Parts of the Amazon have been rainforest for tens of millions of years. No one has been topping up its nutrients or watering it. That is what a stable ecosystem is: a system that keeps itself going, indefinitely, on sunlight alone. This page is about the handful of conditions that make it possible — and how close some of them are to breaking.
📘 What you need to know
A stable ecosystem resists change: population sizes stay roughly steady and resources are not used up.
Stable ecosystems have efficient nutrient cycling, high biodiversity and high rates of photosynthesis.
Four requirements: a reliable energy supply, nutrient recycling, genetic diversity, and climatic variables staying within tolerance limits.
Nutrients are recycled by decomposers breaking down carbon compounds in dead organisms and waste.
Genetic diversity is the number of different alleles present in a population; it is what lets a population adapt.
If conditions move outside tolerance limits, a species must migrate or face extinction.
Stable does not mean static — natural selection keeps acting, so species keep evolving.
What “stable” actually means
Stability here means resistance to change. In a stable ecosystem, consumer population sizes do not swing wildly, so resources are never overused, and the system carries on without any input from outside except energy.
You can recognise a stable ecosystem by four features:
Efficient nutrient cycling, which makes the system self-supporting.
High biodiversity.
Stability itself — populations do not change significantly, so resources are not overused.
High rates of photosynthesis, which keep energy flowing into the system.
Watch the wording in exam questions. “Stable” is about resisting change; “sustainable” is about being able to keep going. They overlap, but they are not synonyms, and a question asking for one will not accept the other.
The four requirements
The caption is worth memorising. Energy is used once and lost as heat, so it has to be replaced constantly. Atoms are used again and again, so they only have to be released.
1. Supply of energy
A reliable energy source must be present, and for almost every ecosystem that source is sunlight. Light energy is converted into chemical energy by photosynthesis, which means plenty of photosynthetic organisms — plants or algae — must be present. That stored chemical energy is then passed up food chains as organisms eat one another.
2. Recycling of nutrients
An ecosystem can only support itself if nutrients are cycled. If they are not, the supply simply runs out. Cycling happens when decomposers such as bacteria and fungi break down the carbon compounds — proteins, nucleic acids and so on — in dead organisms and waste matter. Carbon returns to the atmosphere as carbon dioxide, while minerals such as nitrates and phosphates are released into the soil, where producers take them up again.
Two things follow from that, and both are examinable:
Conditions must suit the decomposers. They need enough oxygen and moisture and a suitable temperature. An ecosystem that is too hot or too dry has slower nutrient cycling and is therefore less productive.
Removing material breaks the cycle. If fallen trees are taken away for timber instead of being left to rot, or if crops are harvested and carried off the field, the nutrients in them leave the system and productivity falls.
This is why farming needs fertiliser. A natural woodland returns its nutrients to the soil every autumn. A wheat field has its nutrients driven away in a lorry every August. The gap has to be filled artificially — which is where the next few pages of problems begin.
3. Genetic diversity
Definition to learnGenetic diversity = the number of different alleles of genes present in a population
High genetic diversity means natural selection has plenty of favourable alleles to act on, so the population has the potential to adapt when the environment changes. A population with only a small number of alleles is much less likely to contain one that happens to be useful, so it cannot adapt. In short, genetic diversity is what allows populations to resist the effects of change.
4. Climatic variables within tolerance limits
Genetic variation only buys a population so much. Every species has a range of conditions — temperature, rainfall and so on — within which it can function. Push conditions beyond those tolerance limits and no amount of variation will save it: the species must migrate or face extinction.
Different species have differently shaped curves. A generalist has a wide, flat curve; a specialist has a narrow, tall one and is far more vulnerable when conditions shift.
Human activities are causing climate change so rapidly that climatic variables in some ecosystems are already moving beyond the tolerance limits of the species living there. The speed is the problem: evolution can track a slow change, but not one that happens within a few generations.
