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view/download model file: the_three_species.nlogo
The 3 species model is an experimental model, done by Stylianos Kampakis to explore the possibilities of NetLogo. It is actually a very simple idealizied biological model, where 3 different species, with many parameters, fight for survival.
First of all, you have to see that there 3 species. Sugarmen, meatmen and grassmen. Sugarmen are butterflies and eat sugar, grassmen are cows and eat grass, meatmen are little people and eat meat. Sugar is blue, grass green and meat red. All 3 species have an energy rating which starts at 100. The lose 1 point of energy with each move, and gain energy when they eat.
To eat, they have to step on a patch with the resource they consume. They eat the resource and then the patch turns black.
When their energy levels get double the birth-energy variable, the spawn another identical creature with them. The creature has half the energy they have and their energy is halved as well.
Each type of resource has a probability per round of being spawned on a single patch equal to the probabilities sliders.
The rules of the species for the initial configuration are the following:
1. Move at random one step in 360 degrees angle
2. If you are on a patch with an enemy creature kill it.
3. If you are on a patch with the resource you use, eat the patch, gain energy equal to the energy for the specified resources
4. If you have energy equal to birth-energy x 2, then hatch another creature of your species with half your energy, and then lose half your energy, too.
5. If your energy is lower than 0, die.
In the initial configurations, resources are spawned non-aggresively. That is, they are spawned ONLY on black patches. Note also, that no matter whether the resources spawn aggresively or not, each resource, gets each round either 1 new patch (at most) or none at all. There is no configuration, for example, to spawn 10 new patches of meat at a round.
In the initial configuration, also, agents have no vision and their behavior is random (hunt-resources is turned to off). This means, that they will hunt resources and kill enemies roaming at random. If the hunt-resources is on, then they hunt resources at a distance indicated by vision and and an angle indicated by the respective slider.
There are two sets of variables you can change:
1)The individual species variables
2)The general variables
The individual variables are the following: starting number, birth energy, energy from food, starting resources, probability of each resource spawning each round and which species they attack.
The general variables are these: aggresive resources and hunt resources
The most important of all variables are by far the general variables. The changing of these variables produces the most interesting dynamics.
If you turn both of these off, even though you'll see an interesting set of interactions, in the end the same state is achieved: total extinction.
The model, under this configuration, has a strong stochastic component. Agents move randomly, and resources are spawned randomly each round and at random patches.
When the agents do not hunt resources they can only survive if they fall on them during their random walks. What I witnessed by my experiments is that in the end remains only one species and in the end, it dies too. The reason is simple. If the resources are not aggresive, then each round there is a probability that a patch will spawn one of the three resources, or remain black. With all things equal, it is more probable, that the patch will grow the other resources, rather than remain black.
Therefore, a species, that does not hunt resources, and with resources that do not spawn aggresively, is doomed to extinction.
However, in case that a second species manage to survive for some time, then the two species can survive longer, since they produce an interesting set of dynamics, by allowing more black patches to appear, while the one species could kill the other in order to keep its numbers in track.
Indeed, in this model, should a species numbers get too large, under any configuration, then they are instantly reduced. The limited space, cannot support an unlimited number of agents.
The most interesting dynamics were observed under the hunt resources switch turned on. Even with non aggresive resources, the species could survive for many thousand rounds, each one's population curve following something that seemed like an sine oscillator.
With aggresive resources on and hunt resources on as well, it is self-evident that at least one species could survive forever.
Note that these observations were based on a limited set of runs I did myself. The correct procedure would be to make many runs with many configurations and pass them into mathematica via the MathematicaLink that netlogo provides, in order to draw more safe conclusions. However, this model is experimental and not highly specific and, also, in its first steps, so this was not deemed a priority.
Besides the most interesting cases described above, try playing around with each species seperately, changing which species it kills, how easy it reprodeces etc. and then notice how often it survives or dies.
This model could be extended in two possible directions in my opinion.
1) Make the model more specific. That is, model some more specific procedure, perhaps by taking out some parameters and introducing others. The truth is that the model holds many parameters as I experimented with the possibilities NetLogo offered, but as the parameters get more, this could work in disadvantage of a specific model, making it highly complex.
2) Make the model even more complex, by adding more parameters that could be used in real situations. Even though this would get to complex to represent any real situation, it would be interesting to see the following additions: species getting energy by eating other species, species eating the same resource, species reproducing sexually (not asexually as in the current model) and agent mutations.
The construction of the model was higly helped by the cone primitive in NetLogo which was used to hunt resources.
This model was inspired in part by the Tutorial 3 model and the Wolf Sheep Predation (docked) model.
(C) 2009 Stylianos Kampakis. All rights reserved.
Permission to use, modify or redistribute this model is hereby granted,
provided that both of the following requirements are followed:
a) this copyright notice is included.
b) this model will not be redistributed for profit without permission
from Stylianos Kampakis
Official Site: http://encefalus.com