Key Idea: Topic D4.2 asks how ecosystems stay stable, what pushes them to change, and how life itself has changed the planet. The thread running through it is resilience vs change. A stable ecosystem keeps coming back to a steady state because it has energy, recycled nutrients, genetic diversity, many species and steady climate. A big enough disturbance (fertiliser run-off, fire) can deflect it — and past a tipping point it flips into a new state it cannot easily recover from. The topic then adds three ideas: using resources sustainably, studying ecosystems in mesocosms, and how pollutants biomagnify up a food chain. It finishes with phenotypic plasticity (the environment changing the phenotype without changing the genes) and life's impact on the atmosphere (photosynthesis raising oxygen and lowering CO₂ over geological time). This shows up mostly on Paper 1 (data/graph MCQs) and Paper 2 (outline/explain).
🌐 What makes an ecosystem stable (4.11.1)
Stability is an emergent property — it comes from the whole system working together, not from any single species. A stable ecosystem keeps returning to a steady state because it meets five requirements. The two that examiners stress most are diversity (many species, with overlapping roles) and genetic diversity (variation within each species). Together they give the ecosystem 'backup' — a buffer against losing any one part.
| Requirement for stability | What it provides |
|---|---|
| A constant supply of energy | Sunlight keeps photosynthesis going, so producers keep refilling the food chains. |
| Recycling of nutrients | Decomposers return nitrogen, carbon and minerals to the soil so they are not used up. |
| Genetic diversity within species | A varied gene pool means some individuals survive new diseases or conditions — the population isn't wiped out. |
| A diverse community (many species) | Overlapping roles give 'backup': if one species is lost, another can fill the gap. |
| Climatic variables that stay within limits | Steady temperature, rainfall and so on keep every organism inside the range it can tolerate. |
Visual recap of stability (4.11.1): a diverse web (left) has 'backup' — lose one species and another covers its role, so the ecosystem stays stable. A simple chain (right) has no backup — lose one link and everything above it collapses. More species and more genetic diversity = more resilience.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
In a diverse web, many species share similar roles, so if one is lost another fills the gap and the web keeps working. In a simple chain, every species is essential — lose one link and everything above it collapses. More species and more genetic variation = more resilience.
🔥 Disturbance, eutrophication & tipping points (4.11.2)
A disturbance is any event that knocks an ecosystem out of its steady state. A small one only deflects the ecosystem, which then recovers. But beyond a tipping point the change becomes self-reinforcing and flips the ecosystem into a new stable state it cannot easily come back from. Two disturbances to know in detail are eutrophication (from fertiliser run-off) and wildfire (which leads to soil erosion).
| Disturbance | How it deflects the ecosystem | If it goes too far |
|---|---|---|
| Fertiliser run-off (nitrate/phosphate) | Eutrophication — an algal bloom, then an oxygen crash as decomposers respire (high BOD) | Fish and aerobic life die; the lake can flip to a turbid, low-oxygen state |
| Wildfire | Burns away vegetation and roots that held the soil and intercepted rain | Bare soil washes away (erosion); recovery is slow and the community may not return |
| A small natural disturbance | Deflects the ecosystem slightly, then it returns to its steady state | — (stays below the tipping point, so it recovers) |
The eutrophication chain (cause → effect)
- Fertiliser (nitrate and phosphate) runs off the land into a lake or river.
- The extra nutrients cause a rapid algal bloom at the surface.
- The bloom blocks light, so plants below die, and the algae themselves soon die.
- Decomposers (bacteria) multiply and respire as they break down all the dead matter.
- Their respiration uses up the dissolved oxygen — a high biochemical oxygen demand (BOD).
- With little oxygen left, fish and other aerobic organisms suffocate and die.
Give it as a cause → effect chain: The fire destroys the plants and roots that held the soil together and intercepted the rain. With the soil exposed and loosened, rain and wind wash or blow it away — that is erosion. (Two linked points = two marks.)
Eutrophication = over-enrichment of water with nitrate/phosphate → algal bloom → oxygen crash. BOD (biochemical oxygen demand) = the oxygen used up by decomposers respiring; a high BOD means little oxygen is left for fish. Tipping point = the threshold past which the ecosystem flips and cannot easily recover.
