SystemsOehlke 2009

Negative Feedback MechanismsMaintain 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 mechanismsAbility to sense output in systemInfluence a process in the systemReturn the system to a steady-stateObserve cyclic fluctuations over timeA time lag can often be observedProvides stability over time

Positive Feedback MechanismsThe 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 feedbackPositive 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 waterAlbedo decreases (Albedo = reflectivity)More energy is absorbed by Earth’s surfaceGlobal temperature rises More ice melts

Albedo = reflectivityWhite reflects better than black! (Know this

term!)

Positive or negative feedback?As carbon dioxide levels in the atmosphere

rise:Temperature of Earth risesAs Earth warms:The rate of photosynthesis in plants increasesMore 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 availableThe population continues to increaseThe poverty increases

Positive or negative feedback?As Earth warms, upper layers of permafrost

melt, producing waterlogged soil above frozen ground:

Methane gas is releasedGreenhouse effect is enhancedEarth warms, melting more permafrost

Positive or negative feedback?Stream system: heavy rainfall increases stream velocity The increased stream velocity increases

channel erosionThe 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 EarthTemperature fallsRates of evaporation fall

Positive or negative feedback?As Earth warms, organic matter in soil is

decomposed faster:More carbon dioxide is releasedEnhanced greenhouse effect occursEarth warms furtherRates of decomposition increase

Positive or negative feedback?As Earth warms, evaporation increases:Snowfall at high latitudes increasesIcecaps enlargeMore energy is reflected by increased albedo

of ice coverEarth coolsRates 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 retreatWarm currents are re-established