A forest changes constantly and yet stays recognisably the same forest for centuries. That is equilibrium, and it is held in place by feedback. Get the two types of feedback straight here, because almost every later topic — climate, populations, eutrophication — is an application of them.
📘 What you need to know
Equilibrium is a state of balance between the components of a system.
Steady-state equilibrium: no major change over a long period, despite small oscillations. Most open systems in nature.
Static equilibrium: no inputs or outputs, so no change at all. No natural system is in static equilibrium.
In stable equilibrium a system returns to its original state after a disturbance; in unstable equilibrium even a small disturbance shifts it to a new state.
Negative feedback counteracts change and stabilises a system.
Positive feedback amplifies change and destabilises a system, driving it towards a tipping point.
Daisyworld is the model showing how life can regulate planetary temperature through negative feedback.
What equilibrium means
An equilibrium is a state of balance occurring between the separate components of a system. Open systems such as ecosystems usually exist in a stable equilibrium, meaning they generally stay in the same state over time and can return to that state after a disturbance. This balance is maintained by stabilising negative feedback loops.
Look at the middle graph: a flat line means no inputs or outputs at all, which is why no living system can ever be in static equilibrium.
Steady-state equilibrium
The main type of stable equilibrium is steady-state equilibrium. The system shows no major change over a long time period, even though small oscillating changes happen over shorter periods. Those fluctuations stay within closely defined limits, and the system always returns to its average state.
Most open systems in nature are in steady-state equilibrium. A forest has constant inputs and outputs of energy and matter that vary over time, so populations of species rise and fall in the short term — but the forest as a whole stays stable in the long term.
Static equilibrium
In static equilibrium there are no inputs or outputs of energy or matter, so the system shows no change over time. No natural system is in static equilibrium, because all natural systems have inputs and outputs. Inanimate objects such as a chair or a desk are the usual examples.
Unstable equilibrium
A system can also be in unstable equilibrium, where even a small disturbance causes it to shift suddenly to a new system state or average state — a new equilibrium.
Feedback loops
Most systems involve feedback loops. These are the mechanisms that make a system react to disturbances, and they are what allow systems to self-regulate. Changes to processes in a system alter its outputs, which in turn affect its inputs.
The red dot marks the same disturbance on both graphs. Everything after it is the feedback loop doing its work.
Negative feedback
Negative feedback is any mechanism in a system that counteracts a change away from equilibrium. The output of a process inhibits or reverses that same process, bringing the system back to its average state. Negative feedback is therefore stabilising.
Predator-prey cycles rely on negative feedback to keep populations relatively stable over time.
Cloud formation in the hydrological cycle is controlled by negative feedback mechanisms.
Positive feedback
Positive feedback is any mechanism that leads to additional and increased change away from equilibrium. The output of a process feeds back into the system in a way that moves it further from its average state. Positive feedback is destabilising: it amplifies deviation and drives systems towards a tipping point, where the system state suddenly shifts to a new equilibrium.
Melting ice caps: global warming melts ice, less ice means less reflection, so more heat is absorbed and more ice melts.
Thawing permafrost: warming releases greenhouse gases from permafrost, which drives further warming.
Positive feedback can amplify a change in either direction — up or down:
Direction
The loop
Population decline
Population falls → reproductive potential drops → population falls further. The decline accelerates
Population growth
Population grows → reproductive potential rises → population grows further. The expansion accelerates
🧩
Negative cancels, positive adds
Think of the maths, not the mood. Negative feedback subtracts from the change and cancels it out. Positive feedback adds to the change and makes it bigger. A runaway population crash is positive feedback, even though nothing about it is good.
The Daisyworld model
James Lovelock and Andrew Watson created Daisyworld as a computer simulation in the 1980s. It models a theoretical planet with only two organisms: black daisies and white daisies, which affect the planet’s albedo — how much solar radiation it reflects away.
Follow the arrows: each state creates the conditions that favour the opposite state. That is what a negative feedback loop looks like drawn as a circle.
As solar luminosity increases, black daisies thrive because they absorb more sunlight. This decreases albedo, trapping more heat and raising global temperature — which makes the planet more habitable for white daisies. As white daisies spread, albedo increases and temperature falls. The two populations compete and eventually reach a stable steady-state equilibrium that keeps the surface temperature suitable for both.
On a dead planet with no daisies there are no such mechanisms. Without organisms adjusting albedo, the climate becomes progressively more extreme — too hot or too cold depending on starting conditions — producing a planet that cannot sustain life.
Worked examples
WE 1
Types of equilibrium
Distinguish between steady-state and static equilibrium. (3 marks)
Steady-state
The system shows no major change over a long period, but there are small oscillating changes over shorter periods, always returning to the average state.
Static
There are no inputs or outputs of energy or matter, so the system shows no change at all over time.
The key contrast
Most natural open systems, such as a forest, are in steady-state equilibrium. No natural system is in static equilibrium, because all have inputs and outputs.
Steady-state fluctuates around an average; static does not move at allsaying “no natural system is static” is a marking point on its own
WE 2
Feedback in climate
Explain how melting ice caps act as a positive feedback loop. (4 marks)
Step 1: the initial change
Rising global temperatures cause polar ice to melt.
Step 2: the consequence
Ice has a high albedo, so as it is lost, less solar radiation is reflected and more is absorbed by the darker ocean and land beneath.
Step 3: the loop closes
Greater absorption raises temperatures further, which melts more ice.
Step 4: why it is positive feedback
The output of the process amplifies the original change rather than counteracting it, so the system moves further from equilibrium and towards a tipping point.
Warming causes melting, and melting causes more warmingclose the loop explicitly — a feedback answer must return to where it started
WE 3
Daisyworld
Outline how the Daisyworld model demonstrates negative feedback. (4 marks)
Step 1: the set-up
Daisyworld is a simulation of a planet with only black and white daisies, which differ in how much solar radiation they reflect, and so affect the planet’s albedo.
Step 2: warming phase
As solar luminosity rises, black daisies thrive and absorb more sunlight. Albedo falls, more heat is trapped, and temperature rises.
Step 3: cooling phase
Higher temperatures favour white daisies, which reflect more sunlight. Albedo rises and temperature falls again.
Step 4: the outcome
Each change triggers the response that counteracts it, so the populations settle into a steady-state equilibrium that stabilises surface temperature.
Each daisy type creates the conditions that favour the other, holding temperature steadycontrast with a dead planet, whose climate drifts to an extreme, for the final mark
💡 Exam tips
Use the exact words: negative feedback counteracts change; positive feedback amplifies it.
Say stabilising and destabilising. They are the terms mark schemes use.
When explaining a feedback loop, close the loop by returning to the starting change.
Remember positive feedback can amplify a decrease as well as an increase.
Know Daisyworld including the albedo mechanism and the dead-planet contrast.
State that no natural system is in static equilibrium.
⚠ Common mistakes
Reading negative as bad and positive as good. They describe direction, not desirability.
Calling a population crash negative feedback. If the decline accelerates itself, it is positive feedback.
Saying steady-state means unchanging. It fluctuates; it just returns to the average.
Describing an ecosystem as being in static equilibrium. It has inputs and outputs, so it cannot be.
Leaving a feedback explanation open-ended. Without returning to the start it is a chain, not a loop.
Forgetting albedo in Daisyworld. Albedo is the mechanism that makes the model work.
Up next: Resilience and Tipping Points. Positive feedback pushes systems towards a threshold. The next page is about where that threshold sits, what decides how far a system can be pushed, and what happens once it crosses.
Want this explained one-to-one?
Book a free session with an experienced IB ESS tutor and get your trickiest topics made simple.