♻️ Sustainability, mesocosms & biomagnification (4.11.3)
Sustainable use means taking no more than is naturally replaced, so the resource lasts indefinitely. A sustainable harvest (of fish or timber) is small enough that the population can replace what is removed; an unsustainable one removes faster than the stock can regrow, so it declines and may collapse. A mesocosm is a small, enclosed experimental ecosystem (e.g. a sealed tank or a fenced plot) used to study how an ecosystem behaves under controlled conditions — and a sealed one that survives for months shows it has the energy, recycling and balance that real stability needs. Biomagnification is the rise in concentration of a pollutant up a food chain.
What makes a harvest sustainable
- The amount removed is less than or equal to the amount the population naturally replaces.
- The breeding stock is left intact, so the population can recover (e.g. quotas, size limits, closed seasons).
- Over-harvesting removes individuals faster than they regrow → the stock falls and can collapse.
Visual recap of biomagnification (4.11.3): a fat-soluble pollutant that is not broken down or excreted is dilute in the producers but multiplies at each trophic level, because each predator eats many prey and keeps what they carried. So it is most concentrated in the top predator.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Key Idea: It builds up the chain only if it is persistent (not broken down) and not excreted (often fat-soluble). Each predator eats many prey and keeps the pollutant they carried, so the amount stacks up at every trophic level — leaving the top predator with the highest concentration.
🦋 Phenotypic plasticity (4.11.4)
Phenotypic plasticity is the ability of one genotype to produce different phenotypes in response to different environments, within an individual's lifetime — with no change to the DNA. It is the opposite of a genetic change: the alleles stay the same, but the environment changes how the genes are expressed. Plastic changes are often reversible and are never inherited.
| Feature | Phenotypic plasticity | Genetic (heritable) change |
|---|---|---|
| What changes | The phenotype (the visible trait) only | The genotype (the alleles) — across generations |
| Cause | The environment (e.g. temperature, light, season) | Mutation and natural selection |
| DNA altered? | No — same genotype throughout | Yes — the allele frequencies change |
| Passed to offspring? | No — not inherited | Yes — inherited |
| Example | Same-genotype moths reared cool develop darker wings than those reared warm | A population evolving darker wings over many generations because dark survivors breed more |
When a question gives clones (or 'same genotype') that look different, the marked points are almost always: (1) the genotype is the same — so the difference is not genetic / not a mutation; (2) the environment (temperature, light, season) caused the different phenotype. Hit both.
🌍 Life's impact on Earth's atmosphere (4.11.5)
Living organisms have changed the composition of the atmosphere over geological time. The early atmosphere had almost no oxygen and a high level of carbon dioxide. Then, billions of years ago, photosynthesis by cyanobacteria (and later algae and plants) began releasing oxygen — the Great Oxidation Event — and removing carbon dioxide. Over time this built today's air: about 21% oxygen and very little CO₂. Much of the removed carbon was locked away by living things into limestone and fossil fuels.
| Early (reducing) atmosphere | Modern (oxidising) atmosphere | |
|---|---|---|
| Oxygen (O₂) | Almost none | About 21% — built up by photosynthesis |
| Carbon dioxide (CO₂) | Very high (volcanic gases) | Very low — much of it locked into limestone and fossil fuels by living things |
| What changed it | — | Photosynthesis by cyanobacteria, algae and plants released oxygen and removed CO₂ |
Key Idea: Oxygen rose and carbon dioxide fell — and the cause of both is the same: photosynthesis, which takes in CO₂ and gives out O₂. The rise in oxygen also allowed aerobic respiration and an ozone layer to form, opening the way for complex life on land.
✍️ Worked examples
IB-style question — predict the effect of fertiliser run-off
Nitrogen-rich fertiliser repeatedly leaches from a field into a shallow lake. Predict, with reasons, how the dissolved oxygen and the fish population in the lake will change over time. [4]
Model answer:
Name the process. The extra nitrate (and phosphate) causes eutrophication — it triggers a rapid algal bloom.
Why oxygen falls. The algae (and the plants shaded out beneath them) die; decomposers multiply and respire to break them down, using up the dissolved oxygen — a high BOD.
Predict oxygen. Dissolved oxygen falls / crashes.
Predict the fish. With little oxygen left, the fish (and other aerobic organisms) suffocate and the population falls / dies off. (1 mark each: eutrophication/algal bloom; decomposers respire and use oxygen; oxygen falls; fish population falls.)
The nitrate causes eutrophication and an algal bloom; the algae and shaded plants die, decomposers multiply and respire, using up the dissolved oxygen (high BOD); so dissolved oxygen falls and the fish population falls as they suffocate.
