Explain that nearly all energy in an ecosystem food web traces back to sunlight captured by producers, and describe how that energy moves and is lost between organisms.
Matter cycles through an ecosystem, but energy moves differently: it flows through in one direction and does not cycle back. Almost all of that energy starts as sunlight. Producers capture the sun energy and use it to build their own food. When a consumer eats a producer, some of that stored energy transfers into the consumer body, and the chain continues each time one organism eats another.
Not all the energy passes on, though. At each step, an organism uses a lot of its energy just to move, grow, and stay warm, and that energy leaves as heat instead of getting passed to the next eater. This is why food chains rarely have more than four or five links: there simply is not enough usable energy left after several steps.
Mastery looks like: They can trace the energy in any food chain back to sunlight and explain why energy is lost, not cycled, at each step.
Common stumbles: Thinking energy cycles back to producers the way matter does; forgetting that plants themselves use up energy before a consumer even eats them.