IB ESS SL1.2 SystemsPaper 1 & 2Core idea~14 min read
Equilibrium and Feedback Loops
A forest looks the same year after year, even though leaves fall, animals die and rain pours through it constantly. Nothing is holding it still by magic. Loops inside the system keep pulling it back towards its average — and other loops can shove it further away.
📚 What you need to know
Equilibrium is a state of balance between the parts of a system.
Steady-state equilibrium: small changes happen, but the long-term average stays the same. Typical of open systems.
Static equilibrium: no inputs or outputs at all, so nothing changes. Only non-living objects.
A stable equilibrium returns to the same state after a disturbance. An unstable one shifts to a new one.
Negative feedback counteracts change and stabilises the system.
Positive feedback amplifies change and destabilises the system, driving it towards a tipping point.
Feedback loops are how systems self-regulate.
What equilibrium means
Definition
An equilibrium is a state of balance between the different parts of a system.
Open systems such as ecosystems usually sit in a stable equilibrium. They generally stay in the same state over time, and if something knocks them, they come back. That balance is held in place by stabilising negative feedback loops.
Steady-state and static
Zoom in on the green line and it looks chaotic. Zoom out and it is flat. That is the whole idea of a steady state.
Steady-state equilibrium
Static equilibrium
Inputs and outputs
Yes, constantly
None at all
Short-term change
Small oscillations within closely defined limits
None
Long-term change
No major change; always returns to its average state
No change
Found in
Most open systems in nature
Non-living objects only
Example
A forest, where species rise and fall but the forest stays a forest
A chair, a desk, a rock on a shelf
The mistake I see most often is students saying an ecosystem is in static equilibrium because “it looks the same”. It is not. A forest is swallowing sunlight and rain and pumping out heat and oxygen every second. Nothing about it is static — it is steady.
Stable and unstable equilibrium
Now push the system and see what happens. That tells you whether the equilibrium is stable or unstable.
Nothing about the ball changes between the two pictures. What changes is the shape it is sitting on — and that is the system’s own structure.
In an unstable equilibrium, even a small disturbance can make the system suddenly shift to a new system state or a new average. Once it has moved, it settles at a different equilibrium and stays there.
Feedback loops
Feedback is what makes a system react to a disturbance. It is what lets a system self-regulate without anyone controlling it.
The basic loop
input → process → output → feedback → back into the input
A change in a process changes the outputs. Those outputs then change the inputs, which changes the process again. Round and round. What matters is which direction the loop pushes.
Negative feedback
DefinitionNegative feedback is any mechanism that counteracts a change away from equilibrium, bringing the system back to its average state.
Negative feedback is stabilising. The output of a process inhibits or reverses that same process. “Negative” here has nothing to do with being bad — it means the loop pushes in the opposite direction to the change.
🧩 Predator and prey, step by step
The zebra population increases.
More prey is available, so the lion population increases.
More lions means more predation, so the zebra population decreases.
Less prey is available, so the lion population decreases.
Fewer lions means less predation, so the zebras recover — and you are back at step 1.
Neither population runs away. Each rise causes the fall that follows it, so both wobble around a long-term average. That is a steady state being held in place by negative feedback.
A second example: cloud cover
The Earth’s surface temperature increases.
Warmer conditions mean more evaporation from lakes and oceans.
More water vapour means more cloud in the atmosphere.
More cloud means a higher albedo, so more solar radiation is reflected back to space.
Less energy reaches the surface, so the surface cools again, back towards the average.
Daisyworld in one line. Lovelock and Watson’s computer model put black and white daisies on an imaginary planet. Black daisies absorb heat and warm the planet, which suits white daisies; white daisies reflect heat and cool it, which suits black daisies. The two populations settle into a steady state that holds the temperature stable. It is a model of negative feedback, and you will meet it again in the notes on models.
Positive feedback
DefinitionPositive feedback is any mechanism that leads to additional and increased change away from equilibrium.
Positive feedback is destabilising. The output of a process feeds back in a way that moves the system further from its average, faster and faster, until it reaches a tipping point and lands in a new equilibrium.