IB-style question — explain phenotypic plasticity
Two groups of genetically identical water-flea clones were raised in the same water, but one group was exposed to chemical traces of a predator. That group grew protective spines; the other did not. Explain why this is phenotypic plasticity rather than evolution. [3]
Model answer:
Same genotype. The water fleas are clones, so they share the same genotype — the spines are not caused by a different gene or a mutation.
Environment causes the phenotype. The predator cue (the environment) changed how the genes were expressed, producing a different phenotype (spines) within the individual's lifetime.
Why not evolution. Evolution is a heritable change in allele frequency across generations; here the DNA is unchanged and the change is not inherited, so it is plasticity, not evolution. (Mark 1: same genotype/clones. Mark 2: environment causes the different phenotype. Mark 3: not heritable / no DNA change.)
The water fleas are clones with the same genotype, so the spines are not genetic. The predator cue (environment) changed gene expression, giving a different phenotype within the individual's lifetime. Because the DNA is unchanged and the trait is not inherited, this is phenotypic plasticity, not evolution.
IB-style question — outline changes to the atmosphere
Outline how living organisms have changed the composition of Earth's atmosphere over geological time. [4]
Model answer:
The starting point. The early atmosphere had almost no oxygen and a high level of carbon dioxide.
Oxygen rose. Photosynthesis — first by cyanobacteria, then algae and plants — released oxygen (the Great Oxidation Event), building today's level of about 21%.
Carbon dioxide fell. The same photosynthesis removed CO₂, and carbon was locked away by organisms into limestone and fossil fuels, so atmospheric CO₂ became very low.
A consequence. The oxygen allowed aerobic respiration and formed an ozone layer, making complex life on land possible. (1 mark per distinct point, up to 4.)
Early air had little oxygen and much CO₂. Photosynthesis (cyanobacteria, then algae and plants) released oxygen (Great Oxidation Event) up to ~21%, and removed CO₂ — much of it locked into limestone and fossil fuels — so CO₂ fell. The oxygen enabled aerobic respiration and an ozone layer.
✅ Quick self-check
Tap each card to check yourself.
What does an ecosystem need to stay stable? A steady energy supply, recycling of nutrients, genetic diversity within species, many species (a diverse community), and climatic variables that stay within tolerable limits.
Why does high biodiversity make an ecosystem more stable? Many species with overlapping roles give 'backup' — if one species is lost, another can fill the gap, so the ecosystem keeps working and returns to balance.
What is a tipping point? The threshold past which a disturbance triggers a self-reinforcing change that flips the ecosystem into a new stable state it cannot easily recover from.
Why does eutrophication lower oxygen and kill fish? Nutrient run-off causes an algal bloom; the algae and shaded plants die; decomposers multiply and respire, using up dissolved oxygen (high BOD); fish then suffocate.
Why does a pollutant biomagnify up a food chain? It is persistent and not excreted (often fat-soluble); each predator eats many prey and keeps the pollutant, so it stacks up at every level — highest in the top predator.
What is phenotypic plasticity? One genotype producing different phenotypes in response to different environments, within a lifetime — the DNA is unchanged and the change is not inherited.
How did living organisms change the atmosphere? Photosynthesis (cyanobacteria, algae, plants) raised oxygen (Great Oxidation Event) and removed carbon dioxide, much of it locked into limestone and fossil fuels.
Exam Tips
- Stability is an emergent property — name the requirements: energy supply, nutrient recycling, genetic diversity, species diversity, steady climate. Diversity (species + genetic) gives 'backup'.
- A small disturbance only deflects an ecosystem (it recovers); past a tipping point the change is self-reinforcing and it flips to a new state it can't easily come back from.
- Eutrophication chain: nutrient run-off → algal bloom → algae/plants die → decomposers respire (high BOD) → oxygen crashes → fish die. Give it as a cause→effect chain.
- Wildfire → erosion is two linked points: fire destroys the roots/plants holding the soil, then rain or wind washes/blows the bare soil away.
- Sustainable harvest = remove no more than the population replaces, leaving the breeding stock intact. Over-harvesting removes faster than the stock regrows → collapse.
- Biomagnification only happens for a persistent, non-excreted (fat-soluble) pollutant; it is most concentrated in the TOP predator — read up the chain.
- Phenotypic plasticity needs BOTH marks: (1) same genotype (not genetic/no mutation) and (2) the environment causes the different phenotype. It is not inherited.
- Atmosphere changes: photosynthesis RAISED oxygen (Great Oxidation Event, ~21% now) and LOWERED CO₂ (locked into limestone and fossil fuels) — one cause, two changes.