Systems thinking is the tool ESS hands you in the first week and then uses in every topic afterwards. Ecosystems, the carbon cycle, a city, a fishery, the whole planet — all of them get drawn the same way, out of storages and flows.
📘 What you need to know
A systems approach simplifies a complicated set of interactions so they can be understood.
Systems can be environmental, social or economic.
A reductionist approach studies the parts separately. A holistic approach studies the whole.
A system is made of storages and flows. Flows provide inputs and outputs of energy and matter.
Flows are either transfers (no change in form) or transformations (form or quality changes).
In a systems diagram, storages are boxes or circles and flows are arrows.
Emergent properties appear from the interactions between components; the components alone do not have them.
What a systems approach is
A systems approach is a way of simplifying and understanding a complicated set of interactions. The interactions can be environmental or ecological (the water cycle, predator-prey relationships), social (how we live and work) or economic (financial transactions, business deals).
DefinitionSystems approach — a method of simplifying and understanding a complicated set of interactions by treating them as a system of storages and flows.
Two ways to study a system
The circles are identical in both panels. The only difference is whether you draw the lines between them.
A reductionist approach breaks a system down into its parts and studies each individually. Useful for examining specific interactions in detail, but it does not show what is happening in the system as a whole.
A holistic approach looks at all of the system’s processes and interactions together.
Sustainability is the standard example of why the holistic view is needed. Sustainable development depends on a highly complex set of interactions between environmental, social and economic factors — sometimes called the three pillars of sustainability. You cannot understand it by studying any one pillar alone.
Storages and flows
Every system is made up of storages and flows. The flows provide the inputs and outputs of energy and matter, and each flow is either a transfer or a transformation.
Transfer
Transformation
What happens
Matter or energy moves from one component to another with no change in form or quality
Matter or energy changes form or quality as it moves
Example
Water flowing from a river into a lake
Sunlight absorbed by a plant and converted to chemical energy in photosynthesis
On a diagram
Arrow usually labelled with the quantity moved (kg of carbon, kJ of energy)
Arrow may carry the name of the process (photosynthesis, respiration)
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Trans-FER or trans-FORM
A transfer just ferries something from A to B unchanged. A transformation changes its form. If you can name the same substance in the same state at both ends, it is a transfer.
Transfers and transformations happen at every scale, from nutrients moving between individual organisms up to energy moving between whole biomes. Identifying them is what lets you spot where a system could be made more efficient or more sustainable.
Drawing a systems diagram
Systems are usually drawn as simplified diagrams: storages as shapes with defined boundaries such as boxes or circles, and flows as arrows showing the inputs and outputs. The size of shapes and arrows can be drawn in proportion to the size of the storage or flow, though often they are not.
If you are asked to draw a system in the exam, keep the boxes and arrows the same size unless the question says otherwise.
Emergent properties
When you look at a system as a whole, the interactions inside it produce emergent properties. These are properties of the system that appear as the components interact — the components themselves do not have them.
DefinitionEmergent properties — properties of a system that arise from the interactions between its components, and which the individual components do not possess.
A reductionist study of each species separately would never predict a predator-prey cycle. The cycle exists only in the connections.
In an ecosystem, all the ecological interactions taking place shape how that ecosystem looks and behaves. Change the interactions — introduce a new predator, for instance — and the emergent properties change too. Predator-prey cycles and trophic cascades are the standard examples: patterns of change that would not occur in the isolated components.
Worked examples
WE 1
Transfer or transformation?
Distinguish between a transfer and a transformation, giving one example of each. (4 marks)
Transfer
The movement of matter or energy from one component of a system to another without any change in form or quality.
Example
Water flowing from a river into a lake: it is water at both ends, in the same state.
Transformation
The movement of matter or energy that does involve a change in form or quality.
Example
Sunlight absorbed by a plant and converted into chemical energy through photosynthesis.
Transfer moves it; transformation changes itthe phrase “change in form or quality” is what earns the definition mark
WE 2
Emergent properties
Explain what is meant by an emergent property, using an ecological example. (3 marks)
Step 1: the definition
An emergent property is a property of a system that appears as its components interact, and which the components themselves do not have.
Step 2: the example
A predator-prey cycle is emergent: neither population produces a cycle on its own, but their interaction generates repeating oscillations.
Step 3: why it matters
If the interactions change, for example when a new predator is introduced, the emergent properties of the ecosystem change with them.
The property lives in the interactions, not in the partstrophic cascades work equally well as the example
WE 3
Choosing an approach
Suggest why a holistic rather than a reductionist approach is needed to study sustainable development. (3 marks)
Point 1: what the two approaches do
A reductionist approach breaks the system into parts and studies each individually; a holistic approach studies all the processes and interactions together.
Point 2: the nature of the problem
Sustainable development depends on a complex set of interactions between environmental, social and economic factors, the three pillars of sustainability.
Point 3: why reductionism fails here
Studying any one pillar alone would miss how they combine, so the behaviour of the whole system would be invisible.
The interactions between the pillars are the thing being studiedname the three pillars explicitly — it is a marking point
💡 Exam tips
Learn the definitions of transfer and transformation word for word, with one example each.
In a systems diagram, draw storages as boxes and flows as arrows, and label every arrow.
Unless told otherwise, keep boxes and arrows the same size.
Use the term emergent property for behaviour that only appears in the whole system.
Remember systems can be environmental, social or economic, not only ecological.
Name the three pillars of sustainability when a question involves sustainable development.
⚠ Common mistakes
Calling photosynthesis a transfer. The form of energy changes, so it is a transformation.
Leaving arrows unlabelled. An unlabelled flow earns nothing.
Saying reductionism is wrong. It is useful for detail; it just cannot show the whole.
Confusing a storage with a flow. Storages hold; flows move.
Treating emergent properties as just “the total”. They are new behaviours, not sums.
Assuming systems must be ecological. Social and economic systems count too.
Up next: Open, Closed and Isolated Systems. You can now draw a system. The next question is what crosses its boundary — and that single question sorts every system in the course into one of three types.
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