Read it round twice. Each lap leaves the planet warmer than the lap before, which is exactly why this loop worries climate scientists.
More positive feedback loops worth learning
Permafrost: huge amounts of carbon dioxide and methane are locked in permanently frozen ground. Warming thaws the permafrost, the gases escape, more heat is trapped, more permafrost thaws.
Population decline: a smaller population has less reproductive potential, which shrinks the population further, which cuts reproductive potential again. The decline speeds up.
Population growth: the same loop running the other way. More individuals means more reproductive potential, which means faster growth.
That last pair is worth noticing: positive feedback does not always mean “increase”. It means more of whatever change already started, up or down.
Negative feedback
Positive feedback
What it does to change
Counteracts it
Amplifies it
Effect on the system
Stabilising
Destabilising
Where it ends up
Back at the average state
Further away, and possibly past a tipping point
Example
Predator and prey cycles; cloud cover and albedo
Ice melting and albedo; permafrost thawing
🧠
Easy way to remember it
Negative says no to the change and pulls it back. Positive says yes to the change and gives it a shove. Nothing to do with good or bad — a positive feedback loop is usually very bad news.
Worked examples
WE 1
Distinguish steady-state from static equilibrium
Distinguish between a steady-state equilibrium and a static equilibrium, giving an example of each. (4 marks)
Point 1: steady-state
There are continuous inputs and outputs, and the system fluctuates slightly but always returns to its average state over the long term.
Point 2: an example
A forest — populations rise and fall year to year, but the forest as a whole stays stable.
Point 3: static
There are no inputs or outputs of energy or matter, so the system shows no change at all over time.
Point 4: an example
A chair or a desk. No natural system is in static equilibrium.
Steady = flows but no net change. Static = no flows at all.say clearly that no natural system is static — it is often the deciding mark
WE 2
Explain a negative feedback loop
Explain how negative feedback keeps predator and prey populations relatively stable. (4 marks)
Step 1
The prey population increases, so there is more food available for predators.
Step 2
The predator population then increases because more prey can be caught.
Step 3
Higher predation causes the prey population to decrease again.
Step 4
With less prey, the predator population falls, and prey numbers recover.
The output of each step reverses the change that caused it, so both populations stay near their averagefinish the loop — an answer that stops after two steps has not shown feedback at all
WE 3
Identify the type of feedback
Melting sea ice exposes darker ocean water, which absorbs more solar radiation. Identify the type of feedback shown and explain your answer. (3 marks)
Point 1: the identification
This is positive feedback.
Point 2: the reason
Melting reduces the albedo, so more solar radiation is absorbed, temperatures rise further and more ice melts.
Point 3: the consequence
The loop amplifies the original change, moving the system further from equilibrium and towards a tipping point.
Change causes more of the same change = positive feedbackuse the word “albedo” — examiners look for it in every ice question
💡 Exam tips
Write feedback answers as a numbered chain that returns to where it started. Loops that do not close do not score.
Use the marking words: stabilising, destabilising, counteracts, amplifies, average state.
Learn one negative and one positive loop by heart, so you always have an example ready.
Say “steady-state equilibrium” in full. “Stable” on its own is vaguer and can lose you a mark.
If a diagram question appears, draw boxes and arrows and label the direction of each change.
Remember positive feedback can drive a decrease as well as an increase.
⚠ Common mistakes
Thinking positive feedback is good. It usually causes serious damage. The name only describes the direction.
Calling an ecosystem static. It has constant inputs and outputs, so it is steady-state.
Describing a chain of events with no return. Feedback must loop back to the start.
Mixing up unstable equilibrium and no equilibrium. An unstable system still has an equilibrium; it just cannot hold it.
Saying negative feedback stops all change. It counteracts change; small fluctuations still happen.
Leaving out the mechanism. “It balances out” is not an explanation. Say which output changes which input.
Up next: Resilience and Tipping Points — what decides how big a push a system can survive, and what happens when it cannot.
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