Disruption, eutrophication and tipping points
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Flip to reveal answersWhat is a disturbance in an ecosystem?
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Question
What is a disturbance in an ecosystem?
Answer
Any event that knocks an ecosystem out of its steady state — e.g. **fertiliser run-off, wildfire, or pollution**.
Question
What is a tipping point?
Answer
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.
Question
Deflection vs tipping — what is the difference?
Answer
A **deflection** is recoverable (the system bounces back); past a **tipping point** the change feeds itself and the system settles in a new state.
Question
What is eutrophication?
Answer
The over-enrichment of water with **nitrate and phosphate** (often from fertiliser run-off), causing an **algal bloom** and then a fall in dissolved oxygen.
Question
Predict what happens when fertiliser leaches into a lake.
Answer
**Algal bloom** → light blocked, plants die → **decomposers respire** and use up the oxygen (high **BOD**) → **fish die**.
Question
What does a high BOD mean?
Answer
That **decomposers are using up a lot of dissolved oxygen** breaking down dead organic matter, leaving little for fish and other aerobic organisms.
Question
How does a wildfire raise the risk of soil erosion?
Answer
It removes the **vegetation and roots** that bind the soil; the **bare soil** is then **washed away by rain and blown away by wind**.
Question
In eutrophication, why does the oxygen fall?
Answer
Because **decomposers multiply and respire aerobically** as they break down the dead algae and plants (high BOD) — not because the algae 'use it up'.
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Topic 4.11 hub
Stability and change
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