Energy Flow Through Ecosystems
Every ecosystem runs on a budget of energy, and Topic 8.2 is where you learn how that budget is spent. Producers capture energy — sunlight for photosynthesizers, chemical bonds for chemosynthesizers — and store it as the organic molecules that feed everything above them. From there the energy moves up through trophic levels: primary consumers eat producers, secondary consumers eat those, and so on. But the key move is recognizing that this is a one-way flow, not a loop. Energy enters as captured light or chemical energy, passes upward through feeding, and exits the living world as heat — it is used, degraded, and lost, never handed back to the level below.
That loss is quantified by the ~10% rule: on average only about a tenth of the energy at one trophic level is stored as biomass that the next level can eat. The other ~90% is spent on the organism's own metabolism, movement, and heat — most of it dissipating into the surroundings as it is used. This is the crucial contrast with the chemical building blocks of life: matter cycles, but energy does not. Carbon, nitrogen, and water are recycled through ecosystems over and over, while energy makes a single pass and leaves. Because so little energy survives each transfer, the pyramid narrows sharply toward the top — which is exactly why top predators are rare: there simply isn't enough energy left at the highest levels to support many large consumers.
Interactive · Energy Flow
Build a food chain and watch energy move up it one level at a time. Set the transfer efficiency and see how much reaches each trophic level, how much is lost as heat, and why the pyramid narrows so fast — energy flows one way and is never recycled, so the top of the chain is always the thinnest.
Energy Flow · Open the full sandbox →The mistakes clustered here all come from misreading that one-way flow. The first is treating energy like matter — assuming it cycles back through the ecosystem instead of leaving as heat (U8-BIO1). The second is losing track of the ~10% rule, expecting most of a level's energy to carry upward rather than only a small fraction (U8-BIO3). The third is conflating the fate of energy with the fate of matter — forgetting that matter cycles while energy does not, so nutrients are reused but the energy budget is spent once (U8-BIO5). And the last is missing why top predators are rare: not because they are hunted or unfit, but because so little energy survives to the top of the pyramid that it can only support a few of them (U8-BIO4). Every scenario in this topic asks you to keep the flow pointed one way and account for where the energy actually goes.
The work
3 ways in · any order
Lesson
Energy Flow Through Ecosystems
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Energy enters ecosystems through producers, flows one way up through trophic levels, and is lost as heat at every transfer — with only about 10% carrying to the next level. The lesson walks the core misreads: treating energy as if it cycles like matter, expecting most of it to survive each transfer, and missing why top predators are rare. It closes with a ten-scenario applet that keeps you accounting for where energy actually goes — captured, passed on, or lost as heat.
Diagnostic
10-item topic check
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Ten items on how energy moves through ecosystems — that it flows one way and is lost as heat, that only ~10% carries to the next trophic level (U8-BIO3), that matter cycles while energy does not (U8-BIO5), and that top predators are rare because so little energy reaches them (U8-BIO4). Items also probe the trap of treating energy as if it cycles back like matter (U8-BIO1). Take it cold to surface which of these are still tangled, or after the lesson to confirm the flow holds.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.