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How does energy flow through an ecosystem, and why does so little reach the top?

Analyze the flow of energy through ecosystems using food chains, food webs, and energy pyramids, including the roles of producers, consumers, and decomposers and the ten percent rule (GSE SB5.b).

A Georgia Milestones Biology EOC answer on energy flow: producers, consumers, and decomposers, trophic levels, food chains and food webs, the ten percent rule, and why energy pyramids narrow toward the top.

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  1. What this topic is asking
  2. Producers, consumers, and decomposers
  3. Food chains and food webs
  4. The ten percent rule and energy pyramids
  5. Try this

What this topic is asking

Standard SB5.b asks you to analyze the flow of energy through ecosystems. For the Georgia Milestones Biology EOC you must know the roles of producers, consumers, and decomposers, read food chains and food webs, interpret energy pyramids, and apply the ten percent rule (only about 10 percent of energy passes to the next level). A common item gives a food chain and asks you to trace energy or explain why it decreases.

Producers, consumers, and decomposers

All the energy in an ecosystem ultimately comes from the sun, captured by producers. Decomposers are easy to overlook but essential: they recycle the matter (a link to the cycling-of-matter topic) even as energy continues to flow through and out.

Food chains and food webs

A food chain is a single sequence showing who eats whom and the direction energy flows, for example grass to grasshopper to frog to snake. The arrows point in the direction energy flows (from the eaten to the eater). A food web is a more realistic picture: many food chains linked together, because most organisms eat (and are eaten by) more than one species. Food webs show why removing one species can affect many others.

Each feeding level is a trophic level: producers are the first level, primary consumers (herbivores) the second, secondary consumers the third, and so on.

The ten percent rule and energy pyramids

So energy flows one way (sun to producers to consumers, lost as heat) and is not recycled, unlike matter. The top predators have the least energy available, which is why they are usually few in number.

Try this

Q1. State the role of a producer and give an example. [2 points]

  • Cue. A producer makes its own food by photosynthesis and forms the base of the food chain; for example, grass or algae.

Q2. Explain why an energy pyramid narrows toward the top. [2 points]

  • Cue. Only about 10 percent of energy passes to each next level (about 90 percent is lost as heat), so each higher level has much less energy available.

Exam-style practice questions

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

Milestones (style)1 marksIn a food chain, about what percentage of the energy at one trophic level is passed on to the next level? (A) 100 percent (B) 50 percent (C) 10 percent (D) 1 percent
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A 1-point selected-response item on the ten percent rule.

The correct answer is C. Only about 10 percent of the energy at one trophic level is passed to the next; the other roughly 90 percent is lost, mainly as heat through cellular respiration and through movement and life processes. This ten percent rule is why energy pyramids narrow toward the top and why food chains rarely have more than four or five levels. 100 percent (A) ignores the losses, and the other values misstate the rule.

Milestones (style)2 marksA grassland food chain is grass, then grasshopper, then frog, then snake. An energy pyramid shows 10,000 units of energy at the grass. Estimate the energy available to the snake and explain the trend.
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A 2-point item applying the ten percent rule with a calculation.

Using the ten percent rule, about 10 percent passes to each next level: grass 10,000, grasshopper about 1,000, frog about 100, snake about 10 units. So the snake has roughly 10 units of energy available. The trend is a sharp decrease at each level because about 90 percent of the energy is lost (mostly as heat from respiration) and only about 10 percent is stored as new tissue for the next level to eat. Full points need the estimate (about 10 units) and the reason (about 90 percent lost at each step, so energy decreases up the pyramid).

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