Nature's Unifying Patterns

Detailed descriptions and examples

To be attuned to their environment, organisms and ecosystems need to receive information from the environment and be able to act appropriately in response to that information. This includes sending and receiving signals to and from other organisms or even within the body of an organism. This system of send, receive, and respond has been finely tuned through millions of years of evolution. Some living systems work within narrow ranges of optimal conditions, so they need to constantly monitor their environment and respond. Others have broader ranges, but still need to be able to detect and respond when conditions are such that they approach their limits (e.g., maximum survivable temperature or oxygen availability). Using feedback loops is one way to monitor those conditions. Both negative feedback loops (those that slow down a process), and positive feedback loops (those that speed up a process) are important in natural systems.

Biology Examples

 

Songbirds

Young songbirds have brightly colored mouths, including some bright coloration around the edge of the beak, which disappears as they mature. A chick opens its mouth wide to show the color, creating a bright target and signal to its parent that it’s hungry. It supplements this with raucous calls. The hungriest chick makes the most noise and opens its mouth the widest. This stimulates the parent to feed the one most in need. As the chick starts feeling full, it sends a less strong signal, or stops signaling completely, causing the parent to react to the next chick who is signaling the most. This is a feedback loop that takes place repeatedly throughout the day.

Acacia tree

Giraffe browsing on an acacia tree.

On the African plains, groups of acacia trees have evolved a simple yet sophisticated sensory detection strategy to respond to threats from herbivores. When a giraffe, for example, begins browsing the leaves of an acacia tree, the acacia emits ethylene gas to warn the other acacia trees. Those trees receive this signal and respond, as part of a feedback loop, by also releasing ethylene gas, thereby warning other trees nearby. Detection of the ethylene also signals the acacias to manufacture and deliver a toxin, such as hydrogen cyanide, in their leaves, as part of a feedback response to further deter the herbivores.

Design Applications

Bullitt Center

Rather than controlling temperature using a standard energy guzzling HVAC system, the Bullitt Center in Seattle, WA, uses feedback to control indoor temperature in a more efficient way. The building features Schüco windows that open and close automatically, responding to inside temperature. (They also open if CO2 concentrations become too high inside from people exhaling.) Occupants can manually override the windows, providing another feedback system in case there are outside noises or other signals the windows are not attuned to. Occupants can monitor their own energy use and thus find ways to modify and reduce their usage. The system also includes exterior sunshades that roll down if temperatures exceed the desired level.

Nike Fuel Band

Nike makes a variety of products in the Nike+ line that depend on feedback loops. The Fuel Band is essentially a watch combined with an accelerometer that tracks the wearer’s daily activity, including running, walking, basketball, dancing, and dozens of everyday activities. Built-in software measures calories burned and provides that information as feedback to the user via an LED display that lights up from red to green, showing the user’s progress throughout the day. An interactive app allows a great range of data analysis and feedback, helping the user achieve personal fitness goals.

Pandora

The days of listening to a record album from start to finish are over. Now you can call up a song on a program like Pandora, and Pandora will customize a “station” for you based on the underlying harmonic and thematic qualities of the music you’ve chosen. As you listen to the selected songs, you can give the songs a “thumbs-up” or a “thumbs-down,” thereby further customizing the listening experience. The system behind the customization incorporates this feedback, helping the system to adjust to better market it music. As the listener, you also get to learn more about your musical preferences as Pandora introduces you to new music through the virtual “disc jockey.”

Regen Energy

Diagram of the Regen system from the company’s website.

Regen Energy has developed a device that allows electrical appliances in a building to communicate with each other to minimize how much power the appliances collectively use at a given point in time. The device must satisfy any local constraints (e.g., a refrigerator must cycle on to maintain a minimum temperature) but simultaneously satisfy the system objective to reduce peak load. Rather than using a top-down approach, this decentralized approach to energy management offers a more effective means to manage supply and demand in a delicately balanced electricity system.
Image Credits

Baby birds: Ken Slade, CC-BY-NC
Acacia and giraffe: Baron Reznik CC-BY-NC-SA
Bullitt Center: Joe Mabel CC-BY-SA via Wikimedia
Nike Fuel Band: Peter Parkes CC-BY
Regen Energy:  via Regen company website

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