Feedback Loops

A feedback loop occurs when two things change together repeatedly.

Examples:

 * I eat one donut on Monday. On Tuesday I crave sugar, so I eat TWO donuts. On Wednesday I really crave sugar, so I eat THREE donuts. Etc.
 * Carbon in the atmosphere warms climate. Permafrost thaws at the poles, releasing more hydrocarbons into the atmosphere. The climate warms, etc.
 * I do origami for fun and give the models to people. People give me origami paper. I make more models and give them to more people, etc.

Pop Culture Note

 * In casual conversation, people often use the phrase, "vicious cycle" to describe a feedback loop.
 * From what I can tell, this was popularized by quoting a scene from a movie released in 1999 which... did not age well.
 * Ironically, the movie scene describes a POSITIVE FEEDBACK LOOP, but it concerns an unhappy topic, so the phenomenon is characterized as a "vicious cycle".
 * This leads to odd conversations 20 years later, e.g., "It's like a vicious cycle! But, like, not bad, you know?"
 * Using the term feedback loop is much easier and more practical.

Cause and effect are typically difficult to discern in deep time research (fossils, ancient life, etc.). Realistically, most natural events that we characterize as having a simple cause and effect involve much more complex feedback loops.

This class will focus on the Mesozoic-Cenozoic transition to appreciate a major positive feedback loop in Earth history.

Dynamic Equilibrium
This describes systems that tend to settle into a balance.

Casual example:


 * 1:00 PM: Few shoppers are trying to check out at Smiths, so the manager tells some staffers to go on break.
 * 1:10 PM: More shoppers line up, and now there are two long lines.
 * 1:20 PM: As staffers come off of break, the manager sends them to registers.
 * 1:30 PM: The two long lines of shoppers are spread out across five check-out stations.
 * 1:40 PM: Two of the staffers have no one in their line, so the manager asks one to get carts and another to take lunch.
 * 1:50 PM: It's pretty easy to check out at Smiths. Thanks, dynamic equilibrium!

Dynamic means shifting around and responding to stuff.

Equilibrium means balance.

A system with dynamic equilibrium will get out of balance one way, then another, and kind of settle on a balance.

Some systems are always shifting back and forth - the supermarket is quiet, then busy, etc. But on the whole, the system does not get so out of balance in any one direction that it breaks apart.

Many dynamic equilibrium systems have a feature like a buffer to mitigate the responses between the different factors. In the example above, the manager is able to buffer the interaction of shoppers and staffers.

Science example.

Giant kelp is a tall algae that grows into large patches, called forests, along the pacific coast of North America.

Kelp forests are home to spiny purple sea urchins, sea bass fish, and carnivorous mammal sea otters.


 * Urchins eat the kelp. Otters eat the urchins.
 * If the kelp forest grows too small, the urchins are easy to spot and get eaten by otters.
 * If the kelp forest grows large, more urchins can hide there, and they eat the kelp.
 * The size of any kelp forest is always changing, but if you fly over the coast there would be about the same total amount of kelp growing along the coast.

This interaction between kelp and urchins is mitigated by otters.

Positive Feedback Loop
This describes a system where at least two features reinforce each other, growing and growing. This system will not automatically find a balance.

The actual system does not need to involve things that have positive emotional connotations - this is not about how the system FEELS.

Science Example:


 * Humans hunt sea otters (they make great hats!!).
 * Sea urchins grow unchecked, and gnaw through the narrow anchor points of giant kelps.
 * The giant kelps float away, leaving behind an "urchin barren": rocks teaming with spiky purple urchins, and no kelp forest.
 * Urchins graze on short algae that easily grow on the rocks in sunlit water.
 * Urchins continue to go berserk, and obliterate kelp forests up and down the coast.

This is a postive feedback loop. It might sound sad (poor kelp!) or happy (yummy urchins!), but it is OUT of EQUILIBRIUM because more urchins leads to less kelp leads to more urchins leads to less kelp.

Social Example:


 * Sub-prime mortgage lending.
 * Subprime crisis - Foreclosures & Bank Instability.pngSubprime crisis - Foreclosures & Bank Instability.png

Negative Feedback Loop
This describes a system where at least two features reduce each other, shrinking and shrinking in their impact on the system. This system will not automatically find a balance.

The actual system does not need to involve things that have negative emotional connotations - this is not about how the system FEELS.

Science Example:


 * I am cold.
 * I shiver.
 * I am less cold.
 * I shiver a little less.
 * I am fine. Thanks, negative feedback loop!

Illustration via wiki media: