Energy Passes Through, Matter Goes Around

A million units of sunlight, and what is left at the top

Producers capture about one percent of the sunlight that reaches them, and each trophic level after that keeps about a tenth of what the one below it had. Step up the levels and watch the number collapse — then ask the question the pyramid picture never answers, which is where the missing energy actually went.

The missing ninety percent, itemised

This is the part that decides whether a student understands the topic. The energy that does not reach the next level is not stored somewhere else in the ecosystem. Every route below ends in the same place.

Turn the sun up and down

Change how much energy enters at the bottom and count how many trophic levels the ecosystem can still support. One level always goes first, and it is worth working out which before you look.

One ecosystem, two quantities, forty steps

The same model tracks both at once. Carbon moves between four reservoirs every step according to fixed rules; energy arrives from the sun, and a fraction of it is respired away as heat each step. At step twenty the sun goes off. Watch what each quantity does — and watch the total.

Where the carbon actually is

The total never moves. The distribution moves constantly. That is what a cycle looks like from the inside, and it is why “the amount of carbon is constant” and “carbon is being recycled” are the same statement rather than opposite ones.

Four cycles, and what makes each one different

Getting energy into the system, and moving it around

Autotrophs put energy into the ecosystem; heterotrophs move it between levels and eventually out. The distinction is about where the energy comes from, not about whether the organism is a plant.

Two ways to run a body

And what the balance sheet decides

Learning check

Answer these in your own words before you open the answers.

1. Explain the difference between saying that energy flows through an ecosystem and that matter cycles within it.
2. Explain why only about ten percent of the energy at one trophic level reaches the next.
3. A student says the missing ninety percent is “still in the ecosystem somewhere”. Explain what is wrong with that.
4. Explain why food chains rarely have more than four or five levels.
5. Describe what a biogeochemical cycle is, and state what every one of them demonstrates.
6. Describe the carbon cycle at the highest level of organization, naming its four processes.
7. Describe the nitrogen cycle, and explain why its largest reservoir is also the least useful one.
8. Explain what makes the phosphorus cycle different from the other three.
9. Describe how autotrophs capture energy, naming both kinds.
10. Distinguish between an endotherm and an ectotherm in terms of energy.
11. Explain what a net gain and a net loss of energy each do to an organism.
12. Explain how a fall in energy availability affects the trophic levels of an ecosystem, and which is affected first.