The big idea: A stable ecosystem is one that stays roughly the same over time and returns to balance after a small disturbance.
No single organism is 'stable' — stability belongs to the whole community and its surroundings working together. We call this an emergent property: it appears only at the level of the whole system, not from any one part on its own.
An ecosystem stays stable only when four conditions are met — and the exam is built around those four conditions.
- Ecosystem
- A community of living organisms together with the non-living (abiotic) environment they interact with.
- Stability
- The tendency of an ecosystem to stay roughly the same over time and to return to balance after a disturbance.
- Emergent property
- A feature that belongs to a whole system, not to any of its individual parts — stability is one.
- Resilience
- The ability of an ecosystem to recover and return to its normal state after a disturbance.
Why 'emergent' matters: You cannot point to a single tree, beetle or bacterium and say 'that is the stable part'.
Stability emerges from many species, many interactions and a steady environment all working at once.
Lose enough of those parts and the property disappears — the ecosystem can no longer hold itself in balance.
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An ecosystem stays stable only if four requirements are met at the same time.
Work through them one at a time — for each, ask what would happen if it were missing? That is exactly how the exam frames it.
The four requirements for stability
- A continuous supply of energy — sunlight reaching the producers. Energy is lost as heat at every level and cannot be recycled, so it must keep coming in.
- Recycling of nutrients — decomposers break down dead matter and return elements (carbon, nitrogen, phosphorus) to the soil and water for producers to reuse. The chemical elements are limited, so they must be cycled.
- Genetic diversity within populations — a range of alleles means some individuals survive a new disease or stress, so a whole population is not wiped out by one change.
- Steady climatic / abiotic variables — temperature, rainfall, pH and oxygen staying within a narrow range, because organisms are adapted to a particular range and large swings push them outside it.
| Requirement | What it means | Why a stable ecosystem needs it |
|---|---|---|
| A continuous supply of energy | Sunlight reaching producers, captured by photosynthesis | Energy is lost as heat at every trophic level and cannot be recycled — without a constant input the food chains run down |
| Recycling of nutrients | Decomposers break down dead matter and return elements (C, N, P) to the soil/water | Unlike energy, the chemical elements are finite — they must be reused or the supply of nutrients for producers runs out |
| Genetic diversity within populations | A range of alleles among the individuals of each species | Variation means some individuals can survive a new disease or stress, so populations are not wiped out by one change |
| Steady climatic / abiotic variables | Temperature, rainfall, pH and oxygen staying within a narrow range | Organisms are adapted to a particular range; large swings push conditions outside their tolerance and break the community |
Energy flows, but nutrients cycle: This contrast is the heart of the topic.
Energy enters as sunlight, passes along the food chain and is lost as heat — it makes a one-way flow, so it must be constantly resupplied.
Nutrients (the chemical elements) are not lost — decomposers recycle them back to the start, so the same atoms are used again and again.
If you remember nothing else: energy flows through; matter cycles round.
Why diversity gives resilience: Genetic diversity (within a species) and biodiversity (across species) both act as backup.
If many species have overlapping roles, losing one species need not break the ecosystem — another can fill the gap. If each role is filled by only one species, losing it makes everything that depended on it collapse.
So more diversity → more backup → more resilience → more stability.
Resilience close-up: the faded species has been removed from both webs. The diverse web (left) still functions because another consumer fills the role; the simple chain (right) breaks.
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How this is tested: This new-syllabus idea is examined as an Outline / Explain question worth 3–4 marks: outline the conditions an ecosystem needs to remain stable, or explain why high biodiversity makes an ecosystem more stable.
The marks come from separate, distinct points — name the requirement and say why it matters. For 4 marks you need four different requirements, not one idea written four ways.
It also underpins Paper 1 items in 4.11.2 (predict what happens when a disturbance — like fertiliser run-off — pushes an ecosystem past a tipping point).
IB-style question — conditions for a stable ecosystem
A tropical rainforest has remained largely unchanged for thousands of years. Outline the conditions needed for an ecosystem such as this to remain stable. [4]
How to score all four marks
- Energy supply. There must be a continuous supply of energy (sunlight captured by producers in photosynthesis), because energy is lost as heat at each level and cannot be recycled.
- Nutrient recycling. Nutrients must be recycled — decomposers return elements such as carbon and nitrogen to the soil so producers can reuse them (the chemical elements are finite).
- Genetic diversity. Populations need genetic diversity so that some individuals can survive disease or environmental change, keeping populations from being wiped out.
- Steady abiotic conditions. Climatic/abiotic variables (temperature, rainfall, pH) must stay relatively constant, within the range the organisms are adapted to. (Award 1 mark per distinct condition, up to 4.)
Final answer
Continuous supply of energy (sunlight → producers); recycling of nutrients by decomposers; genetic diversity within populations; and relatively constant climatic/abiotic variables.
Why this scores full marks: Each sentence is a separate requirement — energy, nutrients, genetic diversity, steady climate.
A 4-mark 'Outline' needs four distinct scoring points, each naming a condition and giving a brief reason. Listing 'lots of energy' three different ways would score only once.
A stable ecosystem
- Energy keeps flowing in (sunlight → producers)
- Nutrients are recycled by decomposers
- High biodiversity and genetic diversity
- Climate/abiotic conditions stay roughly constant
- Returns to balance after a small disturbance (resilient)
An ecosystem losing stability
- Energy input interrupted (e.g. heavy shading, loss of producers)
- Nutrient cycle broken (e.g. decomposers lost)
- Low biodiversity — few backup species
- Climate/abiotic conditions swing sharply
- A disturbance pushes it past a tipping point (links to 4.11.2)