System Dynamics (NetLogo)
Introduction
This page will describe some elements of NetLogo's system dynamics module.
Modeling elements
Like other system dynamics environments, NetLogo diagrams describing the structure of model, includes stocks (levels), in/out flows, and constants and/or variables. More precisely, there four kinds of elements, stocks, variables, flows and links. According to the NetLogo System Dynamics Guide manual:
- A Stock is a collection of stuff, an aggregate. For example, a Stock can represent a population of sheep, the water in a lake, or the number of widgets in a factory.
- A Flow brings things into, or out of a Stock. Flows look like pipes with a faucet because the faucet controls how much stuff passes through the pipe.
- A Variable is a value used in the diagram. It can be an equation that depends on other Variables, or it can be a constant.
- A Link makes a value from one part of the diagram available to another. A link transmits a number from a Variable or a Stock into a Stock or a Flow.
Predation examples
According to Kumar Venkat Predator-Prey Dynamics and Wildlife Management: A System Dynamics Model, a simple two-species system is described by the Lotka-Volerra model:
dN/dt = r * N - s * P * N dP/dt = f * s * P * N – q * P
“N is the prey population (number of individuals or total biomass), P is the predator population, r is the fractional birth rate of the prey population, s is the search efficiency (or attack rate) of the predator, f is the food conversion efficiency of the predator (i.e., how good the predator is at turning food into offspring), and q is the fractional death rate of the predator.”
The system works in the following way as shown in the wolf-sheep model [1] found in the NetLogo library.
N (prey, e.g. sheep) have a natural birth rate which will increase their numbers. Sheep death are caused by wolves eating them. P (predators) population will increase if its food, i.e. the N (the sheep) population, increases. This is implemented with a delay, i.e. eaten sheep is converted into offspring. After a while, the sheep population will decrease which in turn will decrease the predator population.
The following NetLogo simulation diagram shows the interdependencies
Links
- ↑ Wilensky, U. (2005). NetLogo Wolf Sheep Predation (System Dynamics) model. http://ccl.northwestern.edu/netlogo/models/WolfSheepPredation(SystemDynamics). Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.