4.5. PREYS AND PREDATORS. RABBITS AND FOXES This model gathers the dynamics of two typical populations of prey and predator. We will use this example to see if it is possible to create a model without having drawn the causal diagram. For this we can start with a type of problem "the population of rabbits has important oscillations that we want to reduce". A subsequent investigation has demonstrated that foxes are the cause of this oscillation. Here is a list of elements that can be related to a problem of this type: - population of rabbits - birth of foxes - populations of foxes - decrease in the number of foxes - birth of rabbits - life expectancy of foxes - death rate of foxes - birth rate of foxes - life expectancy of rabbits - dietary requirements of foxes - birth rate of rabbits - sustainable population of rabbits A good way of identifying the levels of a system is to make a mental photograph of the system and assign the characteristic levels to those elements that appear in the image. In this case, the populations of foxes and rabbits would be the levels. The fluxes are temporary variations in the levels and here we will consider the births and decreases in the populations of foxes and rabbits. The rest of the elements are auxiliary variables. In our case we are going to design the model so that it reproduces a stable situation of the populations of foxes and rabbits and then we will simulate the effect of a minor change in one of the variables, namely the birth of rabbits. In this way we can see with clarity if the structure of the system decreases or increases this small change or if oscillations are created. 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