What is a system?

• Systems are sets of interacting or interdependent components organized into a functional whole.

• A systems approach is a holistic way of viewing complex environmental or social situations.

• In ESS, systems are described using storages and flows of energy and matter.

• Think in terms of parts + interactions + overall behaviour rather than isolated facts.

System diagrams: storages, flows, inputs and outputs

• In system diagrams, storages are usually shown as rectangular boxes.

• Flows are shown as arrows; arrow direction shows the direction of movement.

• Flows may be inputs, outputs or internal transfers within the system.

• Box size and arrow size may be used to represent the magnitude of a storage or flow.

• ESS exam skill: be able to draw and interpret a system diagram for an ecosystem or environmental issue.

This diagram shows a classic stock-and-flow model: storages are held in compartments and changed by flows in or out. It is useful for ESS because it matches how system diagrams represent storages and flows. Use it to remember that arrows change the size of boxes over time. Source

Transfers and transformations

• Flows are processes that may be either transfers or transformations.

• Transfers = a change in location of energy or matter.

• Transformations = a change in chemical nature, state or energy form.

• Quick distinction: movement = transfer; change = transformation.

• Common exam task: identify whether a named process is a transfer or a transformation.

Open and closed systems

• Open systems exchange both energy and matter across their boundary.

• Closed systems exchange energy only, not matter, across their boundary.

• Most systems are open systems.

• Global geochemical cycles are the closest natural examples to closed systems.

• Biosphere 2 is an example of a closed system; a local ecosystem is an example of an open system.

Earth as an integrated system

• The Earth can be viewed as one integrated system made up of the biosphere, hydrosphere, cryosphere, geosphere, atmosphere and anthroposphere.

• The Gaia hypothesis models Earth as a single integrated system.

• Gaia emphasizes that atmospheric composition and temperature are linked through feedback control mechanisms.

• Key idea: changes in one Earth sphere can trigger effects in others.

These Daisyworld plots show how life can help regulate planetary temperature through feedback. The graph compares temperature on a living planet with a barren one, helping explain the Gaia hypothesis and negative feedback. It is especially useful for ESS questions about how systems can self-regulate. Source

Scale in systems

• The concept of a system can be applied at a range of scales.

• Examples include a small local ecosystem, a large biome or rainforest, or a global Earth system.

• ESS questions may move between local, regional and global scales.

• Always identify the boundary and scale of the system being discussed.

Negative feedback and stability

• Negative feedback loops happen when the output of a process inhibits or reverses that process.

• They are stabilizing because they counteract deviation and reduce change.

• In ecosystems, negative feedback often helps maintain equilibrium.

• An open system such as an ecosystem commonly exists in stable equilibrium.

• Stable equilibrium = the system tends to return to its previous state after disturbance.

• Steady-state equilibrium = inputs balance outputs even though flows continue.

• ESS must-know example: Daisyworld, showing temperature regulation on a planet with life.

This diagram shows the structure of a negative feedback loop: change triggers responses that push the system back toward a stable condition. Even though the example is biological, the same loop logic applies to environmental systems. Use it to revise why negative feedback is stabilizing. Source

Positive feedback and tipping points

• Positive feedback loops happen when a disturbance causes amplification of that disturbance.

• They are destabilizing and drive the system away from equilibrium.

• Positive feedback can increase or decrease a system component, as long as the change is self-amplifying.

• Example: melting ice lowers albedo, causing more heat absorption and further melting.

• Example: a declining population may have lower reproductive potential, causing further decline.

• Positive feedback loops tend to push systems towards a tipping point.

• A tipping point is the minimum amount of change that causes destabilization and a shift to a new equilibrium or stable state.

• Tipping points can produce regime shifts between alternative stable states.

• Named syllabus example: increased nitrate/phosphate concentrations leading to eutrophication.

This figure shows a system being pushed past a tipping point from one stable state into another. The ball-and-valley model is a clear way to picture stability, loss of resilience and sudden regime shift. It is ideal for ESS answers explaining why small changes can sometimes cause large system-wide effects. Source

Models in ESS systems

• A model is a simplified representation of reality.

• Models help us understand how systems work and predict responses to change.

• Models can be graphs, diagrams, equations, simulations or verbal descriptions.

• All models involve simplification, so they involve approximation and some loss of accuracy.

• This means model predictions can differ from what happens in real ecosystems.

• ESS exam point: always link a model’s usefulness with its limitations.

Emergent properties

• Emergent properties arise from interactions between components in a system.

• These properties are not found in the components when viewed in isolation.

• Examples include predator–prey oscillations and trophic cascades.

• Exam idea: the whole system can show patterns that individual parts do not.

Resilience, diversity and time lags

• Resilience is a system’s tendency to avoid tipping points and maintain stability.

• It is the capacity to resist damage and recover from or adapt to disturbance.

• Diversity generally increases resilience because a wider range of components can maintain function after disturbance.

• The size of storages also matters: larger storages usually give slower responses and greater stability.

• This creates time lags, where the effect of change is delayed.

• Example: a lake is usually more stable than a puddle because it has a larger storage.

• Human activities can reduce resilience by lowering diversity and shrinking storages.

• ESS example: deforestation reduces storages and biodiversity, making systems less resilient.