Sustainability has a much sharper definition in biology than in everyday use. It is not about being green — it is arithmetic. If you take a resource out faster than the ecosystem puts it back, the resource runs out. If you take it out no faster than it is replaced, you can keep going forever. Everything on this page follows from that one comparison.
📘 What you need to know
A resource is sustainably harvested if it is replaced as rapidly as it is harvested, so it does not run out.
Sustainability requires the use of a resource to be carefully regulated and monitored.
Black cherry timber: sustainable through selective felling, leaving seed-producing trees, and monitoring regrowth against logging.
Alaska pollock: the largest sustainably certified fishery, thanks to fast growth, low seabed impact, very low bycatch and close monitoring.
Agriculture faces four sustainability problems: soil erosion, leaching, agrochemical pollution and carbon footprint.
Fertiliser supply is a problem in its own right — production is energy-intensive, so cost tracks global energy prices.
What sustainable harvesting actually means
Humans rely on the natural environment for resources such as food and timber. A sustainably harvested resource is one that is replaced as rapidly as it is taken, which means the stock never runs down. Getting there requires two things that are easy to say and hard to do: the harvest must be regulated, and it must be monitored so that the regulations can be adjusted when the stock changes.
The green stock does not stay at 100 %. It drops to a new level and stays there, because a slightly smaller population grows back faster. That is the level a well-managed fishery aims to sit at.
A terrestrial plant: black cherry
Black cherry is a North American hardwood, popular for furniture. Hardwood species grow slowly, which makes the harvesting method critical: clear felling a stand would leave no timber for many years, because nothing regrows quickly enough to replace it.
Sustainable harvesting of black cherry involves three things:
Selective felling — choosing specific individual trees to harvest. The gaps left in the canopy let light reach the forest floor, which encourages more plants to grow there.
Leaving enough individuals in the forest to flower and produce seeds, so new saplings germinate.
Regular monitoring, to check that new growth is keeping up with logging.
Selective felling has a second benefit the source often skips: the canopy gaps are not just a side effect, they actively increase plant diversity on the forest floor. A well-managed selectively felled forest can be more diverse than an untouched one.
A marine fish: Alaska pollock
Pollock live across the North Pacific, and the Alaska pollock fishery is the largest sustainably certified fishery in the world, certified by the Marine Stewardship Council (MSC). It qualifies for several reasons, and a good answer lists more than one:
Pollock are fast-growing and can reproduce from the age of 3–4 years, so the stock replaces itself quickly.
The nets have minimal contact with the sea bed, so that habitat is not damaged.
Bycatch — the proportion of the catch that is not pollock — is very low, under 1 %.
By regulation, any bycatch that is caught cannot be sold commercially, so it is in the fishermen’s own interest to avoid catching it in the first place.
Close monitoring is carried out by science research vessels and by trained individuals on board the commercial vessels themselves.
Areas where a higher than usual number of salmon are caught as bycatch are closed off to fishing vessels.
The clever bit. Banning the sale of bycatch turns an environmental rule into a financial one. Fishermen avoid bycatch not because they are told to but because catching it earns them nothing. Regulations that align with self-interest are far easier to enforce.
Sustainability in agriculture
Agriculture has to supply enough food and materials for the whole human population, and doing that sustainably is genuinely difficult. Four problems come up again and again, and each has a recognised response.
Questions on this topic usually go one of two ways: describe a problem, or suggest an improvement. Learning the pairs means you are ready for both.
Soil erosion
Growing crops or keeping large numbers of grazing animals means clearing land. Removing larger trees and shrubs removes the roots that hold the soil together, so the soil becomes less stable and can be washed or blown away. The layer most at risk is the nutrient-rich topsoil — exactly the part crops need.
Crops and grass do give some protection, but often only partial cover, and they may be removed at harvest or lost to overgrazing or bad weather. Farmers sometimes plant cover crops to hold the topsoil together between growing seasons.
Leaching and nutrient run-off
The minerals in synthetic fertilisers are highly soluble, which is what makes them work — and also what makes them a problem. Rainfall dissolves them and washes them out of the soil into nearby bodies of water. This is leaching, and it leads to nutrient run-off.
