Seventh Grade · Science · week 10

Energy pyramids

Explain how energy flows in one direction through an ecosystem's trophic levels and use the 10-percent rule to explain the shape of an energy pyramid.

Almost all energy in an ecosystem starts with the sun. Producers, mainly plants and algae, capture light energy and store it as chemical energy through photosynthesis. Primary consumers eat producers; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers. Decomposers break down dead organisms and waste at every level. Each feeding position is called a trophic level, and energy moves through these levels in one direction only -- it is never recycled back up the chain the way matter is.

At each step up the pyramid, only about 10 percent of the energy stored at one trophic level ends up stored in the bodies of the next level up. The other roughly 90 percent is used by the organisms themselves for moving, growing, and staying warm, and is ultimately lost as heat, or it is simply never eaten at all. That steep loss is why an energy pyramid is shaped like a pyramid: lots of energy (and usually lots of organisms) at the producer base, and very little energy left by the time you reach top predators.

📋 For the teacher
Teach it (10–15 min):
  • Build a real pyramid together with a local food chain: grass, then a plant-eating insect, then a small predator, then a larger predator, labeling roughly how the energy shrinks at each step.
  • Use money as an analogy if it helps: imagine losing 90% of your allowance every time you passed it to the next person -- it disappears fast.
  • Be precise about what "lost" means: it is not destroyed, it becomes heat and motion, following the same energy rules from earlier physical science.
  • Ask them to guess, before being told, why big fierce predators are rare -- most kids assume it is about danger, not energy math.
Talk about it:
  • Why does energy flow only one direction through an ecosystem instead of cycling?
  • What would happen to a food chain if a step in the middle disappeared?
  • Why might a vegetarian diet support more people, energy-wise, than a diet based only on top predators like large fish?

Mastery looks like: They can apply the 10-percent rule to estimate energy at a given trophic level and explain why energy pyramids narrow sharply toward the top.

Common stumbles: Assuming energy is recycled like matter; thinking the "lost" 90% simply vanishes rather than being used for life processes and released as heat.

Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
Records attendance + 15 min of Science for this learner.
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