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## NetLogo User Community Models

## WHAT IS IT?

Klahs, P. and Karsai, I (2014): Population regulation via feedback. Netlogo v. 5.01 simulation. http://ccl.northwestern.edu/netlogo/models/community/Population regulation via feedback.nlogo

This is a simple model to demonstrate the effect of feedback on birth and death to population growth. I use this model to teach students on understanding the effect of feedback (positive and negative) on systems.

## HOW IT WORKS

The simulation works with 2 populations (red and black); they die and propagate according to the rules. The agents are not affecting each other directly.
Death is a simple elimination of the individual (make a patch empty).
Birth is a simple occupation of an empty patch.
Feedback is simply coming from the population sizes. For example, if positive birth strategy is implemented, then the population with more individuals has a higher probability to occupy the empty patch.

## HOW TO USE IT

You set up the size of the arena and the type of feedback for birth and death. For example if both is set to neutral (=) then you will observe random walk of the population sizes. Positive death strategy tends to stabilize the populations and tends
to lead to an equlibrium. On the other hand, positive birth strategy tends to drive the system to chatastrophy: due to the strong positive feedback effect, the population with
less individuals will die out quickly and the population with more individuals will go through an explosive increase.

## THINGS TO NOTICE

Experiment with different setups and follow the fate of the 2 populations. Understand the effect of negative and positive feedback on the populations.

## THINGS TO TRY

Try to run the simulation on different sizes of arena and study the effect of habitat size on stability of populations.

## EXTENDING THE MODEL

The model is deliberately oversimplified. The main goal of this model to teach the effect of feedback on a simple system. Our more complicated 2 populations model can be found here: http://ccl.northwestern.edu/netlogo/models/community/HabitatFragmentation

## NETLOGO FEATURES

See notes in the code for details

## RELATED MODELS

This model is inspired by the book of Eigen and Winkler: Das Spiel: Naturgesetze steuern den Zufall.

## CREDITS AND REFERENCES

This model was described and analysed in detailed in this book:
Zsíros, V. and Karsai, I. (1997). Death and Life, Randomity and Rule: Modeling some Fundamental Biological Mechanisms. Synbiologia Hungaria 3, Scientia, Budapest. (in Hungarian).

Eigen, M. and Winkler, R. (1975). Das Spiel: Naturgesetze steuern den Zufall, ISBN-13: 978-3492021517 Piper. pp. 403. (in German).