Take exactly what a population replaces each year and you can harvest it forever. Take more and the stock shrinks. That single idea is what maximum sustainable yield means, and net productivity is the number that sets the limit.
📘 What you need to know
Sustainable yield is the rate at which a resource can be harvested without stopping the system regenerating it.
Maximum sustainable yield (MSY) is the highest rate of NPP or NSP that can be harvested without depleting the system.
In agricultural, aquacultural and silvicultural systems, MSY is the most biomass that can be taken without degrading soil, water or the wider ecosystem.
Sustainable yields are higher at lower trophic levels, because less energy has been lost.
Plant-based food is the most energy-efficient and sustainable option for human consumption.
What sustainable yield actually means
Sustainable yield is about balance. Harvest at or below the rate at which the resource regenerates and two things follow:
The resource is not over-exploited, so it lasts.
It stays available for future generations, because natural regeneration keeps up.
Maximum sustainable yield is the point right at that limit — the largest harvest the system can support indefinitely.
The middle panel is the target, but it leaves no margin for error. Real management usually aims slightly below MSY, because estimates of productivity are never perfect.
Notice how this connects straight back to net productivity. NPP and NSP are not just numbers on a diagram — they are the reason a fishery quota is set where it is.
Why lower trophic levels give higher yields
This is the part exams reward most, and the logic is a chain you should write out in full.
🧩 The chain of reasoning
Energy is lost as heat at every transfer between trophic levels.
So less energy is lost from the system at lower trophic levels — there have been fewer transfers.
That makes lower levels more efficient for resource production.
Higher trophic levels, such as carnivores, are therefore much less efficient to harvest.
So sustainable yields are higher for organisms closer to the base of the food chain.
Applied to food production, that means plant-based foods at lower trophic levels are the most energy-efficient and sustainable option for humans. They need far fewer resources — land, water and energy — than animals at higher trophic levels.
Growing wheat or rice for people to eat directly is more sustainable than raising livestock for meat, because raising livestock requires plants to be grown as feed first, adding a whole extra trophic level and its losses.
System
Trophic levels involved
Efficiency
Plant-based diet
Producer to human — one transfer
Higher: no intermediate level, so losses are minimised and more people can be fed from the same crop
Meat-based diet
Producer to livestock to human — two transfers
Lower: energy is lost at two trophic levels, so fewer people are supported by the same crop area
MSY in managed systems
In agricultural, aquacultural and silvicultural systems, MSY is the highest amount of biomass — crops, livestock, seafood or timber — that can be sustainably harvested over time without degrading the soil, water or wider ecosystem. That final clause matters: a harvest that hits the biomass target but wrecks the soil is not sustainable.
Why fisheries collapse. MSY depends on knowing the real net productivity of the stock, which is genuinely hard to measure. If the estimate is too high, quotas are set above the true limit and the population falls — and because the stock is now smaller, next year’s productivity is lower too. That feedback is what turns overfishing into collapse.
WORKED EXAMPLE
A fish population has a net secondary productivity of 640 tonnes per year. The current annual catch is 810 tonnes. Comment on the sustainability of this fishery.
Step 1: Compare catch with productivity810 taken, 640 replacedStep 2: Find the shortfall810 − 640 = 170 tonnes per yearStep 3: State the consequenceThe stock declines by 170 tonnes each year, so next year’s productivity will be lower stillNot sustainable – the catch exceeds MSY
WORKED EXAMPLE
Explain why a hectare of farmland feeds more people when planted with wheat than when used to graze cattle.
Step 1: Count the transferswheat → human: one transferwheat → cattle → human: two transfersStep 2: Apply the energy lossAround 90% is lost as heat, faeces and uneaten material at each transferStep 3: ConcludeFar more of the crop’s energy reaches people directly, so more people are fedSay “lower trophic level” explicitly – it is the key phrase
💡 Exam tip
Define MSY using NPP or NSP — that link is what makes it an HL answer rather than a GCSE one.
Compare harvest with regrowth in any sustainability judgement, and give the difference as a number.
Write the full chain for lower trophic levels: fewer transfers, less heat lost, higher efficiency.
Add the wider condition: without degrading soil, water or the ecosystem.
Give a real example. Fisheries quotas and plant-based versus meat-based diets are both safe choices.
⚠ Common mix-up
Thinking MSY means harvesting everything. It is the maximum that can be taken repeatedly, not the total present.
Comparing catch with total stock instead of with productivity. Productivity is the replacement rate.
Saying meat is unsustainable without explaining why. The energy loss at the extra trophic level is the reason.
Ignoring the feedback. Overharvesting lowers next year’s productivity as well as this year’s stock.
Forgetting soil and water. A yield that degrades the land is not sustainable even if the numbers balance.
Up next: Measuring Ecological Efficiency — putting a percentage on how much energy actually makes it from one level to the next.
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