4.1. POPULATION GROWTH This chapter focuses on the mechanics of getting such a model up and running, and reviewing the results. This chapter helps you get started by: - Moving step-by-step through an example. - Detailing many key functions and activities. - Providing a starting point for further application and exploration. This chapter describes a set of steps you can take to build and analyze a simple model, which lets you try out some of the most important features of Vensim. THE PROBLEM The problem posed is to develop a model of population growth in a situation where are no restrictions on growth. It is a representation of the natural rate of growth in a species population when there are no predators, and no restrictions on habitat, food supply or water supply. The idea underlying the model is simple. Births and deaths are proportional to population, and population is the accumulation of births less deaths. Not surprisingly this is a simple model to build and understand. THE MODEL This model features a simulation model of a rabbit population. The modelling process starts with sketching a model, then writing equations and specifying numerical quantities. Next, the model is simulated and simulation output saved to a dataset. Finally, the simulation data can be examined with analysis tools to discover the dynamic behavior of variables in the model. A system dynamics simulation model is solely determined by the equations that govern the relationships between different variables. The full equation listing completely describes a Vensim model. The structural diagram of a model is a picture of relationships between variables. Graphic system dynamics models should be clearly presented to facilitate model building, analysis, and presentation. LINKS: Theory and Practical Exercises of System Dynamics System Dynamics Online Courses Building Models in Ecology Building models with Vensim Other resources in System Dynamics Boletín de Dinámica de Sistemas