Energy flows through an ecosystem in one direction: from sunlight to producers, then to consumers, and out as heat. It never returns to the producers. Matter is what cycles.
This lesson sits in ecological evidence and population relationships. For the chapter view, see explaining energy flow.
What cycles and what does not?
| Item | Cycles? | Why |
|---|---|---|
| Carbon, nitrogen, water | Yes | Decomposers and other processes return them to the environment |
| Energy | No | It leaves as heat and cannot be reused by producers |
Hold this table in mind. Most wrong answers mix the two columns.
Worked example: a food chain with losses
The numbers are invented and use 10% as a simple assumption. Grass stores 20 000 kJ. A grasshopper population eats grass and gains 2 000 kJ. A frog population eating the grasshoppers gains 200 kJ.
Transfer from grass to grasshopper: 2 000 ÷ 20 000 × 100 = 10%. From grasshopper to frog: 200 ÷ 2 000 × 100 = 10%.
The other 18 000 kJ at the first step went to heat from respiration, to parts not eaten and to waste. The grasshoppers, for example, did not use every blade of grass.
How do you write the explanation?
A full-mark answer names the loss and the direction. “Only about 10% of the energy is transferred to the next level. The rest is lost as heat through respiration, as uneaten parts and as waste, so energy flows one way and is not recycled.”
Notice the answer never says energy returns. The decomposers feed on dead matter and release nutrients, while their own respiration releases heat.
The mistake that costs marks
The common slip is “decomposers return energy to the soil for the plants to use”. Plants cannot use heat, and they get energy only from light.
A second slip is to say energy is “used up” or “destroyed”. Energy is transformed, mostly to heat, and is not destroyed.
Check yourself
Producers store 20 000 kJ and the primary consumers gain 2 600 kJ. Calculate the percentage transferred and name two places the rest went.
Answer
2 600 ÷ 20 000 × 100 = 13%.
The rest went to heat lost in respiration and to parts of the producers that were not eaten (or passed out as waste). Any two of these earn the mark.
What to study next
Go on to comparing two food-web changes. The biological data interpretation trainer has more energy tables.
For a teacher to check your wording, see online one-to-one Biology tuition.