It can be reduced by applying fertiliser in small volumes, at times when rain is not forecast, and by using organic rather than synthetic fertilisers. Hold on to this mechanism — the next page follows what happens once those nutrients reach the water.
Fertiliser supply
Chemical fertilisers are essential to many farmers but are not always easy to obtain. They are expensive, they are used by so many farmers that supply may not meet demand, and their production is very energy-intensive, so their cost is tied to global energy prices. Switching to organic fertilisers eases some of these difficulties.
Pollution from agrochemicals
Some farming relies on chemicals known as agrochemicals: fertilisers, plus pesticides, herbicides and fungicides. These are used to protect yield from insect pests, competition from weeds and fungal disease. The problem is that they enter the natural environment and cause damage, for example by killing non-target species.
Two responses are worth knowing. Biological pest control reduces pesticide use by releasing a pest’s natural predators — ladybirds released to prey on aphids, for instance. And scientists hope genetic modification can produce crop varieties that are resistant to pests and disease, removing the need for some chemicals altogether.
Carbon footprint
Agriculture depends on fossil fuels for transport, machinery and the production of synthetic fertilisers. Burning those fuels releases carbon dioxide, increasing the carbon footprint of farming and contributing to climate change. Moving to renewable energy sources and energy-efficient practices reduces it.
Worked examples
WORKED EXAMPLE 1
Explain why selective felling is a more sustainable way to harvest black cherry than clear felling. [4]
Step 1: the growth rate problem
Black cherry is a hardwood and grows slowly, so it is replaced only slowly after harvesting.
Step 2: what clear felling does
Clear felling removes every tree at once, so no timber is available for many years and there are no trees left to produce seed.
Step 3: what selective felling does instead
Only chosen individuals are removed, leaving enough trees to flower and produce seeds so new saplings germinate.
Step 4: the extra benefitGaps in the canopy let light reach the forest floor, encouraging more plant growth — and regrowth is monitored against the rate of loggingthe mark for “monitoring” is easy to earn and easy to forget
WORKED EXAMPLE 2
A fish stock is harvested at a rate slightly below its replacement rate. Describe and explain what happens to the stock over the following twenty years. [3]
Step 1: describe the shape
The stock falls at first, then levels off and stays roughly constant.
Step 2: explain the fall
Fish are being removed, so numbers drop below the original level.
Step 3: explain the levelling offAt the lower density there is less competition, so the population grows back fast enough to replace what is taken, and stock stabilises“sustainable” does not mean the stock is unchanged — it means it stops falling
WORKED EXAMPLE 3
Suggest two changes a farmer could make to reduce the loss of nitrate fertiliser from their fields, and explain how each works. [4]
Change 1: how much and when
Apply fertiliser in smaller volumes and only when rain is not forecast, so less is dissolved and washed out before the crop takes it up.
Change 2: what kind
Use organic rather than synthetic fertiliser, because it releases nutrients more slowly and is less readily leached.
The underlying reasonNitrates are highly soluble, so any that is not absorbed quickly is carried into nearby watera third option is planting cover crops, which take up nutrients that would otherwise wash away
💡 Exam tip
Define sustainable harvesting as replaced as rapidly as it is harvested — the comparison of two rates is the whole answer.
Give at least three reasons when asked why a fishery is sustainable. There are six available for pollock.
Always include monitoring and regulation; sustainability is a management practice, not a property of the species.
For soil erosion, name the layer: topsoil, and say it is the nutrient-rich one.
Link leaching to solubility. That is the property doing the work.
When suggesting improvements, add a cost or drawback if the question says “evaluate” or “discuss”.
⚠ Common mix-up
Thinking a sustainable stock stays at its original size. It settles at a lower, stable level.
Confusing leaching with run-off of soil. Leaching is dissolved minerals moving through the soil; erosion moves the soil itself.
Saying pesticides are simply bad. They protect yield — the problem is their effect on non-target species.
Forgetting fertiliser production has a carbon cost, not just fuel for tractors.
Assuming organic fertiliser cannot cause eutrophication. It leaches more slowly, but it still contains nutrients.
Writing “clear felling is unsustainable” with no reason. The reason is the slow growth rate and the loss of seed trees.
Up next: Eutrophication — what actually happens when the nitrates leached from those fields arrive in a lake.
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