NetLogo User Community Models

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## WHAT IS IT?

This model is an ecological competition model between two species competing for the same resource. In this model, the patches represent a food resource (grass).

The model is an agent-based modelling approach to the competitive interaction of species for a resource, so is an alternative modelling approach to the equation-based Lotka-Volterra competition model [1].

## HOW IT WORKS

There are two types of agent in the model, with type representing species or subspecies. The patches included in the model denote the resource which the species are competing for. Patches can be consumed by the agents, with agents gaining energy and the patch subsequently turning brown. Patches can be regrown. Agents can reproduce and will expire if they run out of energy.

Numbers of each agent type (species) and numbers of consumable patches are plotted as a function of time. Energies of agent can be shown/hidden via the toggle switch.

## HOW TO USE IT

Click the 'set-up' button to initialise the model and 'go' to run the model. Set the parameters for the two species for birth rate (rate of reproduction) and energy gain from consuming the food resource. Initial numbers of the two species and time for resource regeneration can also be set.

## THINGS TO NOTICE

When running the model, the two species can coexist for a time, if no differences between the two occur (i.e the two species are identical). If there are differences between the two species, one of the species will drive the other to extinction. This furthers the principle of an competitive exlusion made by Gause, which states that two species competing for the same resource cannot coexist at constant population values. One will drive the other to extinction, unless the species can find an ecological niche (see [3]).

Notice that the numbers of the two species fluctuate with the numbers of consumable patches. The carrying capacity is the total number of agents that the food resource can support.

## THINGS TO TRY

The model can be tested with different initial conditions. Can coexistence occur? Does patch regrowth rate effect the outcome.

## NETLOGO FEATURES

A key aspect of the model is that neither of the species gets priority when consuming the resource. If the behaviours had been done systematically, this would have given a species preferential treatment, meaning that species would always win. We use and agentset to randomise the order of actions at each timestep.

## RELATED MODELS

This model is an adaptation of the Wolf-Sheep model created by Wilensky [2], included in the NetLogo model library, which defined a predator-prey relationship - another model of interacting species.

## CREDITS AND REFERENCES
[1] A.J. Lotka. 1925. Elements of Physical Biology. Williams and Wilkins, Baltimore.

[2] Wilensky, U. (1997). NetLogo Wolf Sheep Predation model. http://ccl.northwestern.edu/netlogo/models/WolfSheepPredation. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.
https://ccl.northwestern.edu/netlogo/models/WolfSheepPredation

[3] https://www.sciencedaily.com/terms/competitive_exclusion_principle.htm