The big idea: A stable ecosystem stays roughly the same over time because energy keeps flowing and nutrients keep being recycled in balance.
A disturbance is anything that knocks this balance — for example fertiliser run-off, a wildfire, or a sudden input of a pollutant.
A disturbance deflects the ecosystem away from its steady state. A small one is absorbed and the system recovers. But a large or repeated one can push it past a tipping point into a new, often worse, stable state from which it cannot easily return.
- Stability
- The tendency of an ecosystem to stay roughly the same over time, with energy flow and nutrient recycling kept in balance.
- Disturbance
- Any event that knocks an ecosystem out of its steady state — e.g. fertiliser run-off, fire, flooding, or pollution.
- Deflection
- A shift of an ecosystem away from its normal steady state caused by a disturbance.
- Tipping point
- A threshold beyond which a disturbance causes a self-reinforcing change that flips the ecosystem into a new stable state it cannot easily recover from.
Deflect vs tip — the key distinction: Deflected = pushed off course but still able to bounce back.
Tipped = pushed so far that the change feeds itself and the ecosystem settles into a different state — removing the cause is no longer enough to fix it.
The exam wants you to know that not every disturbance is reversible.
The two disturbances the exam tests most are eutrophication (from fertiliser run-off) and wildfire.
Both are best learned as a chain of cause and effect — each step triggers the next.
Eutrophication: nutrients → oxygen crash: Eutrophication is the enrichment of water with nitrate and phosphate (usually from fertiliser run-off) that triggers excessive growth and then an oxygen crash.
The chain runs: extra nutrients → algal bloom → light is blocked and plants die → decomposers multiply and respire → they use up the dissolved oxygen (high BOD) → fish and aerobic organisms die.
If you are asked to predict what happens when nitrogen fertiliser leaches into a lake, the answer is this chain — ending in low oxygen and dead fish.
The eutrophication chain (cause → effect)
- Fertiliser leaches / runs off farmland into a lake or river, adding nitrate and phosphate (mineral nutrients).
- The extra nutrients let algae and water plants grow rapidly — an algal bloom covers the surface.
- The thick bloom blocks light from reaching plants below, so they cannot photosynthesise and they die.
- The algae themselves are short-lived and soon die too, leaving a lot of dead organic matter.
- Decomposers (bacteria) multiply and feed on the dead material, respiring aerobically.
- Their respiration uses up the dissolved oxygen — the water now has a high biochemical oxygen demand (BOD).
- With little oxygen left, fish and other aerobic organisms suffocate and die — biodiversity falls and the food web collapses.
- Eutrophication
- The over-enrichment of water with nutrients (nitrate/phosphate), causing an algal bloom and then a fall in dissolved oxygen.
- Algal bloom
- A rapid overgrowth of algae on the water surface, fuelled by the extra nutrients; it blocks light from the water below.
- Biochemical oxygen demand (BOD)
- The amount of dissolved oxygen used up by decomposers respiring as they break down dead organic matter. A high BOD means little oxygen is left for other organisms.
Wildfire → soil erosion (a 2-mark Outline): A wildfire removes the vegetation and plant roots that normally cover and bind the soil.
With the plants gone, the soil is left bare and root-free, so it is no longer held in place.
Rain washes it away and wind blows it away — this is soil erosion. The lost topsoil takes nutrients and habitat with it, making recovery slow.
For the 2-mark version: (1) fire destroys the plant cover / roots that hold the soil; (2) the exposed bare soil is then eroded by rain and wind.
Wildfire → erosion (cause → effect)
- Fire burns away the vegetation and kills the plant roots that bind the soil together.
- The soil surface is left bare and exposed, with nothing holding it in place.
- Rain runs off the bare ground and washes soil away; wind blows away the loose dry topsoil.
- The eroded soil carries off nutrients and seeds, so the land recovers slowly — and the run-off can even cause eutrophication downstream.
Eutrophication
- Trigger: fertiliser run-off (nitrate/phosphate)
- Algal bloom blocks light → plants die
- Decomposers respire → oxygen used up (high BOD)
- Result: fish and aerobic life die
Wildfire
- Trigger: fire removes vegetation
- Roots/plant cover that bound the soil are gone
- Bare soil eroded by rain and wind
- Result: topsoil and nutrients lost, slow recovery
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How this is tested: On Paper 1A a 1-mark item asks you to predict the change in a lake when nitrogen fertiliser leaches in — the answer is the eutrophication chain ending in low oxygen and dead fish.
On Paper 2 / Paper 3 a short Outline (2 marks) asks how wildfires raise the risk of soil erosion — score one mark for fire removing the plants/roots that bind the soil, and one for the exposed soil being washed or blown away.
Always answer these as a cause → effect chain, and link the change back to a loss of stability.
IB-style question — outline how wildfires raise the risk of soil erosion
A grassland is destroyed by a wildfire.
Outline how the wildfire raises the risk of soil erosion. [2]
How to score both marks
- Removal of the protective cover. The fire burns away the vegetation and the plant roots that normally cover and bind the soil together.
- Exposed soil is eroded. The bare, root-free soil is then washed away by rain and blown away by wind, so it is eroded. (Mark 1: fire removes plants/roots that hold the soil. Mark 2: exposed soil is washed/blown away.)
Final answer
The fire removes the vegetation and roots that bind and cover the soil; the exposed bare soil is then washed away by rain and blown away by wind, so erosion increases.
✓ Why this scores full marks: It is a clean two-step chain: first the cause (plants/roots that held the soil are gone), then the effect (bare soil is eroded by rain and wind).
A common slip is to only say 'the soil is exposed' without the second step — say what actually erodes it (rain and wind).
| Disturbance | What it does to the ecosystem | Why stability is lost |
|---|---|---|
| Fertiliser run-off → eutrophication | Nitrate/phosphate washes into a lake → algal bloom → algae die → decomposers use up the dissolved oxygen | Oxygen crash (high BOD) kills fish and other aerobic organisms — the food web collapses |
| Wildfire | Burns away the vegetation and plant roots that cover and bind the soil | Bare, root-free soil is exposed, so rain and wind erode it away — nutrients and habitat are lost |
| Nutrient / pollutant input (general) | Adds a substance an ecosystem cannot recycle fast enough (e.g. excess nitrogen, a toxic pollutant) | Recycling and energy flow are pushed out of balance, deflecting the ecosystem from its steady state |