A worked case: tropical rainforest
Some tropical rainforests, including the Amazon in South America and the Congo in Africa, have remained in much their current state for tens of millions of years. They tick every box:
Highly diverse. The Amazon is thought to contain millions of invertebrate species, tens of thousands of plant species, thousands of bird and fish species and hundreds of mammal species.
High photosynthesis. Plenty of light and moisture mean photosynthesis rates are high all year.
Organic matter is cycled by detritivores such as termites, slugs and worms, and by decomposers such as fungi. The released nutrients are taken up again by the trees.
Water is cycled inside the ecosystem: it is lost from the trees by transpiration, condenses, and falls again as rain over the same forest.
That last point is the one students under-use. The rainforest largely makes its own rain. It is not just sitting in a wet place — it is the reason the place is wet, which is exactly why cutting it down is so risky.
Stable is not the same as static
A healthy ecosystem is highly stable, but it is not frozen. Natural selection is always acting on every species in it, so evolutionary change never stops. What stays roughly constant is the overall structure — the population sizes, the nutrient flows, the rates of photosynthesis — not the genetic make-up of the species.
Worked examples
WORKED EXAMPLE 1
Explain why an ecosystem requires a continuous supply of energy but does not require a continuous supply of nutrients. [4]
Step 1: what happens to energy
Energy enters as light and is converted to chemical energy by photosynthesis, but at every transfer some is lost as heat and cannot be reused.
Step 2: the consequence
Energy flows through the ecosystem in one direction only, so it must be constantly replaced by sunlight.
Step 3: what happens to nutrients
Decomposers break down carbon compounds in dead organisms and waste, releasing carbon dioxide and minerals such as nitrates and phosphates.
Step 4: the consequenceProducers take those nutrients up again, so the same atoms are recycled indefinitelyenergy flows, matter cycles — the phrase examiners want to see
WORKED EXAMPLE 2
A population of a rare plant has very low genetic diversity. Explain why this makes the population vulnerable to environmental change. [3]
Step 1: define what is missing
Low genetic diversity means there are few different alleles present in the population.
Step 2: link to natural selection
If the environment changes, the chance that any individual carries a favourable allele is reduced, so natural selection has little to act on.
Step 3: the outcomeThe population cannot adapt, so it may fall outside its tolerance limits and become extinctthis is exactly why conservation programmes worry about inbreeding, not just about numbers
WORKED EXAMPLE 3
Suggest why an ecosystem in a very hot, dry region is likely to be less productive than a tropical rainforest. [3]
Step 1: the condition that matters
Decomposers need moisture, oxygen and a suitable temperature to work efficiently.
Step 2: the effect on cycling
In hot, dry conditions decomposition is slow, so nutrients stay locked in dead material instead of returning to the soil.
Step 3: the effect on producersFewer nutrients are available for uptake, limiting plant growth and so lowering productivitylack of water also limits photosynthesis directly — both routes earn credit
💡 Exam tip
Learn the four requirements as a list you can recite: energy, nutrient recycling, genetic diversity, climate within tolerance.
Use the phrase energy flows, matter cycles — it answers a whole family of questions.
Name the decomposers (bacteria and fungi) and say what they release: carbon dioxide, nitrates, phosphates.
Define genetic diversity properly as the number of different alleles, not “how varied the species are”.
If a question mentions extreme conditions, mention tolerance limits and the choice between migration and extinction.
Remember the “not static” point: stability and ongoing evolutionary change happen together.
⚠ Common mix-up
Saying energy is recycled. It is not — it is lost as heat at every transfer.
Confusing genetic diversity with biodiversity. Genetic diversity is alleles within a population; biodiversity is variety across species.
Assuming a stable ecosystem never changes. Species are still evolving inside it.
Forgetting decomposers need the right conditions. Cold, dry or waterlogged ground slows cycling right down.
Thinking harvesting only removes the organism. It removes the nutrients locked inside it too.
Treating “tolerance limits” as a single value. It is a range, with a stressed zone before the lethal one.
Up next: Ecosystem Stability (Skills) — how to model an ecosystem in a jar, and how to handle the percentage-change calculations that come with deforestation data.
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