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## NetLogo User Community Models(back to the NetLogo User Community Models) ## Angels and Mortalsby Iain Weaver (Submitted: 02/01/2010)
This is a replication of Solomon's model to demonstrate a system where a description of the means is insufficient to predict it's evolution, as presented in "The importance of being discrete: Life always wins on the surface".
The landscape is initially populated by a uniform cloud of mortals with density nB. These are treated as continuous, and this cloud diffuses at a rate given by DB.
Next we add a distribution of angels with density nA. These are treated as discrete, and diffuse at a rate given DA.
Each tick, a fraction of mortals on each patch die, given by exp(-mu).
Is there is an angel occupying the patch, the mortals multiply by a factor given by exp(lambda).
We might naively predict that the population will evolve according to the differential equation
Setup sets each patches mortals to nB and scatters world-width*world-width*nA across the patches.
Each tick, angels and mortals diffuse, and then the mortals at each patch decay or multiply according to the rules above.
The patches are colored acording to the log of mortals there.
'Concentration' shows the density of mortals predicted by the above equation, compared to the true value determined from the model.
For the following parameters (used in Solomon's paper):
Where the equations predict extinction, the model shows that population can flourish.
Under what conditions does the model behave more, or less like the equations predict?
Increasing the diffusion rates, particularly for angels means the continuum approximation used to construct the equations becomes more accurate.
Conversely, reducing diffusion rates allow the mortals to benefit from the less mobile angels, particularly if they themselves diffuse from these sites more slowly.
Nadav M. Shnerb, Yoram Louzoun, Eldad Bettelheim, and Sorin Solomon - "The importance of being discrete: Life always wins on the surface"
Any suggestions or questions? e-mail: isw3@le.ac.uk |

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