows). (B) From the predator to the prey. (C) In both directions equally. (D) From consumers to producers.
Show worked answer →

A 1-point multiple-choice item on reading a food chain.

The correct answer is A. Arrows in a food chain point in the direction energy flows, from the organism eaten to the organism that eats it (from producer to consumer). B and D reverse this, and C is wrong because energy flows one way.

Follow the arrows the way the energy goes: from the eaten to the eater.

STAAR Biology (2024 SCR style)2 marksAn energy pyramid shows producers storing 10000 kJ of energy, with about 10 percent passing to each next level. Estimate the energy available to the primary consumers, and explain why each level has less energy than the one below it. Support your answer.
Show worked answer →

A 2-point short constructed response using mathematics and energy and matter.

Full credit (2 points): about 10 percent of 10 00010\,000 kJ passes on, so the primary consumers have about 1 0001\,000 kJ. Each level has less energy because most of the energy at a level is used for the organisms' own life processes (respiration, movement) and lost as heat, and not all of each organism is eaten or digested, so only about a tenth is passed on.

Partial credit (1 point): the correct estimate (1 0001\,000 kJ) or the energy-loss explanation, but not both. The science is scored.

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