The short answer
Energy flow, trophic levels and population growth — the relationships that structure every ecosystem.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Ecology is about relationships: who eats whom, who competes, and how energy and matter move through an ecosystem. Two ideas unlock most questions — the ~10% rule for energy, and the difference between exponential and logistic population growth.
The 10% rule
Only about 10% of the energy at one trophic level is passed to the next — the rest is lost as heat, movement and waste. That's why food chains are short (rarely more than 4–5 links) and why there are far fewer top predators than plants.
Exponential vs logistic growth
Exponential (J-curve)
- Unlimited resources assumed
- Population accelerates without limit
- Rare and temporary in nature
Logistic (S-curve)
- Resources are limited
- Growth slows and levels off
- Plateaus at the carrying capacity (K)
Symbiosis in one line
Mutualism (+/+, both benefit), commensalism (+/0, one benefits, the other unaffected), parasitism (+/−, one benefits at the other's expense). Predation and competition shape populations alongside these.
Worked example
Producers at the base of a food chain capture 10,000 units of energy. Roughly how much energy is available to a secondary consumer (two levels up)?
Check yourself
A population growing logistically has levelled off and is no longer increasing. It has most likely reached its:
Key takeaways
- Energy flows up trophic levels at only ~10% efficiency → short food chains.
- Exponential growth (J-curve) assumes unlimited resources; rare and temporary.
- Logistic growth (S-curve) plateaus at the carrying capacity (K).
- Symbiosis: mutualism (+/+), commensalism (+/0), parasitism (+/−).
- Matter cycles (carbon, nitrogen); energy flows through and is lost as heat.
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