GED Science
Ecosystems & Energy Flow
Energy enters ecosystems as sunlight, gets captured by plants, and moves up through whatever eats whatever. At each step about 90% is lost as heat, which is why food chains are short and top predators are rare. Arrows in a food web always point the way the energy flows: toward the eater.
The words first
- producer
- makes its own food from sunlight — plants, algae
- consumer
- eats others: herbivores eat plants, carnivores eat animals
- decomposer
- breaks down the dead, recycling nutrients — fungi, bacteria
- trophic level
- a step in the chain: producer → herbivore → carnivore
- the 10% rule
- only about a tenth of the energy passes to the next level
The method
A CHAIN, READ CORRECTLY
grass → rabbit → fox
The arrow means "is eaten by" — it points AT the eater,
following the energy.
THE ENERGY PYRAMID (the 10% rule)
grass 10,000 units
rabbits 1,000 units (90% lost as heat)
foxes 100 units
eagles 10 units ← why top predators are few
REMOVE A SPECIES — trace BOTH directions
remove rabbits:
grass (their food) → grows unchecked, then over-grazed by others
foxes (their eater) → starve or switch prey
DECOMPOSERS close the loop: dead everything → soil nutrients → grass
Worked all the way through
The problem
In the chain grass → grasshopper → frog → hawk, the grass captures 50,000 units of energy. About how much reaches the hawks?
- Step 1 — Apply the 10% rule per step.Each arrow keeps a tenth and loses the rest.
- Step 2 — Walk the chain.Grasshoppers: 50,000 × 0.10 = 5,000. Frogs: 5,000 × 0.10 = 500. Hawks: 500 × 0.10 = 50.
- Step 3 — Read the meaning off the numbers.Three steps shrank 50,000 to 50 — a thousandfold loss, which is why the hawk population must stay small.
On the board
grass: 50,000
captured from the sun
grasshoppers: 5,000
× 0.10
frogs: 500
× 0.10
hawks: 50
× 0.10
short chains, rare predators
the 10% rule's meaning
Answer
About 50 units
A second one, in a different form
A disease wipes out the frogs. Predict two effects.
frogs' food (grasshoppers)
→ population booms, grass suffers
frogs' eater (hawks)
→ lose a food source, decline or switch prey
trace both directions from the gap
the method
Answer
Grasshoppers increase (over-eating the grass) and hawks decline or switch prey
The usual mistake
Predicting effects in only one direction. Removing prey affects both predators above and plants below.
Check yourself
For any remove-a-species question, put your finger on the gap and trace one arrow backward (its food booms) and one forward (its eater starves). Both directions, every time — one-direction answers are the planted traps.
Practice it
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