System Dynamics Simulation creates unexpected result

Dear all,

I ported a simple system dynamics example to Grasshopper - the sheep and wolves polulation dynamics:http://ccl.northwestern.edu/netlogo/models/WolfSheepPredation(SystemDynamics)

Using the wonderful anemone loop component it works in principle. The result seems to be ok, but is different from the expected outcome compared e.g. with a NetLogo simulation:

NetLogo:

Grasshopper (red = sheep, blue = wolves):

The differnce is obvious, but I don't understand where it comes from. I used the expression component for modeling the individual aspects. Could it be some numerical problem (rounding)?

Thanks for some hints.

Best regards

Reinhard

Attachments:

02_WolvesSheep_AnemoneLoop.gh, 12 KB
WolvesSheepDefinition.PNG, 132 KB

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Replies to This Discussion

Permalink Reply by Joseph Oster on November 1, 2017 at 8:11am

Very cool. I don't now if your model is correct (though it looks very simple), but I can modify input values to produce a more similar result by decreasing 'Sheep Birth Rate' and increasing wolf 'Predation Rate' and 'Predator Efficiency' values. Are you sure you're using the same initial assumptions?

Permalink Reply by Reinhard Koenig on November 1, 2017 at 2:55pm

Thanks. I checked all parameters and formulas, but they are the same as in the NetLogo simulation. I also implemented the whole thing with a C# script (attached) to ensure that also the sequence is the same, but the dynamics are still different to the original NetLogo one...

Attachments:

02_WolvesSheep_Script_timer.gh, 14 KB

Permalink Reply by Reinhard Koenig on November 1, 2017 at 3:31pm

Apologize to all who spend some time looking into this. After writing the last response I checked again the sequence of all depencencies and finally found the issue there. Now, it works as expected. Corrected files are attached.

Best regards

Reinhard

Attachments: