What's a Model and how do I read it?

A model is simply a tool for helping someone think about how the world works.  I use a free educational software called Vensim PLE from Ventana Systems Inc. (www.vensim.com).  This software is very easy to  use, and allows me to think about relationships between birds, birders, and the various activities associated with birding.

Here are some definitions and helpful hints for reading the models in my blog posts. 

A box containing words is called a stock.  Stocks are things that can be counted (like birds or birders) or things that can be measured in some way (like level of frustration or level of credibility).  The box surrounding the words is intended to help the reader think of a stock as either filling up or draining out.

A flow indicates an action is happening.  Flows can occur by themselves in the model as shown to the right.  Most of the time, however, flows are connected to a stock as shown below. 

A flow with the arrow touching (entering) the stock indicates a pathway through which the level of the stock increases.  A flow with the arrow facing away from (leaving) the stock indicates a pathway through which the level of the stock decreases.  I try to use words with teh flows that will make it easy on the reader to understand what is going on in the model.  In the example below, I use the word "coming" to indicate birds coming into a given patch, and I use the word "going" to indicate birds leaving that patch.

In a few situations, you may notice that two or stocks are connected together with flows.  This is called a stock chain.  What this means is that things in the first stock move into the second (and subsequent) stocks over time (depending on certain conditions being met that are described inthe model).  Below iI show an example.  Note in this example that the number of things (i.e., types of birds) in the second stock is less than the number of things (i.e., types of birds) in the first stock, and that the third stock contains even fewer things (i.e., types of birds). 

Straight or curved arrows are used to indicate when one component in the model influences or affects another in some way.  In the example on the right, things like migration and daily movements of birds affect the number of different types of birds that occur in a given patch at any given time.