Trophic levels and energy loss
Trophic levels and pyramids
- A trophic level is an organism's position in a food chain (producers first, then primary consumers…).
- You can draw a food chain as a pyramid:
- pyramid of numbers — counts organisms at each level,
- pyramid of biomass — total mass at each level (usually a better picture),
- (Supplement) pyramid of energy — the energy at each level (the most useful).
The 10% rule (Supplement)
- Only about 10% of the energy at one level passes to the next; the rest is lost as heat, in movement and in waste.
- So food chains usually have fewer than five levels.
- It is more efficient to eat crop plants directly than animals fed on those crops.
Practice
Roughly how much energy passes from one trophic level to the next?
Only about 10% is passed on; the rest is lost as heat, movement and waste — so chains are short.
Practice
Why is it more energy-efficient for people to eat crops directly?
Feeding crops to animals first loses ~90% of the energy at that extra level, so eating plants directly wastes less.
Human impact
- overharvesting (e.g. overfishing) — taking too many, so numbers crash.
- introducing a foreign species — it may have no predators and crowd out native species.
Practice
How can humans damage food webs? (Choose all that apply.)
Overharvesting, overfishing and introducing foreign species all damage food webs; one-way energy flow is just normal.
You've got it
Key idea
- a trophic level = position in the chain; pyramids show numbers, biomass, or energy (Supplement)
- only ~10% of energy passes up each level → chains are short; eating plants is more efficient
- humans harm webs by overharvesting and introducing foreign species