1. Topic 1
Systems and Models

2. What is a System?
• A system consists of:
– Storages
– Flows (inputs and outputs) (of energy or matter)
– Processes (transfer or transformation)
– Feedback mechanisms (to maintain stability)

3. storage
Energy or
matter in
(INPUT)
Energy or
matter out
(OUTPUT)
TRANSFORMATION
(Feedback mechanism to control input –
and maintain EQUILIBRIUM)

4. Try to Produce a System Diagram

5. Ecosystems
• Most of the systems we will look at are
ecosystems (self contained communities of living
things and their surrounding environment)
• Very large ecosystems which span a fairly stable
climate are biomes
• Many biologists say the whole planet is a single
ecosystem (eg Gaia theory). Some of them call it
a CLOSED ecosytem

6. Open, Closed and Isolated Ecosytems
• Open – matter and energy exchanged to
suroundings
• Closed – only energy exchanged to surroundings
• Isolated – neither matter nor energy is exchanged
to surroundings
• Is the Earth really a closed ecosystem?
• Do isolated ecosystems really exist?
• Can closed ecosystems be created artificially?
(eg Biosphere 2 Project)

7. Biomes and Biospheres
• Biome – An open ecosystem in a
geographically defined area with similar
climatic conditions throughout – eg, desert,
grassland (savannah), tropical rainforest
• Biosphere – A closed ecosytem – generally
made up of a range of biomes (i.e. the entire
Earth)

8. A Climograph
Meanannualtemperature(oC)
Rainfall (mm/year)
-15
-10
-5
-0
5
10
15
20
25
1000 2000 3000 4000 5000 6000
desert
grassland Tropical rainforest

9. The Laws of Thermodynamics
• First Law – Energy can neither be created nor
destroyed, but can only be transformed
• Second Law – Energy is always changed from a
concentrated (useful) form, to a dispersed
(less useful) form – we say that ENTROPY
(disorder) always increases (this means an
isolated system cannot exist – there must be
an input of energy to keep entropy low)

10. Equilibrium
• A system needs to be in equilibrium
• If not, entropy will increase so much the
system will destroy itself by becoming too
disordered
• There a 4 kinds of equilibrium:
– Static
– Steady State
– Stable
– unstable

11. Static Equilibrium
time
State of the system
Note: this is not realistic – it could only
occur in an isolated system

12. Steady State Equilibrium
time
State of the system

13. Stable Equilibrium
time
State of the system
disturbance
Weebles
wobble but
they don’t fall
down!

14. Unstable Equilibrium
time
State of the system
disturbance
He’s going
down!

15. Feedback Mechanisms
• This is a way that the INPUT is affected by the
OUTPUT
• In a stable equilibrium, feedback returns the
equilibrium to its original state
• In an unstable equilibrium, feedback returns the
equilibrium to a different state
• Feedback can be
– POSITIVE – input changes to bring the system to a new
equilibrium
– NEGATIVE – input changes in order to bring the
system back to its original equilibrium

16. Negative Feedback
• Your (stable) equilibrium body temperature is
37oC
• Sensors in the skin detect your skin
temperature is rising (you are in Cancún)
• Show what happens in a system diagram

17. Positive Feedback
• Your (stable) equilibrium body temperature is
37oC
• Sensors in the skin detect your skin
temperature is decreasing (you are locked in a
freezer)
• Your body is unable to maintain its stable
equibilibrium and therefore you enter a state
of hypothermia
• Show what happens in a system diagram

18. Case Studies
predator-prey equilibria