In 1991, the Washington, D.C. area experienced a major increase in the number of gray squirrels, due to an unusual amount of acorn production in previous years. With so many squirrels, a large number were forced to leave the area; most of those were run over by cars on the streets.

The Computer Modeling and Simulation class used this as the basis for a STELLA^TM modeling project. We created a basic model of the factors affecting squirrel population, ran it with test data of average acorn production over a period of ten years in order to make sure the model was set up correctly, then used real-world data to simulate the squirrel population from 1989 to 2010.

The simulation began with 200 squirrels, 100 adults and 100 yearlings, in a 100-acre forest. From 1993 to 2000, urban development caused the area of the forest to decrease from 100 to 50 acres. In 1989, the weather was extremely good, resulting in twice as many acorns as normal being produced; in 1993, severe weather resulted in an unusually low number of acorns produced.

• Full model diagram
  • Adult squirrel population submodel
  • Yearling population submodel
  • Food production/consumption submodel
• Equations used in the model
  • Adult squirrel population equations
  • Yearling population equations
  • Food equations
• Graph of model results