Positive and Negative Feedback

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.

Predator prey graph

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

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.

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

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!)

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

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

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

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

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

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

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

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