1. Positive and Negative Feedback

2. Negative Feedback Mechanisms
Maintain a system at a stable equilibrium.
Example: A predator – prey relationship:
The rabbit population in a hillside increases, the foxes eats the rabbits, and the fox population also increases. The rabbit population decreases as a result. The fox population decreases as well.

3. Predator prey graph

4. More on negative feedback mechanisms
Ability to sense output in system
Influence a process in the system
Return the system to a steady-state
Observe cyclic fluctuations over time
A time lag can often be observed
Provides stability over time

5. Positive Feedback Mechanisms
The change is always in the same direction.
It may push a system to a new state of equilibrium
Example human population growth:
As the infant mortality decreased, health care improved and food supply improved, more children lived to be adults, who then had large families.

6. More on positive feedback
Positive feedback can be thought of a vicious cycle
Tends to destabilize the system
The change is always in the same direction, away from the steady state

8. Positive or negative feedback?
As Earth warms:
Ice cover melts, exposing soil or water
Albedo decreases (Albedo = reflectivity)
More energy is absorbed by Earth’s surface
Global temperature rises
More ice melts
Albedo = reflectivity
White reflects better than black! (Know this term!)

10. Positive or negative feedback?
As carbon dioxide levels in the atmosphere rise:
Temperature of Earth rises
As Earth warms:
The rate of photosynthesis in plants increases
More carbon dioxide is therefore removed from the atmosphere by plants, reducing the greenhouse effect and reducing global temperatures

11. Positive or negative feedback?
Poverty leads to illness and poor educational standards
There is a lack of family planning available
The population continues to increase
The poverty increases

12. Positive or negative feedback?
As Earth warms, upper layers of permafrost melt, producing waterlogged soil above frozen ground:
Methane gas is released
Greenhouse effect is enhanced
Earth warms, melting more permafrost

13. Positive or negative feedback?
Stream system:
Heavy rainfall increases stream velocity
The increased stream velocity increases channel erosion
The width of the stream increases
A wider stream has greater bed friction, which decreases stream velocity

14. Positive or negative feedback?
As Earth warms, evaporation increases.
More clouds are formed.
Clouds increase albedo, reflecting more light away from Earth
Temperature falls
Rates of evaporation fall

15. Positive or negative feedback?
As Earth warms, organic matter in soil is decomposed faster
More carbon dioxide is released
Enhanced greenhouse effect occurs
Earth warms further
Rates of decomposition increase

16. Positive or negative feedback?
As Earth warms, evaporation increases:
Snowfall at high latitudes increases
Icecaps enlarge
More energy is reflected by increased albedo of ice cover
Earth cools
Rates of evaporation fall

17. Positive or negative feedback?
As Earth warms, polar icecaps melt releasing large numbers of icebergs into oceans:
Warm ocean currents such as Gulf stream are disrupted by additional fresh water input into ocean
Reduced transfer of energy to poles reduces temperature at high latitudes
Ice sheets reform and icebergs retreat
Warm currents are re-established